Patch "ftrace/x86: Set ftrace_stub to weak to prevent gcc from using short jumps to it" has been added to the 4.4-stable tree

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 <[email protected]> know about it.

From: Steven Rostedt <[email protected]>
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 <[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.

---
 arch/x86/kernel/mcount_64.S |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

@@ -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 [email protected] are

queue-4.4/ftrace-x86-set-ftrace_stub-to-weak-to-prevent-gcc-from-using-short-jumps-to-it.patch