[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>

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:
  *

arch/arm/kernel/traps.c

@@ -509,7 +509,7 @@ asmlinkage int arm_syscall(int no, struct pt_regs *regs)
 	 * existence.  Don't ever use this from user code.
 	 */
 	case 0xfff0:
-	{
+	for (;;) {
 		extern void do_DataAbort(unsigned long addr, unsigned int fsr,
 					 struct pt_regs *regs);
 		unsigned long val;
@@ -545,7 +545,6 @@ asmlinkage int arm_syscall(int no, struct pt_regs *regs)
 		up_read(&mm->mmap_sem);
 		/* simulate a write access fault */
 		do_DataAbort(addr, 15 + (1 << 11), regs);
-		return -1;
 	}
 #endif