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Subjectgit pull request for tip/tracing/urgent


The bug that is fixed by this change can affect users. Most likely it will
not, since the fault should never happen. But this is a protective
mechanism, where if it does, that means there is a bug in the tracer.

As you have previously told me, a bug in the tracer should never crash the
kernel. Since the detection of a fault in the function graph tracer can
lead to a kernel crash (without this change) I think this qualifies as
something for 29.

-- Steve

The following patch is in:


branch: tip/tracing/urgent

Steven Rostedt (1):
tracing, x86: fix fixup section to return to original code

arch/x86/kernel/ftrace.c | 8 +++++---
1 files changed, 5 insertions(+), 3 deletions(-)
commit e3944bfac961cd7fc82f3b3143c55dc375748569
Author: Steven Rostedt <>
Date: Tue Feb 10 13:07:13 2009 -0500

tracing, x86: fix fixup section to return to original code

Impact: fix to prevent a kernel crash on fault

If for some reason the pointer to the parent function on the
stack takes a fault, the fix up code will not return back to
the original faulting code. This can lead to unpredictable
results and perhaps even a kernel panic.

A fault should not happen, but if it does, we should simply
disable the tracer, warn, and continue running the kernel.
It should not lead to a kernel crash.

Signed-off-by: Steven Rostedt <>

diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
index 1b43086..9d549e4 100644
--- a/arch/x86/kernel/ftrace.c
+++ b/arch/x86/kernel/ftrace.c
@@ -491,13 +491,15 @@ void prepare_ftrace_return(unsigned long *parent, unsigned long self_addr)
"1: " _ASM_MOV " (%[parent_old]), %[old]\n"
"2: " _ASM_MOV " %[return_hooker], (%[parent_replaced])\n"
" movl $0, %[faulted]\n"
+ "3:\n"

".section .fixup, \"ax\"\n"
- "3: movl $1, %[faulted]\n"
+ "4: movl $1, %[faulted]\n"
+ " jmp 3b\n"

- _ASM_EXTABLE(1b, 3b)
- _ASM_EXTABLE(2b, 3b)
+ _ASM_EXTABLE(1b, 4b)
+ _ASM_EXTABLE(2b, 4b)

: [parent_replaced] "=r" (parent), [old] "=r" (old),
[faulted] "=r" (faulted)

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