linux-kernel - [PATCH 3.2 075/107] ARM: 7880/1: Clear the IT state independent of the Thumb-2 mode

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Message-ID: <lsq.1444349548.323310640@decadent.org.uk>
Date:	Fri, 09 Oct 2015 01:12:28 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:	akpm@...ux-foundation.org, "T.J. Purtell" <tj@...isocial.us>,
	"Russell King" <rmk+kernel@....linux.org.uk>
Subject: [PATCH 3.2 075/107] ARM: 7880/1: Clear the IT state independent
 of the Thumb-2 mode

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

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

From: "T.J. Purtell" <tj@...isocial.us>

commit 6ecf830e5029598732e04067e325d946097519cb upstream.

The ARM architecture reference specifies that the IT state bits in the
PSR must be all zeros in ARM mode or behavior is unspecified.  On the
Qualcomm Snapdragon S4/Krait architecture CPUs the processor continues
to consider the IT state bits while in ARM mode.  This makes it so
that some instructions are skipped by the CPU.

Signed-off-by: T.J. Purtell <tj@...isocial.us>
[rmk+kernel@....linux.org.uk: fixed whitespace formatting in patch]
Signed-off-by: Russell King <rmk+kernel@....linux.org.uk>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 arch/arm/kernel/signal.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

--- a/arch/arm/kernel/signal.c
+++ b/arch/arm/kernel/signal.c
@@ -486,12 +486,18 @@ setup_return(struct pt_regs *regs, struc
 		 */
 		thumb = handler & 1;
 
-		if (thumb) {
-			cpsr |= PSR_T_BIT;
 #if __LINUX_ARM_ARCH__ >= 7
-			/* clear the If-Then Thumb-2 execution state */
-			cpsr &= ~PSR_IT_MASK;
+		/*
+		 * 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.
+		 */
+		cpsr &= ~PSR_IT_MASK;
 #endif
+
+		if (thumb) {
+			cpsr |= PSR_T_BIT;
 		} else
 			cpsr &= ~PSR_T_BIT;
 	}

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