| From | Willy Tarreau <> | Subject | [PATCH 3.10 080/268] ftrace/x86: Set ftrace_stub to weak to prevent gcc from using short jumps to it | Date | Mon, 19 Jun 2017 20:29:39 +0200 |
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From: Steven Rostedt <rostedt@goodmis.org>
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@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> Signed-off-by: Willy Tarreau <w@1wt.eu> --- arch/x86/kernel/entry_64.S | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S index 6ed8f16..cc89b36 100644 --- a/arch/x86/kernel/entry_64.S +++ b/arch/x86/kernel/entry_64.S @@ -122,7 +122,8 @@ 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.rc2.1.gbe9624a
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