linux-kernel - [PATCH 3.12 057/235] ftrace/x86_32: Set ftrace_stub to weak to prevent gcc from using short jumps to it

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Message-Id: <7a2a5f7c2e9b4653385c2f0094e96ce3abc7b43a.1485514374.git.jslaby@suse.cz>
Date:   Fri, 27 Jan 2017 11:53:10 +0100
From:   Jiri Slaby <jslaby@...e.cz>
To:     stable@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org,
        "Steven Rostedt (Red Hat)" <rostedt@...dmis.org>,
        Jiri Slaby <jslaby@...e.cz>
Subject: [PATCH 3.12 057/235] ftrace/x86_32: Set ftrace_stub to weak to prevent gcc from using short jumps to it

From: "Steven Rostedt (Red Hat)" <rostedt@...dmis.org>

3.12-stable review patch.  If anyone has any objections, please let me know.

===============

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@...onical.com>
Tested-by: Colin Ian King <colin.king@...onical.com>
Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
Signed-off-by: Jiri Slaby <jslaby@...e.cz>
---
 arch/x86/kernel/entry_32.S | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/entry_32.S b/arch/x86/kernel/entry_32.S
index 1f1c33d0a13c..a78db5ed8b3f 100644
--- a/arch/x86/kernel/entry_32.S
+++ b/arch/x86/kernel/entry_32.S
@@ -1113,8 +1113,8 @@ 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)

-- 
2.11.0