This is a note to let you know that I've just added the patch titled
ftrace/x86: Set ftrace_stub to weak to prevent gcc from using short jumps to it
to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
ftrace-x86-set-ftrace_stub-to-weak-to-prevent-gcc-from-using-short-jumps-to-it.patch
and it can be found in the queue-4.4 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From 8329e818f14926a6040df86b2668568bde342ebf Mon Sep 17 00:00:00 2001
From: Steven Rostedt <rostedt@xxxxxxxxxxx>
Date: Mon, 16 May 2016 23:00:35 -0400
Subject: ftrace/x86: Set ftrace_stub to weak to prevent gcc from using short jumps to it
From: Steven Rostedt <rostedt@xxxxxxxxxxx>
commit 8329e818f14926a6040df86b2668568bde342ebf upstream.
Matt Fleming reported seeing crashes when enabling and disabling
function profiling which uses function graph tracer. Later Namhyung Kim
hit a similar issue and he found that the issue was due to the jmp to
ftrace_stub in ftrace_graph_call was only two bytes, and when it was
changed to jump to the tracing code, it overwrote the ftrace_stub that
was after it.
Masami Hiramatsu bisected this down to a binutils change:
8dcea93252a9ea7dff57e85220a719e2a5e8ab41 is the first bad commit
commit 8dcea93252a9ea7dff57e85220a719e2a5e8ab41
Author: H.J. Lu <hjl.tools@xxxxxxxxx>
Date: Fri May 15 03:17:31 2015 -0700
Add -mshared option to x86 ELF assembler
This patch adds -mshared option to x86 ELF assembler. By default,
assembler will optimize out non-PLT relocations against defined non-weak
global branch targets with default visibility. The -mshared option tells
the assembler to generate code which may go into a shared library
where all non-weak global branch targets with default visibility can
be preempted. The resulting code is slightly bigger. This option
only affects the handling of branch instructions.
Declaring ftrace_stub as a weak call prevents gas from using two byte
jumps to it, which would be converted to a jump to the function graph
code.
Link: http://lkml.kernel.org/r/20160516230035.1dbae571@xxxxxxxxxxxxxxxxxx
Reported-by: Matt Fleming <matt@xxxxxxxxxxxxxxxxxxx>
Reported-by: Namhyung Kim <namhyung@xxxxxxxxxx>
Tested-by: Matt Fleming <matt@xxxxxxxxxxxxxxxxxxx>
Reviewed-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Jiri Slaby <jslaby@xxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
arch/x86/kernel/mcount_64.S | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
--- a/arch/x86/kernel/mcount_64.S
+++ b/arch/x86/kernel/mcount_64.S
@@ -180,7 +180,8 @@ GLOBAL(ftrace_graph_call)
jmp ftrace_stub
#endif
-GLOBAL(ftrace_stub)
+/* This is weak to keep gas from relaxing the jumps */
+WEAK(ftrace_stub)
retq
END(ftrace_caller)
Patches currently in stable-queue which might be from rostedt@xxxxxxxxxxx are
queue-4.4/ftrace-x86-set-ftrace_stub-to-weak-to-prevent-gcc-from-using-short-jumps-to-it.patch
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