From: Frederic Weisbecker on

Please pull the perf/urgent branch that can be found at:



Frederic Weisbecker (1):
x86/stacktrace: Don't dereference bad frame pointers

arch/x86/kernel/dumpstack_64.c | 12 +++++++++---
1 files changed, 9 insertions(+), 3 deletions(-)

commit 29044ad1509ecc229f1d5a31aeed7a8dc61a71c4
Author: Frederic Weisbecker <fweisbec(a)>
Date: Wed Mar 3 02:25:22 2010 +0100

x86/stacktrace: Don't dereference bad frame pointers

Callers of a stacktrace might pass bad frame pointers. Those
are usually checked for safety in stack walking helpers before
any dereferencing, but this is not the case when we need to go
through one more frame pointer that backlinks the irq stack to
the previous one, as we don't have any reliable address boudaries
to compare this frame pointer against.

This raises crashes when we record callchains for ftrace events
with perf because we don't use the right helpers to capture
registers there. We get wrong frame pointers as we call
task_pt_regs() even on kernel threads, which is a wrong thing
as it gives us the initial state of any kernel threads freshly
created. This is even not what we want for user tasks. What we want
is a hot snapshot of registers when the ftrace event triggers, not
the state before a task entered the kernel.

This requires more thoughts to do it correctly though.
So first put a guardian to ensure the given frame pointer
can be dereferenced to avoid crashes. We'll think about how to fix
the callers in a subsequent patch.

Signed-off-by: Frederic Weisbecker <fweisbec(a)>
Cc: Ingo Molnar <mingo(a)>
Cc: Thomas Gleixner <tglx(a)>
Cc: H. Peter Anvin <hpa(a)>
Cc: Peter Zijlstra <peterz(a)>
Cc: Paul Mackerras <paulus(a)>
Cc: Steven Rostedt <rostedt(a)>
Cc: 2.6.33.x <stable(a)>
Cc: Arnaldo Carvalho de Melo <acme(a)>

diff --git a/arch/x86/kernel/dumpstack_64.c b/arch/x86/kernel/dumpstack_64.c
index 0ad9597..a6c906c 100644
--- a/arch/x86/kernel/dumpstack_64.c
+++ b/arch/x86/kernel/dumpstack_64.c
@@ -125,9 +125,15 @@ fixup_bp_irq_link(unsigned long bp, unsigned long *stack,
struct stack_frame *frame = (struct stack_frame *)bp;
+ unsigned long next;

- if (!in_irq_stack(stack, irq_stack, irq_stack_end))
- return (unsigned long)frame->next_frame;
+ if (!in_irq_stack(stack, irq_stack, irq_stack_end)) {
+ if (!probe_kernel_address(&frame->next_frame, next))
+ return next;
+ else
+ WARN_ONCE(1, "Perf: bad frame pointer = %p in "
+ "callchain\n", &frame->next_frame);
+ }
return bp;
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