[PATCHv2] lockdep: report broken irq restoration

* [PATCHv2] lockdep: report broken irq restoration
@ 2021-01-11 15:37 Mark Rutland
  2021-01-22 11:06 ` Mark Rutland
  2021-01-22 14:05 ` [tip: locking/core] " tip-bot2 for Mark Rutland
  0 siblings, 2 replies; 5+ messages in thread
From: Mark Rutland @ 2021-01-11 15:37 UTC (permalink / raw)
  To: linux-kernel
  Cc: Mark Rutland, Andy Lutomirski, Ingo Molnar, Juergen Gross,
	Peter Zijlstra, Thomas Gleixner

We generally expect local_irq_save() and local_irq_restore() to be
paired and sanely nested, and so local_irq_restore() expects to be
called with irqs disabled. Thus, within local_irq_restore() we only
trace irq flag changes when unmasking irqs.

This means that a sequence such as:

| local_irq_disable();
| local_irq_save(flags);
| local_irq_enable();
| local_irq_restore(flags);

... is liable to break things, as the local_irq_restore() would mask
irqs without tracing this change. Similar problems may exist for
architectures whose arch_irq_restore() function depends on being called
with irqs disabled.

We don't consider such sequences to be a good idea, so let's define
those as forbidden, and add tooling to detect such broken cases.

This patch adds debug code to WARN() when raw_local_irq_restore() is
called with irqs enabled. As raw_local_irq_restore() is expected to pair
with raw_local_irq_save(), it should never be called with irqs enabled.

To avoid the possibility of circular header dependencies between
irqflags.h and bug.h, the warning is handled in a separate C file.

The new code is all conditional on a new CONFIG_DEBUG_IRQFLAGS symbol
which is independent of CONFIG_TRACE_IRQFLAGS. As noted above such cases
will confuse lockdep, so CONFIG_DEBUG_LOCKDEP now selects
CONFIG_DEBUG_IRQFLAGS.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
---
 include/linux/irqflags.h       | 12 ++++++++++++
 kernel/locking/Makefile        |  1 +
 kernel/locking/irqflag-debug.c | 11 +++++++++++
 lib/Kconfig.debug              |  8 ++++++++
 4 files changed, 32 insertions(+)
 create mode 100644 kernel/locking/irqflag-debug.c

Since v1 [1]:
* Remove per-instance bool in favour of a single WARN_ONCE()
* Move check into raw_local_irq_restore()
* Update Kconfig text and cover

[1] https://lore.kernel.org/r/20201209183337.1912-1-mark.rutland@arm.com

Mark.

diff --git a/include/linux/irqflags.h b/include/linux/irqflags.h
index 8de0e1373de7..600c10da321a 100644
--- a/include/linux/irqflags.h
+++ b/include/linux/irqflags.h
@@ -149,6 +149,17 @@ do {						\
 # define start_critical_timings() do { } while (0)
 #endif
 
+#ifdef CONFIG_DEBUG_IRQFLAGS
+extern void warn_bogus_irq_restore(void);
+#define raw_check_bogus_irq_restore()			\
+	do {						\
+		if (unlikely(!arch_irqs_disabled()))	\
+			warn_bogus_irq_restore();	\
+	} while (0)
+#else
+#define raw_check_bogus_irq_restore() do { } while (0)
+#endif
+
 /*
  * Wrap the arch provided IRQ routines to provide appropriate checks.
  */
@@ -162,6 +173,7 @@ do {						\
 #define raw_local_irq_restore(flags)			\
 	do {						\
 		typecheck(unsigned long, flags);	\
+		raw_check_bogus_irq_restore();		\
 		arch_local_irq_restore(flags);		\
 	} while (0)
 #define raw_local_save_flags(flags)			\
diff --git a/kernel/locking/Makefile b/kernel/locking/Makefile
index 6d11cfb9b41f..8838f1d7c4a2 100644
--- a/kernel/locking/Makefile
+++ b/kernel/locking/Makefile
@@ -15,6 +15,7 @@ CFLAGS_REMOVE_mutex-debug.o = $(CC_FLAGS_FTRACE)
 CFLAGS_REMOVE_rtmutex-debug.o = $(CC_FLAGS_FTRACE)
 endif
 
+obj-$(CONFIG_DEBUG_IRQFLAGS) += irqflag-debug.o
 obj-$(CONFIG_DEBUG_MUTEXES) += mutex-debug.o
 obj-$(CONFIG_LOCKDEP) += lockdep.o
 ifeq ($(CONFIG_PROC_FS),y)
diff --git a/kernel/locking/irqflag-debug.c b/kernel/locking/irqflag-debug.c
new file mode 100644
index 000000000000..9603d207a4ca
--- /dev/null
+++ b/kernel/locking/irqflag-debug.c
@@ -0,0 +1,11 @@
+// SPDX-License-Identifier: GPL-2.0-only
+
+#include <linux/bug.h>
+#include <linux/export.h>
+#include <linux/irqflags.h>
+
+void warn_bogus_irq_restore(void)
+{
+	WARN_ONCE(1, "raw_local_irq_restore() called with IRQs enabled\n");
+}
+EXPORT_SYMBOL(warn_bogus_irq_restore);
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 7937265ef879..5ea0c1773b0a 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1335,6 +1335,7 @@ config LOCKDEP_SMALL
 config DEBUG_LOCKDEP
 	bool "Lock dependency engine debugging"
 	depends on DEBUG_KERNEL && LOCKDEP
+	select DEBUG_IRQFLAGS
 	help
 	  If you say Y here, the lock dependency engine will do
 	  additional runtime checks to debug itself, at the price
@@ -1423,6 +1424,13 @@ config TRACE_IRQFLAGS_NMI
 	depends on TRACE_IRQFLAGS
 	depends on TRACE_IRQFLAGS_NMI_SUPPORT
 
+config DEBUG_IRQFLAGS
+	bool "Debug IRQ flag manipulation"
+	help
+	  Enables checks for potentially unsafe enabling or disabling of
+	  interrupts, such as calling raw_local_irq_restore() when interrupts
+	  are enabled.
+
 config STACKTRACE
 	bool "Stack backtrace support"
 	depends on STACKTRACE_SUPPORT
-- 
2.11.0


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