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

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

[gregkh]

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 <stable@vger.kernel.org> know about it.


>From 847fa1a6d3d00f3bdf68ef5fa4a786f644a0dd67 Mon Sep 17 00:00:00 2001
From: "Steven Rostedt (Red Hat)" <rostedt@goodmis.org>
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) <rostedt@goodmis.org>

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.

Fixes: d61f82d06672 ("ftrace: use dynamic patching for updating mcount calls")
Reported-by: Colin Ian King <colin.king@canonical.com>
Tested-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -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 rostedt@goodmis.org 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