Re: [RFC PATCH v2 0/4] arm64: Implement stack trace reliability checks

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Mon, Apr 12, 2021 at 05:59:33PM +0100, Mark Brown wrote:
> On Fri, Apr 09, 2021 at 05:32:27PM -0500, Josh Poimboeuf wrote:
> 
> > Hm, for that matter, even without renaming things, a comment above
> > stack_trace_save_tsk_reliable() describing the meaning of "reliable"
> > would be a good idea.
> 
> Might be better to place something at the prototype for
> arch_stack_walk_reliable() or cross link the two since that's where any
> new architectures should be starting, or perhaps even better to extend
> the document that Mark wrote further and point to that from both places.  
> 
> Some more explict pointer to live patching as the only user would
> definitely be good but I think the more important thing would be writing
> down any assumptions in the API that aren't already written down and
> we're supposed to be relying on.  Mark's document captured a lot of it
> but it sounds like there's more here, and even with knowing that this
> interface is only used by live patch and digging into what it does it's
> not always clear what happens to work with the code right now and what's
> something that's suitable to be relied on.

Something like so?

From: Josh Poimboeuf <jpoimboe@redhat.com>
Subject: [PATCH] livepatch: Clarify the meaning of 'reliable'

Update the comments and documentation to reflect what 'reliable'
unwinding actually means, in the context of live patching.

Suggested-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
---
 .../livepatch/reliable-stacktrace.rst         | 26 +++++++++++++----
 arch/x86/kernel/stacktrace.c                  |  6 ----
 include/linux/stacktrace.h                    | 29 +++++++++++++++++--
 kernel/stacktrace.c                           |  7 ++++-
 4 files changed, 53 insertions(+), 15 deletions(-)

diff --git a/Documentation/livepatch/reliable-stacktrace.rst b/Documentation/livepatch/reliable-stacktrace.rst
index 67459d2ca2af..e325efc7e952 100644
--- a/Documentation/livepatch/reliable-stacktrace.rst
+++ b/Documentation/livepatch/reliable-stacktrace.rst
@@ -72,7 +72,21 @@ The unwinding process varies across architectures, their respective procedure
 call standards, and kernel configurations. This section describes common
 details that architectures should consider.
 
-4.1 Identifying successful termination
+4.1 Only preemptible code needs reliability detection
+-----------------------------------------------------
+
+The only current user of reliable stacktracing is livepatch, which only
+calls it for a) inactive tasks; or b) the current task in task context.
+
+Therefore, the unwinder only needs to detect the reliability of stacks
+involving *preemptible* code.
+
+Practically speaking, reliability of stacks involving *non-preemptible*
+code is a "don't-care".  It may help to return a wrong reliability
+result for such cases, if it results in reduced complexity, since such
+cases will not happen in practice.
+
+4.2 Identifying successful termination
 --------------------------------------
 
 Unwinding may terminate early for a number of reasons, including:
@@ -95,7 +109,7 @@ architectures verify that a stacktrace ends at an expected location, e.g.
 * On a specific stack expected for a kernel entry point (e.g. if the
   architecture has separate task and IRQ stacks).
 
-4.2 Identifying unwindable code
+4.3 Identifying unwindable code
 -------------------------------
 
 Unwinding typically relies on code following specific conventions (e.g.
@@ -129,7 +143,7 @@ unreliable to unwind from, e.g.
 
 * Identifying specific portions of code using bounds information.
 
-4.3 Unwinding across interrupts and exceptions
+4.4 Unwinding across interrupts and exceptions
 ----------------------------------------------
 
 At function call boundaries the stack and other unwind state is expected to be
@@ -156,7 +170,7 @@ have no such cases) should attempt to unwind across exception boundaries, as
 doing so can prevent unnecessarily stalling livepatch consistency checks and
 permits livepatch transitions to complete more quickly.
 
-4.4 Rewriting of return addresses
+4.5 Rewriting of return addresses
 ---------------------------------
 
 Some trampolines temporarily modify the return address of a function in order
@@ -222,7 +236,7 @@ middle of return_to_handler and can report this as unreliable. Architectures
 are not required to unwind from other trampolines which modify the return
 address.
 
-4.5 Obscuring of return addresses
+4.6 Obscuring of return addresses
 ---------------------------------
 
 Some trampolines do not rewrite the return address in order to intercept
@@ -249,7 +263,7 @@ than the link register as would usually be the case.
 Architectures must either ensure that unwinders either reliably unwind
 such cases, or report the unwinding as unreliable.
 
-4.6 Link register unreliability
+4.7 Link register unreliability
 -------------------------------
 
 On some other architectures, 'call' instructions place the return address into a
diff --git a/arch/x86/kernel/stacktrace.c b/arch/x86/kernel/stacktrace.c
index 8627fda8d993..15b058eefc4e 100644
--- a/arch/x86/kernel/stacktrace.c
+++ b/arch/x86/kernel/stacktrace.c
@@ -29,12 +29,6 @@ void arch_stack_walk(stack_trace_consume_fn consume_entry, void *cookie,
 	}
 }
 
-/*
- * This function returns an error if it detects any unreliable features of the
- * stack.  Otherwise it guarantees that the stack trace is reliable.
- *
- * If the task is not 'current', the caller *must* ensure the task is inactive.
- */
 int arch_stack_walk_reliable(stack_trace_consume_fn consume_entry,
 			     void *cookie, struct task_struct *task)
 {
diff --git a/include/linux/stacktrace.h b/include/linux/stacktrace.h
index 50e2df30b0aa..1b6a65a0ad22 100644
--- a/include/linux/stacktrace.h
+++ b/include/linux/stacktrace.h
@@ -26,7 +26,7 @@ unsigned int stack_trace_save_user(unsigned long *store, unsigned int size);
 #ifdef CONFIG_ARCH_STACKWALK
 
 /**
- * stack_trace_consume_fn - Callback for arch_stack_walk()
+ * stack_trace_consume_fn() - Callback for arch_stack_walk()
  * @cookie:	Caller supplied pointer handed back by arch_stack_walk()
  * @addr:	The stack entry address to consume
  *
@@ -35,7 +35,7 @@ unsigned int stack_trace_save_user(unsigned long *store, unsigned int size);
  */
 typedef bool (*stack_trace_consume_fn)(void *cookie, unsigned long addr);
 /**
- * arch_stack_walk - Architecture specific function to walk the stack
+ * arch_stack_walk() - Architecture specific function to walk the stack
  * @consume_entry:	Callback which is invoked by the architecture code for
  *			each entry.
  * @cookie:		Caller supplied pointer which is handed back to
@@ -52,8 +52,33 @@ typedef bool (*stack_trace_consume_fn)(void *cookie, unsigned long addr);
  */
 void arch_stack_walk(stack_trace_consume_fn consume_entry, void *cookie,
 		     struct task_struct *task, struct pt_regs *regs);
+
+/**
+ * arch_stack_walk_reliable() - Architecture specific function to walk the
+ *				stack, with stack reliability check
+ * @consume_entry:	Callback which is invoked by the architecture code for
+ *			each entry.
+ * @cookie:		Caller supplied pointer which is handed back to
+ *			@consume_entry
+ * @task:		Pointer to a task struct, can be NULL for current
+ *
+ * Return: 0 if the stack trace is considered reliable for livepatch; else < 0.
+ *
+ * NOTE: This interface is only used by livepatch.  The caller must ensure that
+ *	 it's only called in one of the following two scenarios:
+ *
+ *	 a) the task is inactive (and guaranteed to remain so); or
+ *
+ *	 b) the task is 'current', running in task context.
+ *
+ * Effectively, this means the arch unwinder doesn't need to detect the
+ * reliability of stacks involving non-preemptible code.
+ *
+ * For more details, see Documentation/livepatch/reliable-stacktrace.rst.
+ */
 int arch_stack_walk_reliable(stack_trace_consume_fn consume_entry, void *cookie,
 			     struct task_struct *task);
+
 void arch_stack_walk_user(stack_trace_consume_fn consume_entry, void *cookie,
 			  const struct pt_regs *regs);
 
diff --git a/kernel/stacktrace.c b/kernel/stacktrace.c
index 9f8117c7cfdd..a198fd194fed 100644
--- a/kernel/stacktrace.c
+++ b/kernel/stacktrace.c
@@ -185,7 +185,12 @@ unsigned int stack_trace_save_regs(struct pt_regs *regs, unsigned long *store,
  *		stack. Otherwise it guarantees that the stack trace is
  *		reliable and returns the number of entries stored.
  *
- * If the task is not 'current', the caller *must* ensure the task is inactive.
+ * NOTE: This interface is only used by livepatch.  The caller must ensure that
+ *	 it's only called in one of the following two scenarios:
+ *
+ *	 a) the task is inactive (and guaranteed to remain so); or
+ *
+ *	 b) the task is 'current', running in task context.
  */
 int stack_trace_save_tsk_reliable(struct task_struct *tsk, unsigned long *store,
 				  unsigned int size)
-- 
2.30.2