From: Steven Rostedt <[email protected]> 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 <[email protected]> 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/[email protected] Reported-by: Matt Fleming <[email protected]> Reported-by: Namhyung Kim <[email protected]> Tested-by: Matt Fleming <[email protected]> Reviewed-by: Masami Hiramatsu <[email protected]> Signed-off-by: Steven Rostedt <[email protected]> Signed-off-by: Willy Tarreau <[email protected]> --- 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
