[ARM] 3102/1: ARM EABI: stack pointer must be 64-bit aligned after a CPU exception

Patch from Nicolas Pitre

The ARM EABI says that the stack pointer has to be 64-bit aligned for
reasons already mentioned in patch #3101 when calling C functions.

We therefore must verify and adjust sp accordingly when taking an
exception from kernel mode since sp might not necessarily be 64-bit
aligned if the exception occurs in the middle of a kernel function.

If the exception occurs while in user mode then no sp fixup is needed as
long as sizeof(struct pt_regs) as well as any additional syscall data
stack space remain multiples of 8.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

3 files changed, 23 insertions, 3 deletions

diff --git a/arch/arm/kernel/entry-armv.S b/arch/arm/kernel/entry-armv.S
index fb8e7f4c4b37..874e6bb79405 100644
--- a/arch/arm/kernel/entry-armv.S
+++ b/arch/arm/kernel/entry-armv.S
@@ -105,14 +105,24 @@ common_invalid:
 /*
  * SVC mode handlers
  */
+
+#if defined(CONFIG_AEABI) && (__LINUX_ARM_ARCH__ >= 5)
+#define SPFIX(code...) code
+#else
+#define SPFIX(code...)
+#endif
+
         .macro  svc_entry
         sub     sp, sp, #S_FRAME_SIZE
+ SPFIX( tst     sp, #4          )
+ SPFIX( bicne   sp, sp, #4      )
         stmib   sp, {r1 - r12}
 
         ldmia   r0, {r1 - r3}
         add     r5, sp, #S_SP           @ here for interlock avoidance
         mov     r4, #-1                 @  ""  ""      ""       ""
         add     r0, sp, #S_FRAME_SIZE   @  ""  ""      ""       ""
+ SPFIX( addne   r0, r0, #4      )
         str     r1, [sp]                @ save the "real" r0 copied
                                         @ from the exception stack
 
@@ -303,7 +313,14 @@ __pabt_svc:
 
 /*
  * User mode handlers
+ *
+ * EABI note: sp_svc is always 64-bit aligned here, so should S_FRAME_SIZE
  */
+
+#if defined(CONFIG_AEABI) && (__LINUX_ARM_ARCH__ >= 5) && (S_FRAME_SIZE & 7)
+#error "sizeof(struct pt_regs) must be a multiple of 8"
+#endif
+
         .macro  usr_entry
         sub     sp, sp, #S_FRAME_SIZE
         stmib   sp, {r1 - r12}
diff --git a/arch/arm/kernel/entry-header.S b/arch/arm/kernel/entry-header.S
index 648cfff93138..55c99cdab7d6 100644
--- a/arch/arm/kernel/entry-header.S
+++ b/arch/arm/kernel/entry-header.S
@@ -19,6 +19,7 @@
 @
 @ Most of the stack format comes from struct pt_regs, but with
 @ the addition of 8 bytes for storing syscall args 5 and 6.
+@ This _must_ remain a multiple of 8 for EABI.
 @
 #define S_OFF           8
 
diff --git a/include/asm-arm/ptrace.h b/include/asm-arm/ptrace.h
index 4377e22b7e1a..f40948d54448 100644
--- a/include/asm-arm/ptrace.h
+++ b/include/asm-arm/ptrace.h
@@ -60,9 +60,11 @@
 
 #ifndef __ASSEMBLY__
 
-/* this struct defines the way the registers are stored on the
-   stack during a system call. */
-
+/*
+ * This struct defines the way the registers are stored on the
+ * stack during a system call.  Note that sizeof(struct pt_regs)
+ * has to be a multiple of 8.
+ */
 struct pt_regs {
         long uregs[18];
 };