[PATCH AUTOSEL 4.14 092/123] ARM: 8830/1: NOMMU: Toggle only bits in EXC_RETURN we are really care of

Sasha Levin
2019-03-27T18:19:32Z

From: Vladimir Murzin <vladimir.murzin@arm.com>

[ Upstream commit 72cd4064fccaae15ab84d40d4be23667402df4ed ]

ARMv8M introduces support for Security extension to M class, among
other things it affects exception handling, especially, encoding of
EXC_RETURN.

The new bits have been added:

Bit [6] Secure or Non-secure stack
Bit [5] Default callee register stacking
Bit [0] Exception Secure

which conflicts with hard-coded value of EXC_RETURN:

In fact, we only care of few bits:

Bit [3] Mode (0 - Handler, 1 - Thread)
Bit [2] Stack pointer selection (0 - Main, 1 - Process)

We can toggle only those bits and left other bits as they were on
exception entry.

It is basically, what patch does - saves EXC_RETURN when we do
transition form Thread to Handler mode (it is first svc), so later
saved value is used instead of EXC_RET_THREADMODE_PROCESSSTACK.

Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
arch/arm/include/asm/v7m.h | 2 +-
arch/arm/kernel/entry-header.S | 3 ++-
arch/arm/kernel/entry-v7m.S | 4 ++++
arch/arm/mm/proc-v7m.S | 3 +++
4 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/arch/arm/include/asm/v7m.h b/arch/arm/include/asm/v7m.h
index e6d9e29fcae4..6416fd3a3894 100644
--- a/arch/arm/include/asm/v7m.h
+++ b/arch/arm/include/asm/v7m.h
@@ -49,7 +49,7 @@
* (0 -> msp; 1 -> psp). Bits [1:0] are fixed to 0b01.
*/
#define EXC_RET_STACK_MASK 0x00000004
-#define EXC_RET_THREADMODE_PROCESSSTACK 0xfffffffd
+#define EXC_RET_THREADMODE_PROCESSSTACK (3 << 2)

/* Cache related definitions */

diff --git a/arch/arm/kernel/entry-header.S b/arch/arm/kernel/entry-header.S
index 773424843d6e..62db1c9746cb 100644
--- a/arch/arm/kernel/entry-header.S
+++ b/arch/arm/kernel/entry-header.S
@@ -127,7 +127,8 @@
*/
.macro v7m_exception_slow_exit ret_r0
cpsid i
- ldr lr, =EXC_RET_THREADMODE_PROCESSSTACK
+ ldr lr, =exc_ret
+ ldr lr, [lr]

@ read original r12, sp, lr, pc and xPSR
add r12, sp, #S_IP
diff --git a/arch/arm/kernel/entry-v7m.S b/arch/arm/kernel/entry-v7m.S
index abcf47848525..19d2dcd6530d 100644
--- a/arch/arm/kernel/entry-v7m.S
+++ b/arch/arm/kernel/entry-v7m.S
@@ -146,3 +146,7 @@ ENTRY(vector_table)
.rept CONFIG_CPU_V7M_NUM_IRQ
.long __irq_entry @ External Interrupts
.endr
+ .align 2
+ .globl exc_ret
+exc_ret:
+ .space 4
diff --git a/arch/arm/mm/proc-v7m.S b/arch/arm/mm/proc-v7m.S
index 47a5acc64433..92e84181933a 100644
--- a/arch/arm/mm/proc-v7m.S
+++ b/arch/arm/mm/proc-v7m.S
@@ -139,6 +139,9 @@ __v7m_setup_cont:
cpsie i
svc #0
1: cpsid i
+ ldr r0, =exc_ret
+ orr lr, lr, #EXC_RET_THREADMODE_PROCESSSTACK
+ str lr, [r0]
ldmia sp, {r0-r3, r12}
str r5, [r12, #11 * 4] @ restore the original SVC vector entry
mov lr, r6 @ restore LR
--
2.19.1