[PATCH 0/9] ftrace: Have callbacks handle their own recursion

[PATCH 0/9] ftrace: Have callbacks handle their own recursion

[Steven Rostedt]

I found that having the ftrace infrastructure use its own trampoline to
handle recursion and RCU by defaulte unless the ftrace_ops set the
appropriate flags, was an issue that nobody set those flags. But then their
callbacks would suffer from an unnecessary overhead instead of simply
handling the recursion itself.

This series makes it mandatory that ftrace callbacks handle recursion or set
a flag asking ftrace to do it for it. It also creates helper functions to
help these callbacks to have recursion protection.

Steven Rostedt (VMware) (9):
      ftrace: Move the recursion testing into global headers
      ftrace: Add ftrace_test_recursion_trylock() helper function
      ftrace: Optimize testing what context current is in
      pstore/ftrace: Add recursion protection to the ftrace callback
      kprobes/ftrace: Add recursion protection to the ftrace callback
      livepatch/ftrace: Add recursion protection to the ftrace callback
      perf/ftrace: Add recursion protection to the ftrace callback
      perf/ftrace: Check for rcu_is_watching() in callback function
      ftrace: Reverse what the RECURSION flag means in the ftrace_ops

----
 Documentation/trace/ftrace-uses.rst  |  82 +++++++++++----
 arch/csky/kernel/probes/ftrace.c     |  12 ++-
 arch/parisc/kernel/ftrace.c          |  13 ++-
 arch/powerpc/kernel/kprobes-ftrace.c |  11 +-
 arch/s390/kernel/ftrace.c            |  13 ++-
 arch/x86/kernel/kprobes/ftrace.c     |  12 ++-
 fs/pstore/ftrace.c                   |   6 ++
 include/linux/ftrace.h               |  13 +--
 include/linux/trace_recursion.h      | 199 +++++++++++++++++++++++++++++++++++
 kernel/livepatch/patch.c             |   5 +
 kernel/trace/fgraph.c                |   3 +-
 kernel/trace/ftrace.c                |  20 ++--
 kernel/trace/trace.h                 | 156 ---------------------------
 kernel/trace/trace_event_perf.c      |  13 ++-
 kernel/trace/trace_events.c          |   1 -
 kernel/trace/trace_functions.c       |  14 ++-
 kernel/trace/trace_selftest.c        |   7 +-
 kernel/trace/trace_stack.c           |   1 -
 18 files changed, 358 insertions(+), 223 deletions(-)
 create mode 100644 include/linux/trace_recursion.h

[PATCH 1/9] ftrace: Move the recursion testing into global headers

[Steven Rostedt]

From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>

Currently, if a callback is registered to a ftrace function and its
ftrace_ops does not have the RECURSION flag set, it is encapsulated in a
helper function that does the recursion for it.

Really, all the callbacks should have their own recursion protection for
performance reasons. But they should not all implement their own. Move the
recursion helpers to global headers, so that all callbacks can use them.

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
 include/linux/ftrace.h          |   1 +
 include/linux/trace_recursion.h | 167 ++++++++++++++++++++++++++++++++
 kernel/trace/trace.h            | 156 -----------------------------
 3 files changed, 168 insertions(+), 156 deletions(-)
 create mode 100644 include/linux/trace_recursion.h

diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
index 1bd3a0356ae4..0e4164a7f56d 100644
--- a/include/linux/ftrace.h
+++ b/include/linux/ftrace.h
@@ -7,6 +7,7 @@
 #ifndef _LINUX_FTRACE_H
 #define _LINUX_FTRACE_H
 
+#include <linux/trace_recursion.h>
 #include <linux/trace_clock.h>
 #include <linux/kallsyms.h>
 #include <linux/linkage.h>
diff --git a/include/linux/trace_recursion.h b/include/linux/trace_recursion.h
new file mode 100644
index 000000000000..6c94c2ece9f0
--- /dev/null
+++ b/include/linux/trace_recursion.h
@@ -0,0 +1,167 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_TRACE_RECURSION_H
+#define _LINUX_TRACE_RECURSION_H
+
+#include <linux/interrupt.h>
+#include <linux/sched.h>
+
+#ifdef CONFIG_TRACING
+
+/* Only current can touch trace_recursion */
+
+/*
+ * For function tracing recursion:
+ *  The order of these bits are important.
+ *
+ *  When function tracing occurs, the following steps are made:
+ *   If arch does not support a ftrace feature:
+ *    call internal function (uses INTERNAL bits) which calls...
+ *   If callback is registered to the "global" list, the list
+ *    function is called and recursion checks the GLOBAL bits.
+ *    then this function calls...
+ *   The function callback, which can use the FTRACE bits to
+ *    check for recursion.
+ *
+ * Now if the arch does not support a feature, and it calls
+ * the global list function which calls the ftrace callback
+ * all three of these steps will do a recursion protection.
+ * There's no reason to do one if the previous caller already
+ * did. The recursion that we are protecting against will
+ * go through the same steps again.
+ *
+ * To prevent the multiple recursion checks, if a recursion
+ * bit is set that is higher than the MAX bit of the current
+ * check, then we know that the check was made by the previous
+ * caller, and we can skip the current check.
+ */
+enum {
+	/* Function recursion bits */
+	TRACE_FTRACE_BIT,
+	TRACE_FTRACE_NMI_BIT,
+	TRACE_FTRACE_IRQ_BIT,
+	TRACE_FTRACE_SIRQ_BIT,
+
+	/* INTERNAL_BITs must be greater than FTRACE_BITs */
+	TRACE_INTERNAL_BIT,
+	TRACE_INTERNAL_NMI_BIT,
+	TRACE_INTERNAL_IRQ_BIT,
+	TRACE_INTERNAL_SIRQ_BIT,
+
+	TRACE_BRANCH_BIT,
+/*
+ * Abuse of the trace_recursion.
+ * As we need a way to maintain state if we are tracing the function
+ * graph in irq because we want to trace a particular function that
+ * was called in irq context but we have irq tracing off. Since this
+ * can only be modified by current, we can reuse trace_recursion.
+ */
+	TRACE_IRQ_BIT,
+
+	/* Set if the function is in the set_graph_function file */
+	TRACE_GRAPH_BIT,
+
+	/*
+	 * In the very unlikely case that an interrupt came in
+	 * at a start of graph tracing, and we want to trace
+	 * the function in that interrupt, the depth can be greater
+	 * than zero, because of the preempted start of a previous
+	 * trace. In an even more unlikely case, depth could be 2
+	 * if a softirq interrupted the start of graph tracing,
+	 * followed by an interrupt preempting a start of graph
+	 * tracing in the softirq, and depth can even be 3
+	 * if an NMI came in at the start of an interrupt function
+	 * that preempted a softirq start of a function that
+	 * preempted normal context!!!! Luckily, it can't be
+	 * greater than 3, so the next two bits are a mask
+	 * of what the depth is when we set TRACE_GRAPH_BIT
+	 */
+
+	TRACE_GRAPH_DEPTH_START_BIT,
+	TRACE_GRAPH_DEPTH_END_BIT,
+
+	/*
+	 * To implement set_graph_notrace, if this bit is set, we ignore
+	 * function graph tracing of called functions, until the return
+	 * function is called to clear it.
+	 */
+	TRACE_GRAPH_NOTRACE_BIT,
+};
+
+#define trace_recursion_set(bit)	do { (current)->trace_recursion |= (1<<(bit)); } while (0)
+#define trace_recursion_clear(bit)	do { (current)->trace_recursion &= ~(1<<(bit)); } while (0)
+#define trace_recursion_test(bit)	((current)->trace_recursion & (1<<(bit)))
+
+#define trace_recursion_depth() \
+	(((current)->trace_recursion >> TRACE_GRAPH_DEPTH_START_BIT) & 3)
+#define trace_recursion_set_depth(depth) \
+	do {								\
+		current->trace_recursion &=				\
+			~(3 << TRACE_GRAPH_DEPTH_START_BIT);		\
+		current->trace_recursion |=				\
+			((depth) & 3) << TRACE_GRAPH_DEPTH_START_BIT;	\
+	} while (0)
+
+#define TRACE_CONTEXT_BITS	4
+
+#define TRACE_FTRACE_START	TRACE_FTRACE_BIT
+#define TRACE_FTRACE_MAX	((1 << (TRACE_FTRACE_START + TRACE_CONTEXT_BITS)) - 1)
+
+#define TRACE_LIST_START	TRACE_INTERNAL_BIT
+#define TRACE_LIST_MAX		((1 << (TRACE_LIST_START + TRACE_CONTEXT_BITS)) - 1)
+
+#define TRACE_CONTEXT_MASK	TRACE_LIST_MAX
+
+static __always_inline int trace_get_context_bit(void)
+{
+	int bit;
+
+	if (in_interrupt()) {
+		if (in_nmi())
+			bit = 0;
+
+		else if (in_irq())
+			bit = 1;
+		else
+			bit = 2;
+	} else
+		bit = 3;
+
+	return bit;
+}
+
+static __always_inline int trace_test_and_set_recursion(int start, int max)
+{
+	unsigned int val = current->trace_recursion;
+	int bit;
+
+	/* A previous recursion check was made */
+	if ((val & TRACE_CONTEXT_MASK) > max)
+		return 0;
+
+	bit = trace_get_context_bit() + start;
+	if (unlikely(val & (1 << bit)))
+		return -1;
+
+	val |= 1 << bit;
+	current->trace_recursion = val;
+	barrier();
+
+	return bit;
+}
+
+static __always_inline void trace_clear_recursion(int bit)
+{
+	unsigned int val = current->trace_recursion;
+
+	if (!bit)
+		return;
+
+	bit = 1 << bit;
+	val &= ~bit;
+
+	barrier();
+	current->trace_recursion = val;
+}
+
+#endif /* CONFIG_TRACING */
+#endif /* _LINUX_TRACE_RECURSION_H */
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index 34e0c4d5a6e7..2bb30ee4c3c6 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -559,162 +559,6 @@ struct tracer {
 };
 
 
-/* Only current can touch trace_recursion */
-
-/*
- * For function tracing recursion:
- *  The order of these bits are important.
- *
- *  When function tracing occurs, the following steps are made:
- *   If arch does not support a ftrace feature:
- *    call internal function (uses INTERNAL bits) which calls...
- *   If callback is registered to the "global" list, the list
- *    function is called and recursion checks the GLOBAL bits.
- *    then this function calls...
- *   The function callback, which can use the FTRACE bits to
- *    check for recursion.
- *
- * Now if the arch does not support a feature, and it calls
- * the global list function which calls the ftrace callback
- * all three of these steps will do a recursion protection.
- * There's no reason to do one if the previous caller already
- * did. The recursion that we are protecting against will
- * go through the same steps again.
- *
- * To prevent the multiple recursion checks, if a recursion
- * bit is set that is higher than the MAX bit of the current
- * check, then we know that the check was made by the previous
- * caller, and we can skip the current check.
- */
-enum {
-	/* Function recursion bits */
-	TRACE_FTRACE_BIT,
-	TRACE_FTRACE_NMI_BIT,
-	TRACE_FTRACE_IRQ_BIT,
-	TRACE_FTRACE_SIRQ_BIT,
-
-	/* INTERNAL_BITs must be greater than FTRACE_BITs */
-	TRACE_INTERNAL_BIT,
-	TRACE_INTERNAL_NMI_BIT,
-	TRACE_INTERNAL_IRQ_BIT,
-	TRACE_INTERNAL_SIRQ_BIT,
-
-	TRACE_BRANCH_BIT,
-/*
- * Abuse of the trace_recursion.
- * As we need a way to maintain state if we are tracing the function
- * graph in irq because we want to trace a particular function that
- * was called in irq context but we have irq tracing off. Since this
- * can only be modified by current, we can reuse trace_recursion.
- */
-	TRACE_IRQ_BIT,
-
-	/* Set if the function is in the set_graph_function file */
-	TRACE_GRAPH_BIT,
-
-	/*
-	 * In the very unlikely case that an interrupt came in
-	 * at a start of graph tracing, and we want to trace
-	 * the function in that interrupt, the depth can be greater
-	 * than zero, because of the preempted start of a previous
-	 * trace. In an even more unlikely case, depth could be 2
-	 * if a softirq interrupted the start of graph tracing,
-	 * followed by an interrupt preempting a start of graph
-	 * tracing in the softirq, and depth can even be 3
-	 * if an NMI came in at the start of an interrupt function
-	 * that preempted a softirq start of a function that
-	 * preempted normal context!!!! Luckily, it can't be
-	 * greater than 3, so the next two bits are a mask
-	 * of what the depth is when we set TRACE_GRAPH_BIT
-	 */
-
-	TRACE_GRAPH_DEPTH_START_BIT,
-	TRACE_GRAPH_DEPTH_END_BIT,
-
-	/*
-	 * To implement set_graph_notrace, if this bit is set, we ignore
-	 * function graph tracing of called functions, until the return
-	 * function is called to clear it.
-	 */
-	TRACE_GRAPH_NOTRACE_BIT,
-};
-
-#define trace_recursion_set(bit)	do { (current)->trace_recursion |= (1<<(bit)); } while (0)
-#define trace_recursion_clear(bit)	do { (current)->trace_recursion &= ~(1<<(bit)); } while (0)
-#define trace_recursion_test(bit)	((current)->trace_recursion & (1<<(bit)))
-
-#define trace_recursion_depth() \
-	(((current)->trace_recursion >> TRACE_GRAPH_DEPTH_START_BIT) & 3)
-#define trace_recursion_set_depth(depth) \
-	do {								\
-		current->trace_recursion &=				\
-			~(3 << TRACE_GRAPH_DEPTH_START_BIT);		\
-		current->trace_recursion |=				\
-			((depth) & 3) << TRACE_GRAPH_DEPTH_START_BIT;	\
-	} while (0)
-
-#define TRACE_CONTEXT_BITS	4
-
-#define TRACE_FTRACE_START	TRACE_FTRACE_BIT
-#define TRACE_FTRACE_MAX	((1 << (TRACE_FTRACE_START + TRACE_CONTEXT_BITS)) - 1)
-
-#define TRACE_LIST_START	TRACE_INTERNAL_BIT
-#define TRACE_LIST_MAX		((1 << (TRACE_LIST_START + TRACE_CONTEXT_BITS)) - 1)
-
-#define TRACE_CONTEXT_MASK	TRACE_LIST_MAX
-
-static __always_inline int trace_get_context_bit(void)
-{
-	int bit;
-
-	if (in_interrupt()) {
-		if (in_nmi())
-			bit = 0;
-
-		else if (in_irq())
-			bit = 1;
-		else
-			bit = 2;
-	} else
-		bit = 3;
-
-	return bit;
-}
-
-static __always_inline int trace_test_and_set_recursion(int start, int max)
-{
-	unsigned int val = current->trace_recursion;
-	int bit;
-
-	/* A previous recursion check was made */
-	if ((val & TRACE_CONTEXT_MASK) > max)
-		return 0;
-
-	bit = trace_get_context_bit() + start;
-	if (unlikely(val & (1 << bit)))
-		return -1;
-
-	val |= 1 << bit;
-	current->trace_recursion = val;
-	barrier();
-
-	return bit;
-}
-
-static __always_inline void trace_clear_recursion(int bit)
-{
-	unsigned int val = current->trace_recursion;
-
-	if (!bit)
-		return;
-
-	bit = 1 << bit;
-	val &= ~bit;
-
-	barrier();
-	current->trace_recursion = val;
-}
-
 static inline struct ring_buffer_iter *
 trace_buffer_iter(struct trace_iterator *iter, int cpu)
 {
-- 
2.28.0

Re: [PATCH 1/9] ftrace: Move the recursion testing into global headers

[Miroslav Benes]

Hi,

> +static __always_inline int trace_get_context_bit(void)
> +{
> +	int bit;
> +
> +	if (in_interrupt()) {
> +		if (in_nmi())
> +			bit = 0;
> +
> +		else if (in_irq())
> +			bit = 1;
> +		else
> +			bit = 2;
> +	} else
> +		bit = 3;
> +
> +	return bit;
> +}
> +
> +static __always_inline int trace_test_and_set_recursion(int start, int max)
> +{
> +	unsigned int val = current->trace_recursion;
> +	int bit;
> +
> +	/* A previous recursion check was made */
> +	if ((val & TRACE_CONTEXT_MASK) > max)
> +		return 0;
> +
> +	bit = trace_get_context_bit() + start;
> +	if (unlikely(val & (1 << bit)))
> +		return -1;
> +
> +	val |= 1 << bit;
> +	current->trace_recursion = val;
> +	barrier();
> +
> +	return bit;
> +}

how does this work in case of NMI? trace_get_context_bit() returns 0 (it 
does not change later in the patch set). "start" in 
trace_test_and_set_recursion() is 0 zero too as used later in the patch 
set by ftrace_test_recursion_trylock(). So trace_test_and_set_recursion() 
returns 0. That is perfectly sane, but then...

> +static __always_inline void trace_clear_recursion(int bit)
> +{
> +	unsigned int val = current->trace_recursion;
> +
> +	if (!bit)
> +		return;

... the bit is not cleared here.

> +	bit = 1 << bit;
> +	val &= ~bit;
> +
> +	barrier();
> +	current->trace_recursion = val;
> +}

Thanks
Miroslav

Re: [PATCH 1/9] ftrace: Move the recursion testing into global headers

[Steven Rostedt]

On Fri, 30 Oct 2020 10:13:50 +0100 (CET)
Miroslav Benes <mbenes@suse.cz> wrote:

> how does this work in case of NMI? trace_get_context_bit() returns 0 (it 
> does not change later in the patch set). "start" in 
> trace_test_and_set_recursion() is 0 zero too as used later in the patch 
> set by ftrace_test_recursion_trylock(). So trace_test_and_set_recursion() 
> returns 0. That is perfectly sane, but then...
> 
> > +static __always_inline void trace_clear_recursion(int bit)
> > +{
> > +	unsigned int val = current->trace_recursion;
> > +
> > +	if (!bit)
> > +		return;  
> 
> ... the bit is not cleared here.

Yeah, I found that and fixed it yesterday, which discovered stack
overflow in perf:

  https://lore.kernel.org/r/20201030002722.766a22df@oasis.local.home

There's another bug that causes a false positive during interrupt
context transition. I have a fix for that too.

-- Steve

[PATCH 2/9] ftrace: Add ftrace_test_recursion_trylock() helper function

[Steven Rostedt]

From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>

To make it easier for ftrace callbacks to have recursion protection, provide
a ftrace_test_recursion_trylock() and ftrace_test_recursion_unlock() helper
that tests for recursion.

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
 include/linux/trace_recursion.h | 25 +++++++++++++++++++++++++
 kernel/trace/trace_functions.c  | 12 +++++-------
 2 files changed, 30 insertions(+), 7 deletions(-)

diff --git a/include/linux/trace_recursion.h b/include/linux/trace_recursion.h
index 6c94c2ece9f0..da17a1144bf4 100644
--- a/include/linux/trace_recursion.h
+++ b/include/linux/trace_recursion.h
@@ -163,5 +163,30 @@ static __always_inline void trace_clear_recursion(int bit)
 	current->trace_recursion = val;
 }
 
+/**
+ * ftrace_test_recursion_trylock - tests for recursion in same context
+ *
+ * Use this for ftrace callbacks. This will detect if the function
+ * tracing recursed in the same context (normal vs interrupt),
+ *
+ * Returns: -1 if a recursion happened.
+ *           >= 0 if no recursion
+ */
+static __always_inline int ftrace_test_recursion_trylock(void)
+{
+	return trace_test_and_set_recursion(TRACE_FTRACE_START, TRACE_FTRACE_MAX);
+}
+
+/**
+ * ftrace_test_recursion_unlock - called when function callback is complete
+ * @bit: The return of a successful ftrace_test_recursion_trylock()
+ *
+ * This is used at the end of a ftrace callback.
+ */
+static __always_inline void ftrace_test_recursion_unlock(int bit)
+{
+	trace_clear_recursion(bit);
+}
+
 #endif /* CONFIG_TRACING */
 #endif /* _LINUX_TRACE_RECURSION_H */
diff --git a/kernel/trace/trace_functions.c b/kernel/trace/trace_functions.c
index 2c2126e1871d..943756c01190 100644
--- a/kernel/trace/trace_functions.c
+++ b/kernel/trace/trace_functions.c
@@ -141,22 +141,20 @@ function_trace_call(unsigned long ip, unsigned long parent_ip,
 	if (unlikely(!tr->function_enabled))
 		return;
 
+	bit = ftrace_test_recursion_trylock();
+	if (bit < 0)
+		return;
+
 	pc = preempt_count();
 	preempt_disable_notrace();
 
-	bit = trace_test_and_set_recursion(TRACE_FTRACE_START, TRACE_FTRACE_MAX);
-	if (bit < 0)
-		goto out;
-
 	cpu = smp_processor_id();
 	data = per_cpu_ptr(tr->array_buffer.data, cpu);
 	if (!atomic_read(&data->disabled)) {
 		local_save_flags(flags);
 		trace_function(tr, ip, parent_ip, flags, pc);
 	}
-	trace_clear_recursion(bit);
-
- out:
+	ftrace_test_recursion_unlock(bit);
 	preempt_enable_notrace();
 }
 
-- 
2.28.0

[PATCH 3/9] ftrace: Optimize testing what context current is in

[Steven Rostedt]

From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>

The preempt_count() is not a simple location in memory, it could be part of
per_cpu code or more. Each access to preempt_count(), or one of its accessor
functions (like in_interrupt()) takes several cycles. By reading
preempt_count() once, and then doing tests to find the context against the
value return is slightly faster than using in_nmi() and in_interrupt().

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
 include/linux/trace_recursion.h | 33 ++++++++++++++++++++-------------
 1 file changed, 20 insertions(+), 13 deletions(-)

diff --git a/include/linux/trace_recursion.h b/include/linux/trace_recursion.h
index da17a1144bf4..04d21e9ec48b 100644
--- a/include/linux/trace_recursion.h
+++ b/include/linux/trace_recursion.h
@@ -111,22 +111,29 @@ enum {
 
 #define TRACE_CONTEXT_MASK	TRACE_LIST_MAX
 
+/*
+ * Used for setting context
+ *  NMI     = 0
+ *  IRQ     = 1
+ *  SOFTIRQ = 2
+ *  NORMAL  = 3
+ */
+enum {
+	TRACE_CTX_NMI,
+	TRACE_CTX_IRQ,
+	TRACE_CTX_SOFTIRQ,
+	TRACE_CTX_NORMAL,
+};
+
 static __always_inline int trace_get_context_bit(void)
 {
-	int bit;
-
-	if (in_interrupt()) {
-		if (in_nmi())
-			bit = 0;
+	unsigned long pc = preempt_count();
 
-		else if (in_irq())
-			bit = 1;
-		else
-			bit = 2;
-	} else
-		bit = 3;
-
-	return bit;
+	if (!(pc & (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_OFFSET)))
+		return TRACE_CTX_NORMAL;
+	else
+		return pc & NMI_MASK ? TRACE_CTX_NMI :
+			pc & HARDIRQ_MASK ? TRACE_CTX_IRQ : TRACE_CTX_SOFTIRQ;
 }
 
 static __always_inline int trace_test_and_set_recursion(int start, int max)
-- 
2.28.0

[PATCH 4/9] pstore/ftrace: Add recursion protection to the ftrace callback

[Steven Rostedt]

From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>

If a ftrace callback does not supply its own recursion protection and
does not set the RECURSION_SAFE flag in its ftrace_ops, then ftrace will
make a helper trampoline to do so before calling the callback instead of
just calling the callback directly.

The default for ftrace_ops is going to assume recursion protection unless
otherwise specified.

Cc: Thomas Meyer <thomas@m3y3r.de>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
 fs/pstore/ftrace.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/pstore/ftrace.c b/fs/pstore/ftrace.c
index 5c0450701293..816210fc5d3a 100644
--- a/fs/pstore/ftrace.c
+++ b/fs/pstore/ftrace.c
@@ -28,6 +28,7 @@ static void notrace pstore_ftrace_call(unsigned long ip,
 				       struct ftrace_ops *op,
 				       struct pt_regs *regs)
 {
+	int bit;
 	unsigned long flags;
 	struct pstore_ftrace_record rec = {};
 	struct pstore_record record = {
@@ -40,6 +41,10 @@ static void notrace pstore_ftrace_call(unsigned long ip,
 	if (unlikely(oops_in_progress))
 		return;
 
+	bit = ftrace_test_recursion_trylock();
+	if (bit < 0)
+		return;
+
 	local_irq_save(flags);
 
 	rec.ip = ip;
@@ -49,6 +54,7 @@ static void notrace pstore_ftrace_call(unsigned long ip,
 	psinfo->write(&record);
 
 	local_irq_restore(flags);
+	ftrace_test_recursion_unlock(bit);
 }
 
 static struct ftrace_ops pstore_ftrace_ops __read_mostly = {
-- 
2.28.0

Re: [PATCH 4/9] pstore/ftrace: Add recursion protection to the ftrace callback

[Kees Cook]

On Wed, Oct 28, 2020 at 07:52:48AM -0400, Steven Rostedt wrote:
> From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
> 
> If a ftrace callback does not supply its own recursion protection and
> does not set the RECURSION_SAFE flag in its ftrace_ops, then ftrace will
> make a helper trampoline to do so before calling the callback instead of
> just calling the callback directly.
> 
> The default for ftrace_ops is going to assume recursion protection unless
> otherwise specified.
> 
> Cc: Thomas Meyer <thomas@m3y3r.de>
> Cc: Kees Cook <keescook@chromium.org>
> Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

Reviewed-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook

[PATCH 5/9] kprobes/ftrace: Add recursion protection to the ftrace callback

[Steven Rostedt]

From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>

If a ftrace callback does not supply its own recursion protection and
does not set the RECURSION_SAFE flag in its ftrace_ops, then ftrace will
make a helper trampoline to do so before calling the callback instead of
just calling the callback directly.

The default for ftrace_ops is going to assume recursion protection unless
otherwise specified.

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Guo Ren <guoren@kernel.org>
Cc: "James E.J. Bottomley" <James.Bottomley@HansenPartnership.com>
Cc: Helge Deller <deller@gmx.de>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Borislav Petkov <bp@alien8.de>
Cc: x86@kernel.org
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: "Naveen N. Rao" <naveen.n.rao@linux.ibm.com>
Cc: Anil S Keshavamurthy <anil.s.keshavamurthy@intel.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: linux-csky@vger.kernel.org
Cc: linux-parisc@vger.kernel.org
Cc: linuxppc-dev@lists.ozlabs.org
Cc: linux-s390@vger.kernel.org
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
 arch/csky/kernel/probes/ftrace.c     | 12 ++++++++++--
 arch/parisc/kernel/ftrace.c          | 13 +++++++++++--
 arch/powerpc/kernel/kprobes-ftrace.c | 11 ++++++++++-
 arch/s390/kernel/ftrace.c            | 13 +++++++++++--
 arch/x86/kernel/kprobes/ftrace.c     | 12 ++++++++++--
 5 files changed, 52 insertions(+), 9 deletions(-)

diff --git a/arch/csky/kernel/probes/ftrace.c b/arch/csky/kernel/probes/ftrace.c
index 5264763d05be..5eb2604fdf71 100644
--- a/arch/csky/kernel/probes/ftrace.c
+++ b/arch/csky/kernel/probes/ftrace.c
@@ -13,16 +13,21 @@ int arch_check_ftrace_location(struct kprobe *p)
 void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
 			   struct ftrace_ops *ops, struct pt_regs *regs)
 {
+	int bit;
 	bool lr_saver = false;
 	struct kprobe *p;
 	struct kprobe_ctlblk *kcb;
 
-	/* Preempt is disabled by ftrace */
+	bit = ftrace_test_recursion_trylock();
+	if (bit < 0)
+		return;
+
+	preempt_disable_notrace();
 	p = get_kprobe((kprobe_opcode_t *)ip);
 	if (!p) {
 		p = get_kprobe((kprobe_opcode_t *)(ip - MCOUNT_INSN_SIZE));
 		if (unlikely(!p) || kprobe_disabled(p))
-			return;
+			goto out;
 		lr_saver = true;
 	}
 
@@ -56,6 +61,9 @@ void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
 		 */
 		__this_cpu_write(current_kprobe, NULL);
 	}
+out:
+	preempt_enable_notrace();
+	ftrace_test_recursion_unlock(bit);
 }
 NOKPROBE_SYMBOL(kprobe_ftrace_handler);
 
diff --git a/arch/parisc/kernel/ftrace.c b/arch/parisc/kernel/ftrace.c
index 4bab21c71055..5f7742b225a5 100644
--- a/arch/parisc/kernel/ftrace.c
+++ b/arch/parisc/kernel/ftrace.c
@@ -208,13 +208,19 @@ void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
 {
 	struct kprobe_ctlblk *kcb;
 	struct kprobe *p = get_kprobe((kprobe_opcode_t *)ip);
+	int bit;
 
-	if (unlikely(!p) || kprobe_disabled(p))
+	bit = ftrace_test_recursion_trylock();
+	if (bit < 0)
 		return;
 
+	preempt_disable_notrace();
+	if (unlikely(!p) || kprobe_disabled(p))
+		goto out;
+
 	if (kprobe_running()) {
 		kprobes_inc_nmissed_count(p);
-		return;
+		goto out;
 	}
 
 	__this_cpu_write(current_kprobe, p);
@@ -235,6 +241,9 @@ void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
 		}
 	}
 	__this_cpu_write(current_kprobe, NULL);
+out:
+	preempt_enable_notrace();
+	ftrace_test_recursion_unlock(bit);
 }
 NOKPROBE_SYMBOL(kprobe_ftrace_handler);
 
diff --git a/arch/powerpc/kernel/kprobes-ftrace.c b/arch/powerpc/kernel/kprobes-ftrace.c
index 972cb28174b2..5df8d50c65ae 100644
--- a/arch/powerpc/kernel/kprobes-ftrace.c
+++ b/arch/powerpc/kernel/kprobes-ftrace.c
@@ -18,10 +18,16 @@ void kprobe_ftrace_handler(unsigned long nip, unsigned long parent_nip,
 {
 	struct kprobe *p;
 	struct kprobe_ctlblk *kcb;
+	int bit;
 
+	bit = ftrace_test_recursion_trylock();
+	if (bit < 0)
+		return;
+
+	preempt_disable_notrace();
 	p = get_kprobe((kprobe_opcode_t *)nip);
 	if (unlikely(!p) || kprobe_disabled(p))
-		return;
+		goto out;
 
 	kcb = get_kprobe_ctlblk();
 	if (kprobe_running()) {
@@ -52,6 +58,9 @@ void kprobe_ftrace_handler(unsigned long nip, unsigned long parent_nip,
 		 */
 		__this_cpu_write(current_kprobe, NULL);
 	}
+out:
+	preempt_enable_notrace();
+	ftrace_test_recursion_unlock(bit);
 }
 NOKPROBE_SYMBOL(kprobe_ftrace_handler);
 
diff --git a/arch/s390/kernel/ftrace.c b/arch/s390/kernel/ftrace.c
index b388e87a08bf..88466d7fb6b2 100644
--- a/arch/s390/kernel/ftrace.c
+++ b/arch/s390/kernel/ftrace.c
@@ -202,13 +202,19 @@ void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
 {
 	struct kprobe_ctlblk *kcb;
 	struct kprobe *p = get_kprobe((kprobe_opcode_t *)ip);
+	int bit;
 
-	if (unlikely(!p) || kprobe_disabled(p))
+	bit = ftrace_test_recursion_trylock();
+	if (bit < 0)
 		return;
 
+	preempt_disable_notrace();
+	if (unlikely(!p) || kprobe_disabled(p))
+		goto out;
+
 	if (kprobe_running()) {
 		kprobes_inc_nmissed_count(p);
-		return;
+		goto out;
 	}
 
 	__this_cpu_write(current_kprobe, p);
@@ -228,6 +234,9 @@ void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
 		}
 	}
 	__this_cpu_write(current_kprobe, NULL);
+out:
+	preempt_enable_notrace();
+	ftrace_test_recursion_unlock(bit);
 }
 NOKPROBE_SYMBOL(kprobe_ftrace_handler);
 
diff --git a/arch/x86/kernel/kprobes/ftrace.c b/arch/x86/kernel/kprobes/ftrace.c
index 681a4b36e9bb..a40a6cdfcca3 100644
--- a/arch/x86/kernel/kprobes/ftrace.c
+++ b/arch/x86/kernel/kprobes/ftrace.c
@@ -18,11 +18,16 @@ void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
 {
 	struct kprobe *p;
 	struct kprobe_ctlblk *kcb;
+	int bit;
 
-	/* Preempt is disabled by ftrace */
+	bit = ftrace_test_recursion_trylock();
+	if (bit < 0)
+		return;
+
+	preempt_disable_notrace();
 	p = get_kprobe((kprobe_opcode_t *)ip);
 	if (unlikely(!p) || kprobe_disabled(p))
-		return;
+		goto out;
 
 	kcb = get_kprobe_ctlblk();
 	if (kprobe_running()) {
@@ -52,6 +57,9 @@ void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
 		 */
 		__this_cpu_write(current_kprobe, NULL);
 	}
+out:
+	preempt_enable_notrace();
+	ftrace_test_recursion_unlock(bit);
 }
 NOKPROBE_SYMBOL(kprobe_ftrace_handler);
 
-- 
2.28.0

[PATCH 5/9] kprobes/ftrace: Add recursion protection to the ftrace callback

[Steven Rostedt]

From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>

If a ftrace callback does not supply its own recursion protection and
does not set the RECURSION_SAFE flag in its ftrace_ops, then ftrace will
make a helper trampoline to do so before calling the callback instead of
just calling the callback directly.

The default for ftrace_ops is going to assume recursion protection unless
otherwise specified.

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Guo Ren <guoren@kernel.org>
Cc: "James E.J. Bottomley" <James.Bottomley@HansenPartnership.com>
Cc: Helge Deller <deller@gmx.de>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Borislav Petkov <bp@alien8.de>
Cc: x86@kernel.org
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: "Naveen N. Rao" <naveen.n.rao@linux.ibm.com>
Cc: Anil S Keshavamurthy <anil.s.keshavamurthy@intel.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: linux-csky@vger.kernel.org
Cc: linux-parisc@vger.kernel.org
Cc: linuxppc-dev@lists.ozlabs.org
Cc: linux-s390@vger.kernel.org
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
 arch/csky/kernel/probes/ftrace.c     | 12 ++++++++++--
 arch/parisc/kernel/ftrace.c          | 13 +++++++++++--
 arch/powerpc/kernel/kprobes-ftrace.c | 11 ++++++++++-
 arch/s390/kernel/ftrace.c            | 13 +++++++++++--
 arch/x86/kernel/kprobes/ftrace.c     | 12 ++++++++++--
 5 files changed, 52 insertions(+), 9 deletions(-)

diff --git a/arch/csky/kernel/probes/ftrace.c b/arch/csky/kernel/probes/ftrace.c
index 5264763d05be..5eb2604fdf71 100644
--- a/arch/csky/kernel/probes/ftrace.c
+++ b/arch/csky/kernel/probes/ftrace.c
@@ -13,16 +13,21 @@ int arch_check_ftrace_location(struct kprobe *p)
 void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
 			   struct ftrace_ops *ops, struct pt_regs *regs)
 {
+	int bit;
 	bool lr_saver = false;
 	struct kprobe *p;
 	struct kprobe_ctlblk *kcb;
 
-	/* Preempt is disabled by ftrace */
+	bit = ftrace_test_recursion_trylock();
+	if (bit < 0)
+		return;
+
+	preempt_disable_notrace();
 	p = get_kprobe((kprobe_opcode_t *)ip);
 	if (!p) {
 		p = get_kprobe((kprobe_opcode_t *)(ip - MCOUNT_INSN_SIZE));
 		if (unlikely(!p) || kprobe_disabled(p))
-			return;
+			goto out;
 		lr_saver = true;
 	}
 
@@ -56,6 +61,9 @@ void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
 		 */
 		__this_cpu_write(current_kprobe, NULL);
 	}
+out:
+	preempt_enable_notrace();
+	ftrace_test_recursion_unlock(bit);
 }
 NOKPROBE_SYMBOL(kprobe_ftrace_handler);
 
diff --git a/arch/parisc/kernel/ftrace.c b/arch/parisc/kernel/ftrace.c
index 4bab21c71055..5f7742b225a5 100644
--- a/arch/parisc/kernel/ftrace.c
+++ b/arch/parisc/kernel/ftrace.c
@@ -208,13 +208,19 @@ void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
 {
 	struct kprobe_ctlblk *kcb;
 	struct kprobe *p = get_kprobe((kprobe_opcode_t *)ip);
+	int bit;
 
-	if (unlikely(!p) || kprobe_disabled(p))
+	bit = ftrace_test_recursion_trylock();
+	if (bit < 0)
 		return;
 
+	preempt_disable_notrace();
+	if (unlikely(!p) || kprobe_disabled(p))
+		goto out;
+
 	if (kprobe_running()) {
 		kprobes_inc_nmissed_count(p);
-		return;
+		goto out;
 	}
 
 	__this_cpu_write(current_kprobe, p);
@@ -235,6 +241,9 @@ void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
 		}
 	}
 	__this_cpu_write(current_kprobe, NULL);
+out:
+	preempt_enable_notrace();
+	ftrace_test_recursion_unlock(bit);
 }
 NOKPROBE_SYMBOL(kprobe_ftrace_handler);
 
diff --git a/arch/powerpc/kernel/kprobes-ftrace.c b/arch/powerpc/kernel/kprobes-ftrace.c
index 972cb28174b2..5df8d50c65ae 100644
--- a/arch/powerpc/kernel/kprobes-ftrace.c
+++ b/arch/powerpc/kernel/kprobes-ftrace.c
@@ -18,10 +18,16 @@ void kprobe_ftrace_handler(unsigned long nip, unsigned long parent_nip,
 {
 	struct kprobe *p;
 	struct kprobe_ctlblk *kcb;
+	int bit;
 
+	bit = ftrace_test_recursion_trylock();
+	if (bit < 0)
+		return;
+
+	preempt_disable_notrace();
 	p = get_kprobe((kprobe_opcode_t *)nip);
 	if (unlikely(!p) || kprobe_disabled(p))
-		return;
+		goto out;
 
 	kcb = get_kprobe_ctlblk();
 	if (kprobe_running()) {
@@ -52,6 +58,9 @@ void kprobe_ftrace_handler(unsigned long nip, unsigned long parent_nip,
 		 */
 		__this_cpu_write(current_kprobe, NULL);
 	}
+out:
+	preempt_enable_notrace();
+	ftrace_test_recursion_unlock(bit);
 }
 NOKPROBE_SYMBOL(kprobe_ftrace_handler);
 
diff --git a/arch/s390/kernel/ftrace.c b/arch/s390/kernel/ftrace.c
index b388e87a08bf..88466d7fb6b2 100644
--- a/arch/s390/kernel/ftrace.c
+++ b/arch/s390/kernel/ftrace.c
@@ -202,13 +202,19 @@ void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
 {
 	struct kprobe_ctlblk *kcb;
 	struct kprobe *p = get_kprobe((kprobe_opcode_t *)ip);
+	int bit;
 
-	if (unlikely(!p) || kprobe_disabled(p))
+	bit = ftrace_test_recursion_trylock();
+	if (bit < 0)
 		return;
 
+	preempt_disable_notrace();
+	if (unlikely(!p) || kprobe_disabled(p))
+		goto out;
+
 	if (kprobe_running()) {
 		kprobes_inc_nmissed_count(p);
-		return;
+		goto out;
 	}
 
 	__this_cpu_write(current_kprobe, p);
@@ -228,6 +234,9 @@ void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
 		}
 	}
 	__this_cpu_write(current_kprobe, NULL);
+out:
+	preempt_enable_notrace();
+	ftrace_test_recursion_unlock(bit);
 }
 NOKPROBE_SYMBOL(kprobe_ftrace_handler);
 
diff --git a/arch/x86/kernel/kprobes/ftrace.c b/arch/x86/kernel/kprobes/ftrace.c
index 681a4b36e9bb..a40a6cdfcca3 100644
--- a/arch/x86/kernel/kprobes/ftrace.c
+++ b/arch/x86/kernel/kprobes/ftrace.c
@@ -18,11 +18,16 @@ void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
 {
 	struct kprobe *p;
 	struct kprobe_ctlblk *kcb;
+	int bit;
 
-	/* Preempt is disabled by ftrace */
+	bit = ftrace_test_recursion_trylock();
+	if (bit < 0)
+		return;
+
+	preempt_disable_notrace();
 	p = get_kprobe((kprobe_opcode_t *)ip);
 	if (unlikely(!p) || kprobe_disabled(p))
-		return;
+		goto out;
 
 	kcb = get_kprobe_ctlblk();
 	if (kprobe_running()) {
@@ -52,6 +57,9 @@ void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
 		 */
 		__this_cpu_write(current_kprobe, NULL);
 	}
+out:
+	preempt_enable_notrace();
+	ftrace_test_recursion_unlock(bit);
 }
 NOKPROBE_SYMBOL(kprobe_ftrace_handler);
 
-- 
2.28.0

Re: [PATCH 5/9] kprobes/ftrace: Add recursion protection to the ftrace callback

[Masami Hiramatsu]

Hi Steve,

On Wed, 28 Oct 2020 07:52:49 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
> 
> If a ftrace callback does not supply its own recursion protection and
> does not set the RECURSION_SAFE flag in its ftrace_ops, then ftrace will
> make a helper trampoline to do so before calling the callback instead of
> just calling the callback directly.

So in that case the handlers will be called without preempt disabled?


> The default for ftrace_ops is going to assume recursion protection unless
> otherwise specified.

This seems to skip entier handler if ftrace finds recursion.
I would like to increment the missed counter even in that case.

[...]
e.g.

> diff --git a/arch/csky/kernel/probes/ftrace.c b/arch/csky/kernel/probes/ftrace.c
> index 5264763d05be..5eb2604fdf71 100644
> --- a/arch/csky/kernel/probes/ftrace.c
> +++ b/arch/csky/kernel/probes/ftrace.c
> @@ -13,16 +13,21 @@ int arch_check_ftrace_location(struct kprobe *p)
>  void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
>  			   struct ftrace_ops *ops, struct pt_regs *regs)
>  {
> +	int bit;
>  	bool lr_saver = false;
>  	struct kprobe *p;
>  	struct kprobe_ctlblk *kcb;
>  
> -	/* Preempt is disabled by ftrace */
> +	bit = ftrace_test_recursion_trylock();

> +
> +	preempt_disable_notrace();
>  	p = get_kprobe((kprobe_opcode_t *)ip);
>  	if (!p) {
>  		p = get_kprobe((kprobe_opcode_t *)(ip - MCOUNT_INSN_SIZE));
>  		if (unlikely(!p) || kprobe_disabled(p))
> -			return;
> +			goto out;
>  		lr_saver = true;
>  	}

	if (bit < 0) {
		kprobes_inc_nmissed_count(p);
		goto out;
	}

>  
> @@ -56,6 +61,9 @@ void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
>  		 */
>  		__this_cpu_write(current_kprobe, NULL);
>  	}
> +out:
> +	preempt_enable_notrace();

	if (bit >= 0)
		ftrace_test_recursion_unlock(bit);

>  }
>  NOKPROBE_SYMBOL(kprobe_ftrace_handler);
>  

Or, we can also introduce a support function,

static inline void kprobes_inc_nmissed_ip(unsigned long ip)
{
	struct kprobe *p;

	preempt_disable_notrace();
	p = get_kprobe((kprobe_opcode_t *)ip);
	if (p)
		kprobes_inc_nmissed_count(p);
	preempt_enable_notrace();
}

> diff --git a/arch/parisc/kernel/ftrace.c b/arch/parisc/kernel/ftrace.c
> index 4bab21c71055..5f7742b225a5 100644
> --- a/arch/parisc/kernel/ftrace.c
> +++ b/arch/parisc/kernel/ftrace.c
> @@ -208,13 +208,19 @@ void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
>  {
>  	struct kprobe_ctlblk *kcb;
>  	struct kprobe *p = get_kprobe((kprobe_opcode_t *)ip);

(BTW, here is a bug... get_kprobe() must be called with preempt disabled.)

> +	int bit;
>  
> -	if (unlikely(!p) || kprobe_disabled(p))
> +	bit = ftrace_test_recursion_trylock();

	if (bit < 0) {
		kprobes_inc_nmissed_ip(ip);
>  		return;
	}

This may easier for you ?

Thank you,

>  
> +	preempt_disable_notrace();
> +	if (unlikely(!p) || kprobe_disabled(p))
> +		goto out;
> +
>  	if (kprobe_running()) {
>  		kprobes_inc_nmissed_count(p);
> -		return;
> +		goto out;
>  	}
>  
>  	__this_cpu_write(current_kprobe, p);
> @@ -235,6 +241,9 @@ void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
>  		}
>  	}
>  	__this_cpu_write(current_kprobe, NULL);
> +out:
> +	preempt_enable_notrace();
> +	ftrace_test_recursion_unlock(bit);
>  }
>  NOKPROBE_SYMBOL(kprobe_ftrace_handler);
>  
> diff --git a/arch/powerpc/kernel/kprobes-ftrace.c b/arch/powerpc/kernel/kprobes-ftrace.c
> index 972cb28174b2..5df8d50c65ae 100644
> --- a/arch/powerpc/kernel/kprobes-ftrace.c
> +++ b/arch/powerpc/kernel/kprobes-ftrace.c
> @@ -18,10 +18,16 @@ void kprobe_ftrace_handler(unsigned long nip, unsigned long parent_nip,
>  {
>  	struct kprobe *p;
>  	struct kprobe_ctlblk *kcb;
> +	int bit;
>  
> +	bit = ftrace_test_recursion_trylock();
> +	if (bit < 0)
> +		return;
> +
> +	preempt_disable_notrace();
>  	p = get_kprobe((kprobe_opcode_t *)nip);
>  	if (unlikely(!p) || kprobe_disabled(p))
> -		return;
> +		goto out;
>  
>  	kcb = get_kprobe_ctlblk();
>  	if (kprobe_running()) {
> @@ -52,6 +58,9 @@ void kprobe_ftrace_handler(unsigned long nip, unsigned long parent_nip,
>  		 */
>  		__this_cpu_write(current_kprobe, NULL);
>  	}
> +out:
> +	preempt_enable_notrace();
> +	ftrace_test_recursion_unlock(bit);
>  }
>  NOKPROBE_SYMBOL(kprobe_ftrace_handler);
>  
> diff --git a/arch/s390/kernel/ftrace.c b/arch/s390/kernel/ftrace.c
> index b388e87a08bf..88466d7fb6b2 100644
> --- a/arch/s390/kernel/ftrace.c
> +++ b/arch/s390/kernel/ftrace.c
> @@ -202,13 +202,19 @@ void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
>  {
>  	struct kprobe_ctlblk *kcb;
>  	struct kprobe *p = get_kprobe((kprobe_opcode_t *)ip);
> +	int bit;
>  
> -	if (unlikely(!p) || kprobe_disabled(p))
> +	bit = ftrace_test_recursion_trylock();
> +	if (bit < 0)
>  		return;
>  
> +	preempt_disable_notrace();
> +	if (unlikely(!p) || kprobe_disabled(p))
> +		goto out;
> +
>  	if (kprobe_running()) {
>  		kprobes_inc_nmissed_count(p);
> -		return;
> +		goto out;
>  	}
>  
>  	__this_cpu_write(current_kprobe, p);
> @@ -228,6 +234,9 @@ void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
>  		}
>  	}
>  	__this_cpu_write(current_kprobe, NULL);
> +out:
> +	preempt_enable_notrace();
> +	ftrace_test_recursion_unlock(bit);
>  }
>  NOKPROBE_SYMBOL(kprobe_ftrace_handler);
>  
> diff --git a/arch/x86/kernel/kprobes/ftrace.c b/arch/x86/kernel/kprobes/ftrace.c
> index 681a4b36e9bb..a40a6cdfcca3 100644
> --- a/arch/x86/kernel/kprobes/ftrace.c
> +++ b/arch/x86/kernel/kprobes/ftrace.c
> @@ -18,11 +18,16 @@ void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
>  {
>  	struct kprobe *p;
>  	struct kprobe_ctlblk *kcb;
> +	int bit;
>  
> -	/* Preempt is disabled by ftrace */
> +	bit = ftrace_test_recursion_trylock();
> +	if (bit < 0)
> +		return;
> +
> +	preempt_disable_notrace();
>  	p = get_kprobe((kprobe_opcode_t *)ip);
>  	if (unlikely(!p) || kprobe_disabled(p))
> -		return;
> +		goto out;
>  
>  	kcb = get_kprobe_ctlblk();
>  	if (kprobe_running()) {
> @@ -52,6 +57,9 @@ void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
>  		 */
>  		__this_cpu_write(current_kprobe, NULL);
>  	}
> +out:
> +	preempt_enable_notrace();
> +	ftrace_test_recursion_unlock(bit);
>  }
>  NOKPROBE_SYMBOL(kprobe_ftrace_handler);
>  
> -- 
> 2.28.0
> 
> 


-- 
Masami Hiramatsu <mhiramat@kernel.org>

Re: [PATCH 5/9] kprobes/ftrace: Add recursion protection to the ftrace callback

[Masami Hiramatsu]

Hi Steve,

On Wed, 28 Oct 2020 07:52:49 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
> 
> If a ftrace callback does not supply its own recursion protection and
> does not set the RECURSION_SAFE flag in its ftrace_ops, then ftrace will
> make a helper trampoline to do so before calling the callback instead of
> just calling the callback directly.

So in that case the handlers will be called without preempt disabled?


> The default for ftrace_ops is going to assume recursion protection unless
> otherwise specified.

This seems to skip entier handler if ftrace finds recursion.
I would like to increment the missed counter even in that case.

[...]
e.g.

> diff --git a/arch/csky/kernel/probes/ftrace.c b/arch/csky/kernel/probes/ftrace.c
> index 5264763d05be..5eb2604fdf71 100644
> --- a/arch/csky/kernel/probes/ftrace.c
> +++ b/arch/csky/kernel/probes/ftrace.c
> @@ -13,16 +13,21 @@ int arch_check_ftrace_location(struct kprobe *p)
>  void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
>  			   struct ftrace_ops *ops, struct pt_regs *regs)
>  {
> +	int bit;
>  	bool lr_saver = false;
>  	struct kprobe *p;
>  	struct kprobe_ctlblk *kcb;
>  
> -	/* Preempt is disabled by ftrace */
> +	bit = ftrace_test_recursion_trylock();

> +
> +	preempt_disable_notrace();
>  	p = get_kprobe((kprobe_opcode_t *)ip);
>  	if (!p) {
>  		p = get_kprobe((kprobe_opcode_t *)(ip - MCOUNT_INSN_SIZE));
>  		if (unlikely(!p) || kprobe_disabled(p))
> -			return;
> +			goto out;
>  		lr_saver = true;
>  	}

	if (bit < 0) {
		kprobes_inc_nmissed_count(p);
		goto out;
	}

>  
> @@ -56,6 +61,9 @@ void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
>  		 */
>  		__this_cpu_write(current_kprobe, NULL);
>  	}
> +out:
> +	preempt_enable_notrace();

	if (bit >= 0)
		ftrace_test_recursion_unlock(bit);

>  }
>  NOKPROBE_SYMBOL(kprobe_ftrace_handler);
>  

Or, we can also introduce a support function,

static inline void kprobes_inc_nmissed_ip(unsigned long ip)
{
	struct kprobe *p;

	preempt_disable_notrace();
	p = get_kprobe((kprobe_opcode_t *)ip);
	if (p)
		kprobes_inc_nmissed_count(p);
	preempt_enable_notrace();
}

> diff --git a/arch/parisc/kernel/ftrace.c b/arch/parisc/kernel/ftrace.c
> index 4bab21c71055..5f7742b225a5 100644
> --- a/arch/parisc/kernel/ftrace.c
> +++ b/arch/parisc/kernel/ftrace.c
> @@ -208,13 +208,19 @@ void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
>  {
>  	struct kprobe_ctlblk *kcb;
>  	struct kprobe *p = get_kprobe((kprobe_opcode_t *)ip);

(BTW, here is a bug... get_kprobe() must be called with preempt disabled.)

> +	int bit;
>  
> -	if (unlikely(!p) || kprobe_disabled(p))
> +	bit = ftrace_test_recursion_trylock();

	if (bit < 0) {
		kprobes_inc_nmissed_ip(ip);
>  		return;
	}

This may easier for you ?

Thank you,

>  
> +	preempt_disable_notrace();
> +	if (unlikely(!p) || kprobe_disabled(p))
> +		goto out;
> +
>  	if (kprobe_running()) {
>  		kprobes_inc_nmissed_count(p);
> -		return;
> +		goto out;
>  	}
>  
>  	__this_cpu_write(current_kprobe, p);
> @@ -235,6 +241,9 @@ void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
>  		}
>  	}
>  	__this_cpu_write(current_kprobe, NULL);
> +out:
> +	preempt_enable_notrace();
> +	ftrace_test_recursion_unlock(bit);
>  }
>  NOKPROBE_SYMBOL(kprobe_ftrace_handler);
>  
> diff --git a/arch/powerpc/kernel/kprobes-ftrace.c b/arch/powerpc/kernel/kprobes-ftrace.c
> index 972cb28174b2..5df8d50c65ae 100644
> --- a/arch/powerpc/kernel/kprobes-ftrace.c
> +++ b/arch/powerpc/kernel/kprobes-ftrace.c
> @@ -18,10 +18,16 @@ void kprobe_ftrace_handler(unsigned long nip, unsigned long parent_nip,
>  {
>  	struct kprobe *p;
>  	struct kprobe_ctlblk *kcb;
> +	int bit;
>  
> +	bit = ftrace_test_recursion_trylock();
> +	if (bit < 0)
> +		return;
> +
> +	preempt_disable_notrace();
>  	p = get_kprobe((kprobe_opcode_t *)nip);
>  	if (unlikely(!p) || kprobe_disabled(p))
> -		return;
> +		goto out;
>  
>  	kcb = get_kprobe_ctlblk();
>  	if (kprobe_running()) {
> @@ -52,6 +58,9 @@ void kprobe_ftrace_handler(unsigned long nip, unsigned long parent_nip,
>  		 */
>  		__this_cpu_write(current_kprobe, NULL);
>  	}
> +out:
> +	preempt_enable_notrace();
> +	ftrace_test_recursion_unlock(bit);
>  }
>  NOKPROBE_SYMBOL(kprobe_ftrace_handler);
>  
> diff --git a/arch/s390/kernel/ftrace.c b/arch/s390/kernel/ftrace.c
> index b388e87a08bf..88466d7fb6b2 100644
> --- a/arch/s390/kernel/ftrace.c
> +++ b/arch/s390/kernel/ftrace.c
> @@ -202,13 +202,19 @@ void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
>  {
>  	struct kprobe_ctlblk *kcb;
>  	struct kprobe *p = get_kprobe((kprobe_opcode_t *)ip);
> +	int bit;
>  
> -	if (unlikely(!p) || kprobe_disabled(p))
> +	bit = ftrace_test_recursion_trylock();
> +	if (bit < 0)
>  		return;
>  
> +	preempt_disable_notrace();
> +	if (unlikely(!p) || kprobe_disabled(p))
> +		goto out;
> +
>  	if (kprobe_running()) {
>  		kprobes_inc_nmissed_count(p);
> -		return;
> +		goto out;
>  	}
>  
>  	__this_cpu_write(current_kprobe, p);
> @@ -228,6 +234,9 @@ void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
>  		}
>  	}
>  	__this_cpu_write(current_kprobe, NULL);
> +out:
> +	preempt_enable_notrace();
> +	ftrace_test_recursion_unlock(bit);
>  }
>  NOKPROBE_SYMBOL(kprobe_ftrace_handler);
>  
> diff --git a/arch/x86/kernel/kprobes/ftrace.c b/arch/x86/kernel/kprobes/ftrace.c
> index 681a4b36e9bb..a40a6cdfcca3 100644
> --- a/arch/x86/kernel/kprobes/ftrace.c
> +++ b/arch/x86/kernel/kprobes/ftrace.c
> @@ -18,11 +18,16 @@ void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
>  {
>  	struct kprobe *p;
>  	struct kprobe_ctlblk *kcb;
> +	int bit;
>  
> -	/* Preempt is disabled by ftrace */
> +	bit = ftrace_test_recursion_trylock();
> +	if (bit < 0)
> +		return;
> +
> +	preempt_disable_notrace();
>  	p = get_kprobe((kprobe_opcode_t *)ip);
>  	if (unlikely(!p) || kprobe_disabled(p))
> -		return;
> +		goto out;
>  
>  	kcb = get_kprobe_ctlblk();
>  	if (kprobe_running()) {
> @@ -52,6 +57,9 @@ void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
>  		 */
>  		__this_cpu_write(current_kprobe, NULL);
>  	}
> +out:
> +	preempt_enable_notrace();
> +	ftrace_test_recursion_unlock(bit);
>  }
>  NOKPROBE_SYMBOL(kprobe_ftrace_handler);
>  
> -- 
> 2.28.0
> 
> 


-- 
Masami Hiramatsu <mhiramat@kernel.org>

Re: [PATCH 5/9] kprobes/ftrace: Add recursion protection to the ftrace callback

[Steven Rostedt]

On Thu, 29 Oct 2020 16:58:03 +0900
Masami Hiramatsu <mhiramat@kernel.org> wrote:

> Hi Steve,
> 
> On Wed, 28 Oct 2020 07:52:49 -0400
> Steven Rostedt <rostedt@goodmis.org> wrote:
> 
> > From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
> > 
> > If a ftrace callback does not supply its own recursion protection and
> > does not set the RECURSION_SAFE flag in its ftrace_ops, then ftrace will
> > make a helper trampoline to do so before calling the callback instead of
> > just calling the callback directly.  
> 
> So in that case the handlers will be called without preempt disabled?
> 
> 
> > The default for ftrace_ops is going to assume recursion protection unless
> > otherwise specified.  
> 
> This seems to skip entier handler if ftrace finds recursion.
> I would like to increment the missed counter even in that case.

Note, this code does not change the functionality at this point, because
without having the FL_RECURSION flag set (which kprobes does not even in
this patch), it always gets called from the helper function that does this:

	bit = trace_test_and_set_recursion(TRACE_LIST_START, TRACE_LIST_MAX);
	if (bit < 0)
		return;

	preempt_disable_notrace();

	op->func(ip, parent_ip, op, regs);

	preempt_enable_notrace();
	trace_clear_recursion(bit);

Where this function gets called by op->func().

In other words, you don't get that count anyway, and I don't think you want
it. Because it means you traced something that your callback calls.

That bit check is basically a nop, because the last patch in this series
will make the default that everything has recursion protection, but at this
patch the test does this:

	/* A previous recursion check was made */
	if ((val & TRACE_CONTEXT_MASK) > max)
		return 0;

Which would always return true, because this function is called via the
helper that already did the trace_test_and_set_recursion() which, if it
made it this far, the val would always be greater than max.

> 
> [...]
> e.g.
> 
> > diff --git a/arch/csky/kernel/probes/ftrace.c b/arch/csky/kernel/probes/ftrace.c
> > index 5264763d05be..5eb2604fdf71 100644
> > --- a/arch/csky/kernel/probes/ftrace.c
> > +++ b/arch/csky/kernel/probes/ftrace.c
> > @@ -13,16 +13,21 @@ int arch_check_ftrace_location(struct kprobe *p)
> >  void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
> >  			   struct ftrace_ops *ops, struct pt_regs *regs)
> >  {
> > +	int bit;
> >  	bool lr_saver = false;
> >  	struct kprobe *p;
> >  	struct kprobe_ctlblk *kcb;
> >  
> > -	/* Preempt is disabled by ftrace */
> > +	bit = ftrace_test_recursion_trylock();  
> 
> > +
> > +	preempt_disable_notrace();
> >  	p = get_kprobe((kprobe_opcode_t *)ip);
> >  	if (!p) {
> >  		p = get_kprobe((kprobe_opcode_t *)(ip - MCOUNT_INSN_SIZE));
> >  		if (unlikely(!p) || kprobe_disabled(p))
> > -			return;
> > +			goto out;
> >  		lr_saver = true;
> >  	}  
> 
> 	if (bit < 0) {
> 		kprobes_inc_nmissed_count(p);
> 		goto out;
> 	}

If anything called in get_kprobe() or kprobes_inc_nmissed_count() gets
traced here, you have zero recursion protection, and this will crash the
machine with a likely reboot (triple fault).

Note, the recursion handles interrupts and wont stop them. bit < 0 only
happens if you recurse because this function called something that ends up
calling itself. Really, why would you care about missing a kprobe on the
same kprobe?

-- Steve

Re: [PATCH 5/9] kprobes/ftrace: Add recursion protection to the ftrace callback

[Steven Rostedt]

On Thu, 29 Oct 2020 16:58:03 +0900
Masami Hiramatsu <mhiramat@kernel.org> wrote:

> Hi Steve,
> 
> On Wed, 28 Oct 2020 07:52:49 -0400
> Steven Rostedt <rostedt@goodmis.org> wrote:
> 
> > From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
> > 
> > If a ftrace callback does not supply its own recursion protection and
> > does not set the RECURSION_SAFE flag in its ftrace_ops, then ftrace will
> > make a helper trampoline to do so before calling the callback instead of
> > just calling the callback directly.  
> 
> So in that case the handlers will be called without preempt disabled?
> 
> 
> > The default for ftrace_ops is going to assume recursion protection unless
> > otherwise specified.  
> 
> This seems to skip entier handler if ftrace finds recursion.
> I would like to increment the missed counter even in that case.

Note, this code does not change the functionality at this point, because
without having the FL_RECURSION flag set (which kprobes does not even in
this patch), it always gets called from the helper function that does this:

	bit = trace_test_and_set_recursion(TRACE_LIST_START, TRACE_LIST_MAX);
	if (bit < 0)
		return;

	preempt_disable_notrace();

	op->func(ip, parent_ip, op, regs);

	preempt_enable_notrace();
	trace_clear_recursion(bit);

Where this function gets called by op->func().

In other words, you don't get that count anyway, and I don't think you want
it. Because it means you traced something that your callback calls.

That bit check is basically a nop, because the last patch in this series
will make the default that everything has recursion protection, but at this
patch the test does this:

	/* A previous recursion check was made */
	if ((val & TRACE_CONTEXT_MASK) > max)
		return 0;

Which would always return true, because this function is called via the
helper that already did the trace_test_and_set_recursion() which, if it
made it this far, the val would always be greater than max.

> 
> [...]
> e.g.
> 
> > diff --git a/arch/csky/kernel/probes/ftrace.c b/arch/csky/kernel/probes/ftrace.c
> > index 5264763d05be..5eb2604fdf71 100644
> > --- a/arch/csky/kernel/probes/ftrace.c
> > +++ b/arch/csky/kernel/probes/ftrace.c
> > @@ -13,16 +13,21 @@ int arch_check_ftrace_location(struct kprobe *p)
> >  void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
> >  			   struct ftrace_ops *ops, struct pt_regs *regs)
> >  {
> > +	int bit;
> >  	bool lr_saver = false;
> >  	struct kprobe *p;
> >  	struct kprobe_ctlblk *kcb;
> >  
> > -	/* Preempt is disabled by ftrace */
> > +	bit = ftrace_test_recursion_trylock();  
> 
> > +
> > +	preempt_disable_notrace();
> >  	p = get_kprobe((kprobe_opcode_t *)ip);
> >  	if (!p) {
> >  		p = get_kprobe((kprobe_opcode_t *)(ip - MCOUNT_INSN_SIZE));
> >  		if (unlikely(!p) || kprobe_disabled(p))
> > -			return;
> > +			goto out;
> >  		lr_saver = true;
> >  	}  
> 
> 	if (bit < 0) {
> 		kprobes_inc_nmissed_count(p);
> 		goto out;
> 	}

If anything called in get_kprobe() or kprobes_inc_nmissed_count() gets
traced here, you have zero recursion protection, and this will crash the
machine with a likely reboot (triple fault).

Note, the recursion handles interrupts and wont stop them. bit < 0 only
happens if you recurse because this function called something that ends up
calling itself. Really, why would you care about missing a kprobe on the
same kprobe?

-- Steve

Re: [PATCH 5/9] kprobes/ftrace: Add recursion protection to the ftrace callback

[Masami Hiramatsu]

On Thu, 29 Oct 2020 09:40:01 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Thu, 29 Oct 2020 16:58:03 +0900
> Masami Hiramatsu <mhiramat@kernel.org> wrote:
> 
> > Hi Steve,
> > 
> > On Wed, 28 Oct 2020 07:52:49 -0400
> > Steven Rostedt <rostedt@goodmis.org> wrote:
> > 
> > > From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
> > > 
> > > If a ftrace callback does not supply its own recursion protection and
> > > does not set the RECURSION_SAFE flag in its ftrace_ops, then ftrace will
> > > make a helper trampoline to do so before calling the callback instead of
> > > just calling the callback directly.  
> > 
> > So in that case the handlers will be called without preempt disabled?
> > 
> > 
> > > The default for ftrace_ops is going to assume recursion protection unless
> > > otherwise specified.  
> > 
> > This seems to skip entier handler if ftrace finds recursion.
> > I would like to increment the missed counter even in that case.
> 
> Note, this code does not change the functionality at this point, because
> without having the FL_RECURSION flag set (which kprobes does not even in
> this patch), it always gets called from the helper function that does this:
> 
> 	bit = trace_test_and_set_recursion(TRACE_LIST_START, TRACE_LIST_MAX);
> 	if (bit < 0)
> 		return;
> 
> 	preempt_disable_notrace();
> 
> 	op->func(ip, parent_ip, op, regs);
> 
> 	preempt_enable_notrace();
> 	trace_clear_recursion(bit);
> 
> Where this function gets called by op->func().
> 
> In other words, you don't get that count anyway, and I don't think you want
> it. Because it means you traced something that your callback calls.

Got it. So nmissed count increment will be an improvement.

> 
> That bit check is basically a nop, because the last patch in this series
> will make the default that everything has recursion protection, but at this
> patch the test does this:
> 
> 	/* A previous recursion check was made */
> 	if ((val & TRACE_CONTEXT_MASK) > max)
> 		return 0;
> 
> Which would always return true, because this function is called via the
> helper that already did the trace_test_and_set_recursion() which, if it
> made it this far, the val would always be greater than max.

OK, let me check the last patch too.

> 
> > 
> > [...]
> > e.g.
> > 
> > > diff --git a/arch/csky/kernel/probes/ftrace.c b/arch/csky/kernel/probes/ftrace.c
> > > index 5264763d05be..5eb2604fdf71 100644
> > > --- a/arch/csky/kernel/probes/ftrace.c
> > > +++ b/arch/csky/kernel/probes/ftrace.c
> > > @@ -13,16 +13,21 @@ int arch_check_ftrace_location(struct kprobe *p)
> > >  void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
> > >  			   struct ftrace_ops *ops, struct pt_regs *regs)
> > >  {
> > > +	int bit;
> > >  	bool lr_saver = false;
> > >  	struct kprobe *p;
> > >  	struct kprobe_ctlblk *kcb;
> > >  
> > > -	/* Preempt is disabled by ftrace */
> > > +	bit = ftrace_test_recursion_trylock();  
> > 
> > > +
> > > +	preempt_disable_notrace();
> > >  	p = get_kprobe((kprobe_opcode_t *)ip);
> > >  	if (!p) {
> > >  		p = get_kprobe((kprobe_opcode_t *)(ip - MCOUNT_INSN_SIZE));
> > >  		if (unlikely(!p) || kprobe_disabled(p))
> > > -			return;
> > > +			goto out;
> > >  		lr_saver = true;
> > >  	}  
> > 
> > 	if (bit < 0) {
> > 		kprobes_inc_nmissed_count(p);
> > 		goto out;
> > 	}
> 
> If anything called in get_kprobe() or kprobes_inc_nmissed_count() gets
> traced here, you have zero recursion protection, and this will crash the
> machine with a likely reboot (triple fault).

Oops, ok, those can be traced. 

> 
> Note, the recursion handles interrupts and wont stop them. bit < 0 only
> happens if you recurse because this function called something that ends up
> calling itself. Really, why would you care about missing a kprobe on the
> same kprobe?

Usually, sw-breakpoint based kprobes will count that case. Moreover, kprobes
shares one ftrace_ops among all kprobes. I guess in that case any kprobes
in kprobes (e.g. recursive call inside kprobe pre_handlers) will be skipped
by ftrace_test_recursion_trylock(), is that correct?

Thank you,

> 
> -- Steve


-- 
Masami Hiramatsu <mhiramat@kernel.org>

Re: [PATCH 5/9] kprobes/ftrace: Add recursion protection to the ftrace callback

[Masami Hiramatsu]

On Thu, 29 Oct 2020 09:40:01 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Thu, 29 Oct 2020 16:58:03 +0900
> Masami Hiramatsu <mhiramat@kernel.org> wrote:
> 
> > Hi Steve,
> > 
> > On Wed, 28 Oct 2020 07:52:49 -0400
> > Steven Rostedt <rostedt@goodmis.org> wrote:
> > 
> > > From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
> > > 
> > > If a ftrace callback does not supply its own recursion protection and
> > > does not set the RECURSION_SAFE flag in its ftrace_ops, then ftrace will
> > > make a helper trampoline to do so before calling the callback instead of
> > > just calling the callback directly.  
> > 
> > So in that case the handlers will be called without preempt disabled?
> > 
> > 
> > > The default for ftrace_ops is going to assume recursion protection unless
> > > otherwise specified.  
> > 
> > This seems to skip entier handler if ftrace finds recursion.
> > I would like to increment the missed counter even in that case.
> 
> Note, this code does not change the functionality at this point, because
> without having the FL_RECURSION flag set (which kprobes does not even in
> this patch), it always gets called from the helper function that does this:
> 
> 	bit = trace_test_and_set_recursion(TRACE_LIST_START, TRACE_LIST_MAX);
> 	if (bit < 0)
> 		return;
> 
> 	preempt_disable_notrace();
> 
> 	op->func(ip, parent_ip, op, regs);
> 
> 	preempt_enable_notrace();
> 	trace_clear_recursion(bit);
> 
> Where this function gets called by op->func().
> 
> In other words, you don't get that count anyway, and I don't think you want
> it. Because it means you traced something that your callback calls.

Got it. So nmissed count increment will be an improvement.

> 
> That bit check is basically a nop, because the last patch in this series
> will make the default that everything has recursion protection, but at this
> patch the test does this:
> 
> 	/* A previous recursion check was made */
> 	if ((val & TRACE_CONTEXT_MASK) > max)
> 		return 0;
> 
> Which would always return true, because this function is called via the
> helper that already did the trace_test_and_set_recursion() which, if it
> made it this far, the val would always be greater than max.

OK, let me check the last patch too.

> 
> > 
> > [...]
> > e.g.
> > 
> > > diff --git a/arch/csky/kernel/probes/ftrace.c b/arch/csky/kernel/probes/ftrace.c
> > > index 5264763d05be..5eb2604fdf71 100644
> > > --- a/arch/csky/kernel/probes/ftrace.c
> > > +++ b/arch/csky/kernel/probes/ftrace.c
> > > @@ -13,16 +13,21 @@ int arch_check_ftrace_location(struct kprobe *p)
> > >  void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
> > >  			   struct ftrace_ops *ops, struct pt_regs *regs)
> > >  {
> > > +	int bit;
> > >  	bool lr_saver = false;
> > >  	struct kprobe *p;
> > >  	struct kprobe_ctlblk *kcb;
> > >  
> > > -	/* Preempt is disabled by ftrace */
> > > +	bit = ftrace_test_recursion_trylock();  
> > 
> > > +
> > > +	preempt_disable_notrace();
> > >  	p = get_kprobe((kprobe_opcode_t *)ip);
> > >  	if (!p) {
> > >  		p = get_kprobe((kprobe_opcode_t *)(ip - MCOUNT_INSN_SIZE));
> > >  		if (unlikely(!p) || kprobe_disabled(p))
> > > -			return;
> > > +			goto out;
> > >  		lr_saver = true;
> > >  	}  
> > 
> > 	if (bit < 0) {
> > 		kprobes_inc_nmissed_count(p);
> > 		goto out;
> > 	}
> 
> If anything called in get_kprobe() or kprobes_inc_nmissed_count() gets
> traced here, you have zero recursion protection, and this will crash the
> machine with a likely reboot (triple fault).

Oops, ok, those can be traced. 

> 
> Note, the recursion handles interrupts and wont stop them. bit < 0 only
> happens if you recurse because this function called something that ends up
> calling itself. Really, why would you care about missing a kprobe on the
> same kprobe?

Usually, sw-breakpoint based kprobes will count that case. Moreover, kprobes
shares one ftrace_ops among all kprobes. I guess in that case any kprobes
in kprobes (e.g. recursive call inside kprobe pre_handlers) will be skipped
by ftrace_test_recursion_trylock(), is that correct?

Thank you,

> 
> -- Steve


-- 
Masami Hiramatsu <mhiramat@kernel.org>

[PATCH 6/9] livepatch/ftrace: Add recursion protection to the ftrace callback

[Steven Rostedt]

From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>

If a ftrace callback does not supply its own recursion protection and
does not set the RECURSION_SAFE flag in its ftrace_ops, then ftrace will
make a helper trampoline to do so before calling the callback instead of
just calling the callback directly.

The default for ftrace_ops is going to assume recursion protection unless
otherwise specified.

Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Jiri Kosina <jikos@kernel.org>
Cc: Miroslav Benes <mbenes@suse.cz>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Joe Lawrence <joe.lawrence@redhat.com>
Cc: live-patching@vger.kernel.org
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
 kernel/livepatch/patch.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/livepatch/patch.c b/kernel/livepatch/patch.c
index b552cf2d85f8..6c0164d24bbd 100644
--- a/kernel/livepatch/patch.c
+++ b/kernel/livepatch/patch.c
@@ -45,9 +45,13 @@ static void notrace klp_ftrace_handler(unsigned long ip,
 	struct klp_ops *ops;
 	struct klp_func *func;
 	int patch_state;
+	int bit;
 
 	ops = container_of(fops, struct klp_ops, fops);
 
+	bit = ftrace_test_recursion_trylock();
+	if (bit < 0)
+		return;
 	/*
 	 * A variant of synchronize_rcu() is used to allow patching functions
 	 * where RCU is not watching, see klp_synchronize_transition().
@@ -117,6 +121,7 @@ static void notrace klp_ftrace_handler(unsigned long ip,
 
 unlock:
 	preempt_enable_notrace();
+	ftrace_test_recursion_unlock(bit);
 }
 
 /*
-- 
2.28.0

Re: [PATCH 6/9] livepatch/ftrace: Add recursion protection to the ftrace callback

[Miroslav Benes]

On Wed, 28 Oct 2020, Steven Rostedt wrote:

> From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
> 
> If a ftrace callback does not supply its own recursion protection and
> does not set the RECURSION_SAFE flag in its ftrace_ops, then ftrace will
> make a helper trampoline to do so before calling the callback instead of
> just calling the callback directly.
> 
> The default for ftrace_ops is going to assume recursion protection unless
> otherwise specified.

Hm, I've always thought that we did not need any kind of recursion 
protection for our callback. It is marked as notrace and it does not call 
anything traceable. In fact, it does not call anything. I even have a note 
in my todo list to mark the callback as RECURSION_SAFE :)

At the same time, it probably does not hurt and the patch is still better 
than what we have now without RECURSION_SAFE if I understand the patch set 
correctly.
 
> Cc: Josh Poimboeuf <jpoimboe@redhat.com>
> Cc: Jiri Kosina <jikos@kernel.org>
> Cc: Miroslav Benes <mbenes@suse.cz>
> Cc: Petr Mladek <pmladek@suse.com>
> Cc: Joe Lawrence <joe.lawrence@redhat.com>
> Cc: live-patching@vger.kernel.org
> Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> ---
>  kernel/livepatch/patch.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/kernel/livepatch/patch.c b/kernel/livepatch/patch.c
> index b552cf2d85f8..6c0164d24bbd 100644
> --- a/kernel/livepatch/patch.c
> +++ b/kernel/livepatch/patch.c
> @@ -45,9 +45,13 @@ static void notrace klp_ftrace_handler(unsigned long ip,
>  	struct klp_ops *ops;
>  	struct klp_func *func;
>  	int patch_state;
> +	int bit;
>  
>  	ops = container_of(fops, struct klp_ops, fops);
>  
> +	bit = ftrace_test_recursion_trylock();
> +	if (bit < 0)
> +		return;

This means that the original function will be called in case of recursion. 
That's probably fair, but I'm wondering if we should at least WARN about 
it.

Thanks
Miroslav

Re: [PATCH 6/9] livepatch/ftrace: Add recursion protection to the ftrace callback

[Steven Rostedt]

On Thu, 29 Oct 2020 14:51:06 +0100 (CET)
Miroslav Benes <mbenes@suse.cz> wrote:

> > index b552cf2d85f8..6c0164d24bbd 100644
> > --- a/kernel/livepatch/patch.c
> > +++ b/kernel/livepatch/patch.c
> > @@ -45,9 +45,13 @@ static void notrace klp_ftrace_handler(unsigned long ip,
> >  	struct klp_ops *ops;
> >  	struct klp_func *func;
> >  	int patch_state;
> > +	int bit;
> >  
> >  	ops = container_of(fops, struct klp_ops, fops);
> >  
> > +	bit = ftrace_test_recursion_trylock();
> > +	if (bit < 0)
> > +		return;  
> 
> This means that the original function will be called in case of recursion. 
> That's probably fair, but I'm wondering if we should at least WARN about 
> it.

It's probably what happens today. But if you add a WARN_ON_ONCE() it may
not hurt.

I also plan on adding code that reports when recursion has happened,
because even if it's not a problem, recursion adds extra overhead.

-- Steve

Re: [PATCH 6/9] livepatch/ftrace: Add recursion protection to the ftrace callback

[Steven Rostedt]

On Thu, 29 Oct 2020 10:37:44 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> I also plan on adding code that reports when recursion has happened,
> because even if it's not a problem, recursion adds extra overhead.

I did the above (will be posting that later, maybe next week), and
found two bugs with the recursion code. :-/

One was in the nmi handling, where it never cleared the nmi bit
(because it was zero, and thus ignored), and that caused all functions
in NMI handlers to not be traced (because it thought it was a
recursion).
(see https://lore.kernel.org/r/20201030002722.766a22df@oasis.local.home)

The second was the recursion algorithm depends on the preempt_count()
being accurate, but when it transitions between context, and there's
tracing in that transition, it could falsely record it as a recursion.

I have a fix for both of these bugs and will be sending them up marked
for stable after I finish testing them.

This goes to show that the recursion reported should be implemented
(but that will be for the next merge window).

-- Steve

Re: [PATCH 6/9] livepatch/ftrace: Add recursion protection to the ftrace callback

[Petr Mladek]

On Thu 2020-10-29 14:51:06, Miroslav Benes wrote:
> On Wed, 28 Oct 2020, Steven Rostedt wrote:
> 
> > From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
> > 
> > If a ftrace callback does not supply its own recursion protection and
> > does not set the RECURSION_SAFE flag in its ftrace_ops, then ftrace will
> > make a helper trampoline to do so before calling the callback instead of
> > just calling the callback directly.
> > 
> > The default for ftrace_ops is going to assume recursion protection unless
> > otherwise specified.

It might be my lack skills to read English. But the above sentence
sounds ambiguous to me. It is not clear to me who provides the
recursion protection by default. Could you please make it more
explicit, for example by:

"The default for ftrace_ops is going to change. It will expect that
handlers provide their own recursion protection."


> Hm, I've always thought that we did not need any kind of recursion 
> protection for our callback. It is marked as notrace and it does not call 
> anything traceable. In fact, it does not call anything. I even have a note 
> in my todo list to mark the callback as RECURSION_SAFE :)

Well, it calls WARN_ON_ONCE() ;-)

> At the same time, it probably does not hurt and the patch is still better 
> than what we have now without RECURSION_SAFE if I understand the patch set 
> correctly.

And better be on the safe side.


> > Cc: Josh Poimboeuf <jpoimboe@redhat.com>
> > Cc: Jiri Kosina <jikos@kernel.org>
> > Cc: Miroslav Benes <mbenes@suse.cz>
> > Cc: Petr Mladek <pmladek@suse.com>
> > Cc: Joe Lawrence <joe.lawrence@redhat.com>
> > Cc: live-patching@vger.kernel.org
> > Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> > ---
> >  kernel/livepatch/patch.c | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > diff --git a/kernel/livepatch/patch.c b/kernel/livepatch/patch.c
> > index b552cf2d85f8..6c0164d24bbd 100644
> > --- a/kernel/livepatch/patch.c
> > +++ b/kernel/livepatch/patch.c
> > @@ -45,9 +45,13 @@ static void notrace klp_ftrace_handler(unsigned long ip,
> >  	struct klp_ops *ops;
> >  	struct klp_func *func;
> >  	int patch_state;
> > +	int bit;
> >  
> >  	ops = container_of(fops, struct klp_ops, fops);
> >  
> > +	bit = ftrace_test_recursion_trylock();
> > +	if (bit < 0)
> > +		return;
> 
> This means that the original function will be called in case of recursion. 
> That's probably fair, but I'm wondering if we should at least WARN about 
> it.

Yeah, the early return might break the consistency model and
unexpected things might happen. We should be aware of it.
Please use:

	if (WARN_ON_ONCE(bit < 0))
		return;

WARN_ON_ONCE() might be part of the recursion. But it should happen
only once. IMHO, it is worth the risk.

Otherwise it looks good.

Best Regards,
Petr

Re: [PATCH 6/9] livepatch/ftrace: Add recursion protection to the ftrace callback

[Miroslav Benes]

On Thu, 29 Oct 2020, Petr Mladek wrote:

> On Thu 2020-10-29 14:51:06, Miroslav Benes wrote:
> > On Wed, 28 Oct 2020, Steven Rostedt wrote:
> 
> > Hm, I've always thought that we did not need any kind of recursion 
> > protection for our callback. It is marked as notrace and it does not call 
> > anything traceable. In fact, it does not call anything. I even have a note 
> > in my todo list to mark the callback as RECURSION_SAFE :)
> 
> Well, it calls WARN_ON_ONCE() ;-)

Oh my, I learned to ignore these. Of course there is printk hidden 
everywhere.

> > At the same time, it probably does not hurt and the patch is still better 
> > than what we have now without RECURSION_SAFE if I understand the patch set 
> > correctly.
> 
> And better be on the safe side.

Agreed. 
 
> > > Cc: Josh Poimboeuf <jpoimboe@redhat.com>
> > > Cc: Jiri Kosina <jikos@kernel.org>
> > > Cc: Miroslav Benes <mbenes@suse.cz>
> > > Cc: Petr Mladek <pmladek@suse.com>
> > > Cc: Joe Lawrence <joe.lawrence@redhat.com>
> > > Cc: live-patching@vger.kernel.org
> > > Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> > > ---
> > >  kernel/livepatch/patch.c | 5 +++++
> > >  1 file changed, 5 insertions(+)
> > > 
> > > diff --git a/kernel/livepatch/patch.c b/kernel/livepatch/patch.c
> > > index b552cf2d85f8..6c0164d24bbd 100644
> > > --- a/kernel/livepatch/patch.c
> > > +++ b/kernel/livepatch/patch.c
> > > @@ -45,9 +45,13 @@ static void notrace klp_ftrace_handler(unsigned long ip,
> > >  	struct klp_ops *ops;
> > >  	struct klp_func *func;
> > >  	int patch_state;
> > > +	int bit;
> > >  
> > >  	ops = container_of(fops, struct klp_ops, fops);
> > >  
> > > +	bit = ftrace_test_recursion_trylock();
> > > +	if (bit < 0)
> > > +		return;
> > 
> > This means that the original function will be called in case of recursion. 
> > That's probably fair, but I'm wondering if we should at least WARN about 
> > it.
> 
> Yeah, the early return might break the consistency model and
> unexpected things might happen. We should be aware of it.
> Please use:
> 
> 	if (WARN_ON_ONCE(bit < 0))
> 		return;
> 
> WARN_ON_ONCE() might be part of the recursion. But it should happen
> only once. IMHO, it is worth the risk.

Agreed.

Miroslav

Re: [PATCH 6/9] livepatch/ftrace: Add recursion protection to the ftrace callback

[Steven Rostedt]

On Thu, 29 Oct 2020 15:57:09 +0100
Petr Mladek <pmladek@suse.com> wrote:

> On Thu 2020-10-29 14:51:06, Miroslav Benes wrote:
> > On Wed, 28 Oct 2020, Steven Rostedt wrote:
> >   
> > > From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
> > > 
> > > If a ftrace callback does not supply its own recursion protection and
> > > does not set the RECURSION_SAFE flag in its ftrace_ops, then ftrace will
> > > make a helper trampoline to do so before calling the callback instead of
> > > just calling the callback directly.
> > > 
> > > The default for ftrace_ops is going to assume recursion protection unless
> > > otherwise specified.  
> 
> It might be my lack skills to read English. But the above sentence
> sounds ambiguous to me. It is not clear to me who provides the
> recursion protection by default. Could you please make it more
> explicit, for example by:
> 
> "The default for ftrace_ops is going to change. It will expect that
> handlers provide their own recursion protection."

It was originally written as something else, as my first series (that I
didn't post) added the recursion flag, and then I needed one big nasty
patch to remove them. Then I realized it would be fine to just keep the
double recursion testing and remove the flag when it was no longer used. I
then went back and wrote up that sentence, and yeah, it wasn't the best
explanation.

Your sentence is better, I'll update it.

> 
> 
> > Hm, I've always thought that we did not need any kind of recursion 
> > protection for our callback. It is marked as notrace and it does not call 
> > anything traceable. In fact, it does not call anything. I even have a note 
> > in my todo list to mark the callback as RECURSION_SAFE :)  
> 
> Well, it calls WARN_ON_ONCE() ;-)
> 
> > At the same time, it probably does not hurt and the patch is still better 
> > than what we have now without RECURSION_SAFE if I understand the patch set 
> > correctly.  
> 
> And better be on the safe side.

And the WARN_ON_ONCE() use to cause a problem, until I fixed it:

  dfbf2897d0049 ("bug: set warn variable before calling WARN()")

> 
> 
> > > Cc: Josh Poimboeuf <jpoimboe@redhat.com>
> > > Cc: Jiri Kosina <jikos@kernel.org>
> > > Cc: Miroslav Benes <mbenes@suse.cz>
> > > Cc: Petr Mladek <pmladek@suse.com>
> > > Cc: Joe Lawrence <joe.lawrence@redhat.com>
> > > Cc: live-patching@vger.kernel.org
> > > Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> > > ---
> > >  kernel/livepatch/patch.c | 5 +++++
> > >  1 file changed, 5 insertions(+)
> > > 
> > > diff --git a/kernel/livepatch/patch.c b/kernel/livepatch/patch.c
> > > index b552cf2d85f8..6c0164d24bbd 100644
> > > --- a/kernel/livepatch/patch.c
> > > +++ b/kernel/livepatch/patch.c
> > > @@ -45,9 +45,13 @@ static void notrace klp_ftrace_handler(unsigned long ip,
> > >  	struct klp_ops *ops;
> > >  	struct klp_func *func;
> > >  	int patch_state;
> > > +	int bit;
> > >  
> > >  	ops = container_of(fops, struct klp_ops, fops);
> > >  
> > > +	bit = ftrace_test_recursion_trylock();
> > > +	if (bit < 0)
> > > +		return;  
> > 
> > This means that the original function will be called in case of recursion. 
> > That's probably fair, but I'm wondering if we should at least WARN about 
> > it.  
> 
> Yeah, the early return might break the consistency model and
> unexpected things might happen. We should be aware of it.
> Please use:
> 
> 	if (WARN_ON_ONCE(bit < 0))
> 		return;
> 
> WARN_ON_ONCE() might be part of the recursion. But it should happen
> only once. IMHO, it is worth the risk.
> 
> Otherwise it looks good.

Perhaps we can add that as a separate patch, because this patch doesn't add
any real functionality change. It only moves the recursion testing from the
helper function (which ftrace wraps all callbacks that do not have the
RECURSION flags set, including this one) down to your callback.

In keeping with one patch to do one thing principle, the added of
WARN_ON_ONCE() should be a separate patch, as that will change the
functionality.

If that WARN_ON_ONCE() breaks things, I'd like it to be bisected to another
patch other than this one.

-- Steve

Re: [PATCH 6/9] livepatch/ftrace: Add recursion protection to the ftrace callback

[Miroslav Benes]

> > > > +	bit = ftrace_test_recursion_trylock();
> > > > +	if (bit < 0)
> > > > +		return;  
> > > 
> > > This means that the original function will be called in case of recursion. 
> > > That's probably fair, but I'm wondering if we should at least WARN about 
> > > it.  
> > 
> > Yeah, the early return might break the consistency model and
> > unexpected things might happen. We should be aware of it.
> > Please use:
> > 
> > 	if (WARN_ON_ONCE(bit < 0))
> > 		return;
> > 
> > WARN_ON_ONCE() might be part of the recursion. But it should happen
> > only once. IMHO, it is worth the risk.
> > 
> > Otherwise it looks good.
> 
> Perhaps we can add that as a separate patch, because this patch doesn't add
> any real functionality change. It only moves the recursion testing from the
> helper function (which ftrace wraps all callbacks that do not have the
> RECURSION flags set, including this one) down to your callback.
> 
> In keeping with one patch to do one thing principle, the added of
> WARN_ON_ONCE() should be a separate patch, as that will change the
> functionality.
> 
> If that WARN_ON_ONCE() breaks things, I'd like it to be bisected to another
> patch other than this one.

Works for me.

Miroslav

Re: [PATCH 6/9] livepatch/ftrace: Add recursion protection to the ftrace callback

[Petr Mladek]

On Fri 2020-10-30 10:48:58, Miroslav Benes wrote:
> > > > > +	bit = ftrace_test_recursion_trylock();
> > > > > +	if (bit < 0)
> > > > > +		return;  
> > > > 
> > > > This means that the original function will be called in case of recursion. 
> > > > That's probably fair, but I'm wondering if we should at least WARN about 
> > > > it.  
> > > 
> > > Yeah, the early return might break the consistency model and
> > > unexpected things might happen. We should be aware of it.
> > > Please use:
> > > 
> > > 	if (WARN_ON_ONCE(bit < 0))
> > > 		return;
> > > 
> > > WARN_ON_ONCE() might be part of the recursion. But it should happen
> > > only once. IMHO, it is worth the risk.
> > > 
> > > Otherwise it looks good.
> > 
> > Perhaps we can add that as a separate patch, because this patch doesn't add
> > any real functionality change. It only moves the recursion testing from the
> > helper function (which ftrace wraps all callbacks that do not have the
> > RECURSION flags set, including this one) down to your callback.
> > 
> > In keeping with one patch to do one thing principle, the added of
> > WARN_ON_ONCE() should be a separate patch, as that will change the
> > functionality.
> > 
> > If that WARN_ON_ONCE() breaks things, I'd like it to be bisected to another
> > patch other than this one.
> 
> Works for me.

+1

So, with the updated commit message:

Reviewed-by: Petr Mladek <pmladek@suse.com>

Best Regards,
Petr

[PATCH 7/9] perf/ftrace: Add recursion protection to the ftrace callback

[Steven Rostedt]

From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>

If a ftrace callback does not supply its own recursion protection and
does not set the RECURSION_SAFE flag in its ftrace_ops, then ftrace will
make a helper trampoline to do so before calling the callback instead of
just calling the callback directly.

The default for ftrace_ops is going to assume recursion protection unless
otherwise specified.

Cc: Jiri Olsa <jolsa@redhat.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
 kernel/trace/trace_event_perf.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/trace_event_perf.c b/kernel/trace/trace_event_perf.c
index 643e0b19920d..fd58d83861d8 100644
--- a/kernel/trace/trace_event_perf.c
+++ b/kernel/trace/trace_event_perf.c
@@ -439,10 +439,15 @@ perf_ftrace_function_call(unsigned long ip, unsigned long parent_ip,
 	struct hlist_head head;
 	struct pt_regs regs;
 	int rctx;
+	int bit;
 
 	if ((unsigned long)ops->private != smp_processor_id())
 		return;
 
+	bit = ftrace_test_recursion_trylock();
+	if (bit < 0)
+		return;
+
 	event = container_of(ops, struct perf_event, ftrace_ops);
 
 	/*
@@ -463,13 +468,15 @@ perf_ftrace_function_call(unsigned long ip, unsigned long parent_ip,
 
 	entry = perf_trace_buf_alloc(ENTRY_SIZE, NULL, &rctx);
 	if (!entry)
-		return;
+		goto out;
 
 	entry->ip = ip;
 	entry->parent_ip = parent_ip;
 	perf_trace_buf_submit(entry, ENTRY_SIZE, rctx, TRACE_FN,
 			      1, &regs, &head, NULL);
 
+out:
+	ftrace_test_recursion_unlock(bit);
 #undef ENTRY_SIZE
 }
 
-- 
2.28.0

[PATCH 8/9] perf/ftrace: Check for rcu_is_watching() in callback function

[Steven Rostedt]

From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>

If a ftrace callback requires "rcu_is_watching", then it adds the
FTRACE_OPS_FL_RCU flag and it will not be called if RCU is not "watching".
But this means that it will use a trampoline when called, and this slows
down the function tracing a tad. By checking rcu_is_watching() from within
the callback, it no longer needs the RCU flag set in the ftrace_ops and it
can be safely called directly.

Cc: Jiri Olsa <jolsa@redhat.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
 kernel/trace/trace_event_perf.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/trace_event_perf.c b/kernel/trace/trace_event_perf.c
index fd58d83861d8..a2b9fddb8148 100644
--- a/kernel/trace/trace_event_perf.c
+++ b/kernel/trace/trace_event_perf.c
@@ -441,6 +441,9 @@ perf_ftrace_function_call(unsigned long ip, unsigned long parent_ip,
 	int rctx;
 	int bit;
 
+	if (!rcu_is_watching())
+		return;
+
 	if ((unsigned long)ops->private != smp_processor_id())
 		return;
 
@@ -484,7 +487,6 @@ static int perf_ftrace_function_register(struct perf_event *event)
 {
 	struct ftrace_ops *ops = &event->ftrace_ops;
 
-	ops->flags   = FTRACE_OPS_FL_RCU;
 	ops->func    = perf_ftrace_function_call;
 	ops->private = (void *)(unsigned long)nr_cpu_ids;
 
-- 
2.28.0

[PATCH 9/9] ftrace: Reverse what the RECURSION flag means in the ftrace_ops

[Steven Rostedt]

From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>

Now that all callbacks are recursion safe, reverse the meaning of the
RECURSION flag and rename it from RECURSION_SAFE to simply RECURSION.
Now only callbacks that request to have recursion protecting it will
have the added trampoline to do so.

Also remove the outdated comment about "PER_CPU" when determining to
use the ftrace_ops_assist_func.

Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Miroslav Benes <mbenes@suse.cz>
Cc: Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>
Cc: Petr Mladek <pmladek@suse.com>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
 Documentation/trace/ftrace-uses.rst | 82 +++++++++++++++++++++--------
 include/linux/ftrace.h              | 12 ++---
 kernel/trace/fgraph.c               |  3 +-
 kernel/trace/ftrace.c               | 20 ++++---
 kernel/trace/trace_events.c         |  1 -
 kernel/trace/trace_functions.c      |  2 +-
 kernel/trace/trace_selftest.c       |  7 +--
 kernel/trace/trace_stack.c          |  1 -
 8 files changed, 79 insertions(+), 49 deletions(-)

diff --git a/Documentation/trace/ftrace-uses.rst b/Documentation/trace/ftrace-uses.rst
index 2a05e770618a..cce218d65f40 100644
--- a/Documentation/trace/ftrace-uses.rst
+++ b/Documentation/trace/ftrace-uses.rst
@@ -30,8 +30,8 @@ The ftrace context
   This requires extra care to what can be done inside a callback. A callback
   can be called outside the protective scope of RCU.
 
-The ftrace infrastructure has some protections against recursions and RCU
-but one must still be very careful how they use the callbacks.
+There are helper functions to help against recursion, and making sure
+RCU is watching. These are explained below.
 
 
 The ftrace_ops structure
@@ -108,6 +108,50 @@ The prototype of the callback function is as follows (as of v4.14):
 	at the start of the function where ftrace was tracing. Otherwise it
 	either contains garbage, or NULL.
 
+Protect your callback
+=====================
+
+As functions can be called from anywhere, and it is possible that a function
+called by a callback may also be traced, and call that same callback,
+recursion protection must be used. There are two helper functions that
+can help in this regard. If you start your code with:
+
+	int bit;
+
+	bit = ftrace_test_recursion_trylock();
+	if (bit < 0)
+		return;
+
+and end it with:
+
+	ftrace_test_recursion_unlock(bit);
+
+The code in between will be safe to use, even if it ends up calling a
+function that the callback is tracing. Note, on success, 
+ftrace_test_recursion_trylock() will disable preemption, and the
+ftrace_test_recursion_unlock() will enable it again (if it was previously
+enabled).
+
+Alternatively, if the FTRACE_OPS_FL_RECURSION flag is set on the ftrace_ops
+(as explained below), then a helper trampoline will be used to test
+for recursion for the callback and no recursion test needs to be done.
+But this is at the expense of a slightly more overhead from an extra
+function call.
+
+If your callback accesses any data or critical section that requires RCU
+protection, it is best to make sure that RCU is "watching", otherwise
+that data or critical section will not be protected as expected. In this
+case add:
+
+	if (!rcu_is_watching())
+		return;
+
+Alternatively, if the FTRACE_OPS_FL_RCU flag is set on the ftrace_ops
+(as explained below), then a helper trampoline will be used to test
+for rcu_is_watching for the callback and no other test needs to be done.
+But this is at the expense of a slightly more overhead from an extra
+function call.
+
 
 The ftrace FLAGS
 ================
@@ -128,26 +172,20 @@ FTRACE_OPS_FL_SAVE_REGS_IF_SUPPORTED
 	will not fail with this flag set. But the callback must check if
 	regs is NULL or not to determine if the architecture supports it.
 
-FTRACE_OPS_FL_RECURSION_SAFE
-	By default, a wrapper is added around the callback to
-	make sure that recursion of the function does not occur. That is,
-	if a function that is called as a result of the callback's execution
-	is also traced, ftrace will prevent the callback from being called
-	again. But this wrapper adds some overhead, and if the callback is
-	safe from recursion, it can set this flag to disable the ftrace
-	protection.
-
-	Note, if this flag is set, and recursion does occur, it could cause
-	the system to crash, and possibly reboot via a triple fault.
-
-	It is OK if another callback traces a function that is called by a
-	callback that is marked recursion safe. Recursion safe callbacks
-	must never trace any function that are called by the callback
-	itself or any nested functions that those functions call.
-
-	If this flag is set, it is possible that the callback will also
-	be called with preemption enabled (when CONFIG_PREEMPTION is set),
-	but this is not guaranteed.
+FTRACE_OPS_FL_RECURSION
+	By default, it is expected that the callback can handle recursion.
+	But if the callback is not that worried about overehead, then
+	setting this bit will add the recursion protection around the
+	callback by calling a helper function that will do the recursion
+	protection and only call the callback if it did not recurse.
+
+	Note, if this flag is not set, and recursion does occur, it could
+	cause the system to crash, and possibly reboot via a triple fault.
+
+	Not, if this flag is set, then the callback will always be called
+	with preemption disabled. If it is not set, then it is possible
+	(but not guaranteed) that the callback will be called in
+	preemptable context.
 
 FTRACE_OPS_FL_IPMODIFY
 	Requires FTRACE_OPS_FL_SAVE_REGS set. If the callback is to "hijack"
diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
index 0e4164a7f56d..806196345c3f 100644
--- a/include/linux/ftrace.h
+++ b/include/linux/ftrace.h
@@ -98,7 +98,7 @@ ftrace_func_t ftrace_ops_get_func(struct ftrace_ops *ops);
 /*
  * FTRACE_OPS_FL_* bits denote the state of ftrace_ops struct and are
  * set in the flags member.
- * CONTROL, SAVE_REGS, SAVE_REGS_IF_SUPPORTED, RECURSION_SAFE, STUB and
+ * CONTROL, SAVE_REGS, SAVE_REGS_IF_SUPPORTED, RECURSION, STUB and
  * IPMODIFY are a kind of attribute flags which can be set only before
  * registering the ftrace_ops, and can not be modified while registered.
  * Changing those attribute flags after registering ftrace_ops will
@@ -121,10 +121,10 @@ ftrace_func_t ftrace_ops_get_func(struct ftrace_ops *ops);
  *            passing regs to the handler.
  *            Note, if this flag is set, the SAVE_REGS flag will automatically
  *            get set upon registering the ftrace_ops, if the arch supports it.
- * RECURSION_SAFE - The ftrace_ops can set this to tell the ftrace infrastructure
- *            that the call back has its own recursion protection. If it does
- *            not set this, then the ftrace infrastructure will add recursion
- *            protection for the caller.
+ * RECURSION - The ftrace_ops can set this to tell the ftrace infrastructure
+ *            that the call back needs recursion protection. If it does
+ *            not set this, then the ftrace infrastructure will assume
+ *            that the callback can handle recursion on its own.
  * STUB   - The ftrace_ops is just a place holder.
  * INITIALIZED - The ftrace_ops has already been initialized (first use time
  *            register_ftrace_function() is called, it will initialized the ops)
@@ -156,7 +156,7 @@ enum {
 	FTRACE_OPS_FL_DYNAMIC			= BIT(1),
 	FTRACE_OPS_FL_SAVE_REGS			= BIT(2),
 	FTRACE_OPS_FL_SAVE_REGS_IF_SUPPORTED	= BIT(3),
-	FTRACE_OPS_FL_RECURSION_SAFE		= BIT(4),
+	FTRACE_OPS_FL_RECURSION			= BIT(4),
 	FTRACE_OPS_FL_STUB			= BIT(5),
 	FTRACE_OPS_FL_INITIALIZED		= BIT(6),
 	FTRACE_OPS_FL_DELETED			= BIT(7),
diff --git a/kernel/trace/fgraph.c b/kernel/trace/fgraph.c
index 5658f13037b3..73edb9e4f354 100644
--- a/kernel/trace/fgraph.c
+++ b/kernel/trace/fgraph.c
@@ -334,8 +334,7 @@ unsigned long ftrace_graph_ret_addr(struct task_struct *task, int *idx,
 
 static struct ftrace_ops graph_ops = {
 	.func			= ftrace_stub,
-	.flags			= FTRACE_OPS_FL_RECURSION_SAFE |
-				   FTRACE_OPS_FL_INITIALIZED |
+	.flags			= FTRACE_OPS_FL_INITIALIZED |
 				   FTRACE_OPS_FL_PID |
 				   FTRACE_OPS_FL_STUB,
 #ifdef FTRACE_GRAPH_TRAMP_ADDR
diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 4833b6a82ce7..2dcae8251104 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -80,7 +80,7 @@ enum {
 
 struct ftrace_ops ftrace_list_end __read_mostly = {
 	.func		= ftrace_stub,
-	.flags		= FTRACE_OPS_FL_RECURSION_SAFE | FTRACE_OPS_FL_STUB,
+	.flags		= FTRACE_OPS_FL_STUB,
 	INIT_OPS_HASH(ftrace_list_end)
 };
 
@@ -866,7 +866,7 @@ static void unregister_ftrace_profiler(void)
 #else
 static struct ftrace_ops ftrace_profile_ops __read_mostly = {
 	.func		= function_profile_call,
-	.flags		= FTRACE_OPS_FL_RECURSION_SAFE | FTRACE_OPS_FL_INITIALIZED,
+	.flags		= FTRACE_OPS_FL_INITIALIZED,
 	INIT_OPS_HASH(ftrace_profile_ops)
 };
 
@@ -1040,8 +1040,7 @@ struct ftrace_ops global_ops = {
 	.local_hash.notrace_hash	= EMPTY_HASH,
 	.local_hash.filter_hash		= EMPTY_HASH,
 	INIT_OPS_HASH(global_ops)
-	.flags				= FTRACE_OPS_FL_RECURSION_SAFE |
-					  FTRACE_OPS_FL_INITIALIZED |
+	.flags				= FTRACE_OPS_FL_INITIALIZED |
 					  FTRACE_OPS_FL_PID,
 };
 
@@ -2382,7 +2381,7 @@ static void call_direct_funcs(unsigned long ip, unsigned long pip,
 
 struct ftrace_ops direct_ops = {
 	.func		= call_direct_funcs,
-	.flags		= FTRACE_OPS_FL_IPMODIFY | FTRACE_OPS_FL_RECURSION_SAFE
+	.flags		= FTRACE_OPS_FL_IPMODIFY
 			  | FTRACE_OPS_FL_DIRECT | FTRACE_OPS_FL_SAVE_REGS
 			  | FTRACE_OPS_FL_PERMANENT,
 	/*
@@ -6864,8 +6863,7 @@ void ftrace_init_trace_array(struct trace_array *tr)
 
 struct ftrace_ops global_ops = {
 	.func			= ftrace_stub,
-	.flags			= FTRACE_OPS_FL_RECURSION_SAFE |
-				  FTRACE_OPS_FL_INITIALIZED |
+	.flags			= FTRACE_OPS_FL_INITIALIZED |
 				  FTRACE_OPS_FL_PID,
 };
 
@@ -7025,11 +7023,11 @@ NOKPROBE_SYMBOL(ftrace_ops_assist_func);
 ftrace_func_t ftrace_ops_get_func(struct ftrace_ops *ops)
 {
 	/*
-	 * If the function does not handle recursion, needs to be RCU safe,
-	 * or does per cpu logic, then we need to call the assist handler.
+	 * If the function does not handle recursion or needs to be RCU safe,
+	 * then we need to call the assist handler.
 	 */
-	if (!(ops->flags & FTRACE_OPS_FL_RECURSION_SAFE) ||
-	    ops->flags & FTRACE_OPS_FL_RCU)
+	if (ops->flags & (FTRACE_OPS_FL_RECURSION |
+			  FTRACE_OPS_FL_RCU))
 		return ftrace_ops_assist_func;
 
 	return ops->func;
diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index e705f06c68c6..4a9a2a9853bc 100644
--- a/kernel/trace/trace_events.c
+++ b/kernel/trace/trace_events.c
@@ -3712,7 +3712,6 @@ function_test_events_call(unsigned long ip, unsigned long parent_ip,
 static struct ftrace_ops trace_ops __initdata  =
 {
 	.func = function_test_events_call,
-	.flags = FTRACE_OPS_FL_RECURSION_SAFE,
 };
 
 static __init void event_trace_self_test_with_function(void)
diff --git a/kernel/trace/trace_functions.c b/kernel/trace/trace_functions.c
index 943756c01190..89c414ce1388 100644
--- a/kernel/trace/trace_functions.c
+++ b/kernel/trace/trace_functions.c
@@ -48,7 +48,7 @@ int ftrace_allocate_ftrace_ops(struct trace_array *tr)
 
 	/* Currently only the non stack version is supported */
 	ops->func = function_trace_call;
-	ops->flags = FTRACE_OPS_FL_RECURSION_SAFE | FTRACE_OPS_FL_PID;
+	ops->flags = FTRACE_OPS_FL_PID;
 
 	tr->ops = ops;
 	ops->private = tr;
diff --git a/kernel/trace/trace_selftest.c b/kernel/trace/trace_selftest.c
index b5e3496cf803..50dd913d23e7 100644
--- a/kernel/trace/trace_selftest.c
+++ b/kernel/trace/trace_selftest.c
@@ -150,17 +150,14 @@ static void trace_selftest_test_dyn_func(unsigned long ip,
 
 static struct ftrace_ops test_probe1 = {
 	.func			= trace_selftest_test_probe1_func,
-	.flags			= FTRACE_OPS_FL_RECURSION_SAFE,
 };
 
 static struct ftrace_ops test_probe2 = {
 	.func			= trace_selftest_test_probe2_func,
-	.flags			= FTRACE_OPS_FL_RECURSION_SAFE,
 };
 
 static struct ftrace_ops test_probe3 = {
 	.func			= trace_selftest_test_probe3_func,
-	.flags			= FTRACE_OPS_FL_RECURSION_SAFE,
 };
 
 static void print_counts(void)
@@ -448,11 +445,11 @@ static void trace_selftest_test_recursion_safe_func(unsigned long ip,
 
 static struct ftrace_ops test_rec_probe = {
 	.func			= trace_selftest_test_recursion_func,
+	.flags			= FTRACE_OPS_FL_RECURSION,
 };
 
 static struct ftrace_ops test_recsafe_probe = {
 	.func			= trace_selftest_test_recursion_safe_func,
-	.flags			= FTRACE_OPS_FL_RECURSION_SAFE,
 };
 
 static int
@@ -556,7 +553,7 @@ static void trace_selftest_test_regs_func(unsigned long ip,
 
 static struct ftrace_ops test_regs_probe = {
 	.func		= trace_selftest_test_regs_func,
-	.flags		= FTRACE_OPS_FL_RECURSION_SAFE | FTRACE_OPS_FL_SAVE_REGS,
+	.flags		= FTRACE_OPS_FL_SAVE_REGS,
 };
 
 static int
diff --git a/kernel/trace/trace_stack.c b/kernel/trace/trace_stack.c
index c408423e5d65..969db526a563 100644
--- a/kernel/trace/trace_stack.c
+++ b/kernel/trace/trace_stack.c
@@ -318,7 +318,6 @@ stack_trace_call(unsigned long ip, unsigned long parent_ip,
 static struct ftrace_ops trace_ops __read_mostly =
 {
 	.func = stack_trace_call,
-	.flags = FTRACE_OPS_FL_RECURSION_SAFE,
 };
 
 static ssize_t
-- 
2.28.0