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Date:   Tue, 24 Jan 2017 08:55:11 +0100
From:   Greg Kroah-Hartman <>
Cc:     Greg Kroah-Hartman <>,, Matt Fleming <>,
        Namhyung Kim <>,
        Masami Hiramatsu <>,
        Steven Rostedt <>,
        Jiri Slaby <>
Subject: [PATCH 4.4 01/42] ftrace/x86: Set ftrace_stub to weak to prevent gcc from using short jumps to it

4.4-stable review patch.  If anyone has any objections, please let me know.


From: Steven Rostedt <>

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 <>
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


Reported-by: Matt Fleming <>
Reported-by: Namhyung Kim <>
Tested-by: Matt Fleming <>
Reviewed-by: Masami Hiramatsu <>
Signed-off-by: Steven Rostedt <>
Cc: Jiri Slaby <>
Signed-off-by: Greg Kroah-Hartman <>

 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
+/* This is weak to keep gas from relaxing the jumps */

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