linux-kernel - [PATCH 3.16 091/370] ftrace/x86_32: Set ftrace_stub to weak to prevent gcc from using short jumps to it

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Message-ID: <lsq.1489146382.730845415@decadent.org.uk>
Date:   Fri, 10 Mar 2017 11:46:22 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org,
        "Colin Ian King" <colin.king@...onical.com>,
        "Steven Rostedt (Red Hat)" <rostedt@...dmis.org>
Subject: [PATCH 3.16 091/370] ftrace/x86_32: Set ftrace_stub to weak to
 prevent gcc from using short jumps to it

3.16.42-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: "Steven Rostedt (Red Hat)" <rostedt@...dmis.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@...onical.com>
Tested-by: Colin Ian King <colin.king@...onical.com>
Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
[bwh: Backported to 3.16: adjust filename]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 arch/x86/kernel/entry_32.S | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/arch/x86/kernel/entry_32.S
+++ b/arch/x86/kernel/entry_32.S
@@ -1086,8 +1086,8 @@ 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)