Patch "ftrace/x86_32: Set ftrace_stub to weak to prevent gcc from using short jumps to it" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

ftrace/x86_32: Set ftrace_stub to weak to prevent gcc from using short jumps to it

to the 4.9-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_32-set-ftrace_stub-to-weak-to-prevent-gcc-from-using-short-jumps-to-it.patch
and it can be found in the queue-4.9 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 (Red Hat)" <[email protected]>
Date: Thu, 8 Dec 2016 12:48:26 -0500
Subject: ftrace/x86_32: Set ftrace_stub to weak to prevent gcc from using short jumps to it

From: Steven Rostedt (Red Hat) <[email protected]>

commit 847fa1a6d3d00f3bdf68ef5fa4a786f644a0dd67 upstream.

With new binutils, gcc may get smart with its optimization and change a jmp from a 5 byte jump to a 2 byte one even though it was jumping to a global function. But that global function existed within a 2 byte radius, and gcc was able to optimize it. Unfortunately, that jump was also being modified when function graph tracing begins. Since ftrace expected that jump to be 5 bytes, but it was only two, it overwrote code after the jump, causing a crash.

This was fixed for x86_64 with commit 8329e818f149, with the same subject as this commit, but nothing was done for x86_32.

---
 arch/x86/entry/entry_32.S |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

@@ -889,8 +889,8 @@ ftrace_graph_call:
 	jmp	ftrace_stub
 #endif
 
-.globl ftrace_stub
-ftrace_stub:
+/* This is weak to keep gas from relaxing the jumps */
+WEAK(ftrace_stub)
 	ret
 END(ftrace_caller)
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/ftrace-x86_32-set-ftrace_stub-to-weak-to-prevent-gcc-from-using-short-jumps-to-it.patch queue-4.9/fgraph-handle-a-case-where-a-tracer-ignores-set_graph_notrace.patch