[for-next][PATCH 5/8] ftrace/x86_32: Set ftrace_stub to weak to prevent gcc from using short jumps to it

From: Steven Rostedt
Date: Fri Dec 09 2016 - 09:28:53 EST

From: "Steven Rostedt (Red Hat)" <rostedt@xxxxxxxxxxx>

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

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

Cc: stable@xxxxxxxxxxxxxxx
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>
arch/x86/entry/entry_32.S | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index 21b352a11b49..edba8606b99a 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -889,8 +889,8 @@ ftrace_graph_call:
jmp ftrace_stub

-.globl ftrace_stub
+/* This is weak to keep gas from relaxing the jumps */