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Subject[PATCH 3/3] ftrace/x86: Set ftrace_stub to weak to prevent gcc from using short jumps to it
From: Steven Rostedt <rostedt@goodmis.org>

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@gmail.com>
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@gandalf.local.home

Reported-by: Matt Fleming <matt@codeblueprint.co.uk>
Reported-by: Namhyung Kim <namhyung@kernel.org>
Tested-by: Matt Fleming <matt@codeblueprint.co.uk>
Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
arch/x86/kernel/mcount_64.S | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/mcount_64.S b/arch/x86/kernel/mcount_64.S
index ed48a9f465f8..61924222a9e1 100644
--- a/arch/x86/kernel/mcount_64.S
+++ b/arch/x86/kernel/mcount_64.S
@@ -182,7 +182,8 @@ 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)

--
2.8.0.rc3

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