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 4.4-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_32-set-ftrace_stub-to-weak-to-prevent-gcc-from-using-short-jumps-to-it.patch
and it can be found in the queue-4.4 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From 847fa1a6d3d00f3bdf68ef5fa4a786f644a0dd67 Mon Sep 17 00:00:00 2001
From: "Steven Rostedt (Red Hat)" <rostedt@xxxxxxxxxxx>
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@xxxxxxxxxxx>
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@xxxxxxxxxxxxx>
Tested-by: Colin Ian King <colin.king@xxxxxxxxxxxxx>
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
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
@@ -766,8 +766,8 @@ 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@xxxxxxxxxxx are
queue-4.4/ftrace-x86_32-set-ftrace_stub-to-weak-to-prevent-gcc-from-using-short-jumps-to-it.patch
queue-4.4/fgraph-handle-a-case-where-a-tracer-ignores-set_graph_notrace.patch
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