Patch "ARM: 8550/1: protect idiv patching against undefined gcc behavior" has been added to the 4.5-stable tree

Patch "ARM: 8550/1: protect idiv patching against undefined gcc behavior" has been added to the 4.5-stable tree

[gregkh]

This is a note to let you know that I've just added the patch titled

    ARM: 8550/1: protect idiv patching against undefined gcc behavior

to the 4.5-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:
     arm-8550-1-protect-idiv-patching-against-undefined-gcc-behavior.patch
and it can be found in the queue-4.5 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From 208fae5c3b9431013ad7bcea07cbcee114e7d163 Mon Sep 17 00:00:00 2001
From: Nicolas Pitre <nicolas.pitre@linaro.org>
Date: Mon, 14 Mar 2016 02:55:45 +0100
Subject: ARM: 8550/1: protect idiv patching against undefined gcc behavior

From: Nicolas Pitre <nicolas.pitre@linaro.org>

commit 208fae5c3b9431013ad7bcea07cbcee114e7d163 upstream.

It was reported that a kernel with CONFIG_ARM_PATCH_IDIV=y stopped
booting when compiled with the upcoming gcc 6.  Turns out that turning
a function address into a writable array is undefined and gcc 6 decided
it was OK to omit the store to the first word of the function while
still preserving the store to the second word.

Even though gcc 6 is now fixed to behave more coherently, it is a
mystery that gcc 4 and gcc 5 actually produce wanted code in the kernel.
And in fact the reduced test case to illustrate the issue does indeed
break with gcc < 6 as well.

In any case, let's guard the kernel against undefined compiler behavior
by hiding the nature of the array location as suggested by gcc
developers.

Reference: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70128

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Reported-by: Marcin Juszkiewicz <mjuszkiewicz@redhat.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 arch/arm/kernel/setup.c |    2 ++
 1 file changed, 2 insertions(+)

--- a/arch/arm/kernel/setup.c
+++ b/arch/arm/kernel/setup.c
@@ -430,11 +430,13 @@ static void __init patch_aeabi_idiv(void
 	pr_info("CPU: div instructions available: patching division code\n");
 
 	fn_addr = ((uintptr_t)&__aeabi_uidiv) & ~1;
+	asm ("" : "+g" (fn_addr));
 	((u32 *)fn_addr)[0] = udiv_instruction();
 	((u32 *)fn_addr)[1] = bx_lr_instruction();
 	flush_icache_range(fn_addr, fn_addr + 8);
 
 	fn_addr = ((uintptr_t)&__aeabi_idiv) & ~1;
+	asm ("" : "+g" (fn_addr));
 	((u32 *)fn_addr)[0] = sdiv_instruction();
 	((u32 *)fn_addr)[1] = bx_lr_instruction();
 	flush_icache_range(fn_addr, fn_addr + 8);


Patches currently in stable-queue which might be from nicolas.pitre@linaro.org are

queue-4.5/arm-8550-1-protect-idiv-patching-against-undefined-gcc-behavior.patch