[ARM] 4659/1: remove possibilities for spurious false negative with __kuser_cmpxchg

The ARM __kuser_cmpxchg routine is meant to implement an atomic cmpxchg
in user space.  It however can produce spurious false negative if a
processor exception occurs in the middle of the operation.  Normally
this is not a problem since cmpxchg is typically called in a loop until
it succeeds to implement an atomic increment for example.

Some use cases which don't involve a loop require that the operation be
100% reliable though.  This patch changes the implementation so to
reattempt the operation after an exception has occurred in the critical
section rather than abort it.

Here's a simple program to test the fix (don't use CONFIG_NO_HZ in your
kernel as this depends on a sufficiently high interrupt rate):

	#include <stdio.h>

	typedef int (__kernel_cmpxchg_t)(int oldval, int newval, int *ptr);
	#define __kernel_cmpxchg (*(__kernel_cmpxchg_t *)0xffff0fc0)

	int main()
	{
		int i, x = 0;
		for (i = 0; i < 100000000; i++) {
			int v = x;
			if (__kernel_cmpxchg(v, v+1, &x))
				printf("failed at %d: %d vs %d\n", i, v, x);
		}
		printf("done with %d vs %d\n", i, x);
		return 0;
	}

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

1 files changed, 57 insertions, 37 deletions

diff --git a/arch/arm/kernel/entry-armv.S b/arch/arm/kernel/entry-armv.S
index d645897652c2..0d1bbea84df0 100644
--- a/arch/arm/kernel/entry-armv.S
+++ b/arch/arm/kernel/entry-armv.S
@@ -339,16 +339,6 @@ __pabt_svc:
         str     r1, [sp]                @ save the "real" r0 copied
                                         @ from the exception stack
 
-#if __LINUX_ARM_ARCH__ < 6 && !defined(CONFIG_NEEDS_SYSCALL_FOR_CMPXCHG)
-#ifndef CONFIG_MMU
-#warning "NPTL on non MMU needs fixing"
-#else
-        @ make sure our user space atomic helper is aborted
-        cmp     r2, #TASK_SIZE
-        bichs   r3, r3, #PSR_Z_BIT
-#endif
-#endif
-
         @
         @ We are now ready to fill in the remaining blanks on the stack:
         @
@@ -372,9 +362,25 @@ __pabt_svc:
         zero_fp
         .endm
 
+        .macro  kuser_cmpxchg_check
+#if __LINUX_ARM_ARCH__ < 6 && !defined(CONFIG_NEEDS_SYSCALL_FOR_CMPXCHG)
+#ifndef CONFIG_MMU
+#warning "NPTL on non MMU needs fixing"
+#else
+        @ Make sure our user space atomic helper is restarted
+        @ if it was interrupted in a critical region.  Here we
+        @ perform a quick test inline since it should be false
+        @ 99.9999% of the time.  The rest is done out of line.
+        cmp     r2, #TASK_SIZE
+        blhs    kuser_cmpxchg_fixup
+#endif
+#endif
+        .endm
+
         .align  5
 __dabt_usr:
         usr_entry
+        kuser_cmpxchg_check
 
         @
         @ Call the processor-specific abort handler:
@@ -404,6 +410,7 @@ __dabt_usr:
         .align  5
 __irq_usr:
         usr_entry
+        kuser_cmpxchg_check
 
 #ifdef CONFIG_TRACE_IRQFLAGS
         bl      trace_hardirqs_off
@@ -669,7 +676,7 @@ __kuser_helper_start:
  *
  * Clobbered:
  *
- *      the Z flag might be lost
+ *      none
  *
  * Definition and user space usage example:
  *
@@ -730,9 +737,6 @@ __kuser_memory_barrier:				@ 0xffff0fa0
  *
  *    - This routine already includes memory barriers as needed.
  *
- *    - A failure might be transient, i.e. it is possible, although unlikely,
- *      that "failure" be returned even if *ptr == oldval.
- *
  * For example, a user space atomic_add implementation could look like this:
  *
  * #define atomic_add(ptr, val) \
@@ -769,46 +773,62 @@ __kuser_cmpxchg:				@ 0xffff0fc0
 
 #elif __LINUX_ARM_ARCH__ < 6
 
+#ifdef CONFIG_MMU
+
         /*
-         * Theory of operation:
-         *
-         * We set the Z flag before loading oldval. If ever an exception
-         * occurs we can not be sure the loaded value will still be the same
-         * when the exception returns, therefore the user exception handler
-         * will clear the Z flag whenever the interrupted user code was
-         * actually from the kernel address space (see the usr_entry macro).
-         *
-         * The post-increment on the str is used to prevent a race with an
-         * exception happening just after the str instruction which would
-         * clear the Z flag although the exchange was done.
+         * The only thing that can break atomicity in this cmpxchg
+         * implementation is either an IRQ or a data abort exception
+         * causing another process/thread to be scheduled in the middle
+         * of the critical sequence.  To prevent this, code is added to
+         * the IRQ and data abort exception handlers to set the pc back
+         * to the beginning of the critical section if it is found to be
+         * within that critical section (see kuser_cmpxchg_fixup).
          */
-#ifdef CONFIG_MMU
-        teq     ip, ip                  @ set Z flag
-        ldr     ip, [r2]                @ load current val
-        add     r3, r2, #1              @ prepare store ptr
-        teqeq   ip, r0                  @ compare with oldval if still allowed
-        streq   r1, [r3, #-1]!          @ store newval if still allowed
-        subs    r0, r2, r3              @ if r2 == r3 the str occured
+1:      ldr     r3, [r2]                        @ load current val
+        subs    r3, r3, r0                      @ compare with oldval
+2:      streq   r1, [r2]                        @ store newval if eq
+        rsbs    r0, r3, #0                      @ set return val and C flag
+        usr_ret lr
+
+        .text
+kuser_cmpxchg_fixup:
+        @ Called from kuser_cmpxchg_check macro.
+        @ r2 = address of interrupted insn (must be preserved).
+        @ sp = saved regs. r7 and r8 are clobbered.
+        @ 1b = first critical insn, 2b = last critical insn.
+        @ If r2 >= 1b and r2 <= 2b then saved pc_usr is set to 1b.
+        mov     r7, #0xffff0fff
+        sub     r7, r7, #(0xffff0fff - (0xffff0fc0 + (1b - __kuser_cmpxchg)))
+        subs    r8, r2, r7
+        rsbcss  r8, r8, #(2b - 1b)
+        strcs   r7, [sp, #S_PC]
+        mov     pc, lr
+        .previous
+
 #else
 #warning "NPTL on non MMU needs fixing"
         mov     r0, #-1
         adds    r0, r0, #0
-#endif
         usr_ret lr
+#endif
 
 #else
 
 #ifdef CONFIG_SMP
         mcr     p15, 0, r0, c7, c10, 5  @ dmb
 #endif
-        ldrex   r3, [r2]
+1:      ldrex   r3, [r2]
         subs    r3, r3, r0
         strexeq r3, r1, [r2]
+        teqeq   r3, #1
+        beq     1b
         rsbs    r0, r3, #0
+        /* beware -- each __kuser slot must be 8 instructions max */
 #ifdef CONFIG_SMP
-        mcr     p15, 0, r0, c7, c10, 5  @ dmb
-#endif
+        b       __kuser_memory_barrier
+#else
         usr_ret lr
+#endif
 
 #endif
 
@@ -829,7 +849,7 @@ __kuser_cmpxchg:				@ 0xffff0fc0
  *
  * Clobbered:
  *
- *      the Z flag might be lost
+ *      none
  *
  * Definition and user space usage example:
  *