Patch "ftrace/x86_32: Set ftrace_stub to weak to prevent gcc from using short jumps to it" has been added to the 4.4-stable tree

PATCH [email protected] 2 participants 3 messages Jan 6, 2017 – Jan 20, 2017

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[email protected]

author maintainer Applied

2017-01-06 14:55 UTC #0 lore raw

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 <[email protected]> know about it.

Quoted 1 line

From 847fa1a6d3d00f3bdf68ef5fa4a786f644a0dd67 Mon Sep 17 00:00:00 2001

From: "Steven Rostedt (Red Hat)" <[email protected]>
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) <[email protected]>

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 <[email protected]>

Tested-by Colin Ian King <[email protected]>

Signed-off-by Steven Rostedt <[email protected]>

Signed-off-by Greg Kroah-Hartman <[email protected]>

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

arch/x86/entry/entry_32.S +2 -2

@@ -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 [email protected] 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

Jiri Slaby

2017-01-20 09:18 UTC #1 lore raw

Quoted 17 lines · On 01/06/2017, 03:55 PM, [email protected] wrote:

From 847fa1a6d3d00f3bdf68ef5fa4a786f644a0dd67 Mon Sep 17 00:00:00 2001
From: "Steven Rostedt (Red Hat)" <[email protected]>
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) <[email protected]>
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.

Quoted 2 lines

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

Greg,

can we have the x86_64 fix (8329e818f149) in 4.4 too?

thanks,

Signature · 2 lines

js
suse labs

Greg KH

author maintainer

2017-01-20 09:43 UTC #2 lore raw

Quoted 18 lines · Jiri Slaby

Quoted 17 lines · On 01/06/2017, 03:55 PM, [email protected] wrote:
From 847fa1a6d3d00f3bdf68ef5fa4a786f644a0dd67 Mon Sep 17 00:00:00 2001
From: "Steven Rostedt (Red Hat)" <[email protected]>
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) <[email protected]>
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.

Quoted 6 lines

Quoted 2 lines
This was fixed for x86_64 with commit 8329e818f149, with the same subject as this commit, but nothing was done for x86_32.
Greg,
can we have the x86_64 fix (8329e818f149) in 4.4 too?

Ah, yes, I totally missed that. I'll queue it up after this next round of kernels goes out in a few hours.

thanks,

greg k-h