[3.16.y-ckt stable] Patch "ARM: fix Thumb2 signal handling when ARMv6 is enabled" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    ARM: fix Thumb2 signal handling when ARMv6 is enabled

to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.16.y-queue

This patch is scheduled to be released in version 3.16.7-ckt19.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.16.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From 084e340df141d49c20009f5c66d7a1291e346cc8 Mon Sep 17 00:00:00 2001
From: Russell King <rmk+kernel at arm.linux.org.uk>
Date: Fri, 11 Sep 2015 16:44:02 +0100
Subject: ARM: fix Thumb2 signal handling when ARMv6 is enabled

commit 9b55613f42e8d40d5c9ccb8970bde6af4764b2ab upstream.

When a kernel is built covering ARMv6 to ARMv7, we omit to clear the
IT state when entering a signal handler.  This can cause the first
few instructions to be conditionally executed depending on the parent
context.

In any case, the original test for >= ARMv7 is broken - ARMv6 can have
Thumb-2 support as well, and an ARMv6T2 specific build would omit this
code too.

Relax the test back to ARMv6 or greater.  This results in us always
clearing the IT state bits in the PSR, even on CPUs where these bits
are reserved.  However, they're reserved for the IT state, so this
should cause no harm.

Fixes: d71e1352e240 ("Clear the IT state when invoking a Thumb-2 signal handler")
Acked-by: Tony Lindgren <tony at atomide.com>
Tested-by: H. Nikolaus Schaller <hns at goldelico.com>
Tested-by: Grazvydas Ignotas <notasas at gmail.com>
Signed-off-by: Russell King <rmk+kernel at arm.linux.org.uk>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 arch/arm/kernel/signal.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/arch/arm/kernel/signal.c b/arch/arm/kernel/signal.c
index bd1983437205..ea6d69125dde 100644
--- a/arch/arm/kernel/signal.c
+++ b/arch/arm/kernel/signal.c
@@ -354,12 +354,17 @@ setup_return(struct pt_regs *regs, struct ksignal *ksig,
 		 */
 		thumb = handler & 1;

-#if __LINUX_ARM_ARCH__ >= 7
+#if __LINUX_ARM_ARCH__ >= 6
 		/*
-		 * Clear the If-Then Thumb-2 execution state
-		 * ARM spec requires this to be all 000s in ARM mode
-		 * Snapdragon S4/Krait misbehaves on a Thumb=>ARM
-		 * signal transition without this.
+		 * Clear the If-Then Thumb-2 execution state.  ARM spec
+		 * requires this to be all 000s in ARM mode.  Snapdragon
+		 * S4/Krait misbehaves on a Thumb=>ARM signal transition
+		 * without this.
+		 *
+		 * We must do this whenever we are running on a Thumb-2
+		 * capable CPU, which includes ARMv6T2.  However, we elect
+		 * to do this whenever we're on an ARMv6 or later CPU for
+		 * simplicity.
 		 */
 		cpsr &= ~PSR_IT_MASK;
 #endif