linux-kernel - [PATCH 3.16.y-ckt 010/104] ARM: fix Thumb2 signal handling when ARMv6 is enabled

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Message-Id: <1445867006-18048-11-git-send-email-luis.henriques@canonical.com>
Date:	Mon, 26 Oct 2015 13:41:52 +0000
From:	Luis Henriques <luis.henriques@...onical.com>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	kernel-team@...ts.ubuntu.com
Cc:	Russell King <rmk+kernel@....linux.org.uk>,
	Luis Henriques <luis.henriques@...onical.com>
Subject: [PATCH 3.16.y-ckt 010/104] ARM: fix Thumb2 signal handling when ARMv6 is enabled

3.16.7-ckt19 -stable review patch.  If anyone has any objections, please let me know.

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

From: Russell King <rmk+kernel@....linux.org.uk>

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@...mide.com>
Tested-by: H. Nikolaus Schaller <hns@...delico.com>
Tested-by: Grazvydas Ignotas <notasas@...il.com>
Signed-off-by: Russell King <rmk+kernel@....linux.org.uk>
Signed-off-by: Luis Henriques <luis.henriques@...onical.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
--
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