MIPS: Sanitize restart logics

Put the original syscall number into ->regs[0] when we leave syscall
with error.  Use it in restart logics.  Everything else will have
it 0 since we pass through SAVE_SOME on all the ways in.  Note that
in places like bad_stack and inllegal_syscall we leave it 0 - it's not
restartable.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-kernel@vger.kernel.org
Cc: linux-arch@vger.kernel.org
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/1698/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/branch.c

@@ -40,7 +40,6 @@ int __compute_return_epc(struct pt_regs *regs)
 		return -EFAULT;
 	}
 
-	regs->regs[0] = 0;
 	switch (insn.i_format.opcode) {
 	/*
 	 * jr and jalr are in r_format format.

arch/mips/kernel/scall32-o32.S

@@ -63,9 +63,9 @@ stack_done:
 	sw	t0, PT_R7(sp)		# set error flag
 	beqz	t0, 1f
 
+	lw	t1, PT_R2(sp)		# syscall number
 	negu	v0			# error
-	sw	v0, PT_R0(sp)		# set flag for syscall
-					# restarting
+	sw	t1, PT_R0(sp)		# save it for syscall restarting
 1:	sw	v0, PT_R2(sp)		# result
 
 o32_syscall_exit:
@@ -104,9 +104,9 @@ syscall_trace_entry:
 	sw	t0, PT_R7(sp)		# set error flag
 	beqz	t0, 1f
 
+	lw	t1, PT_R2(sp)		# syscall number
 	negu	v0			# error
-	sw	v0, PT_R0(sp)		# set flag for syscall
-					# restarting
+	sw	t1, PT_R0(sp)		# save it for syscall restarting
 1:	sw	v0, PT_R2(sp)		# result
 
 	j	syscall_exit
@@ -170,7 +170,6 @@ stackargs:
 	 */
 bad_stack:
 	negu	v0				# error
-	sw	v0, PT_R0(sp)
 	sw	v0, PT_R2(sp)
 	li	t0, 1				# set error flag
 	sw	t0, PT_R7(sp)

arch/mips/kernel/scall64-64.S

@@ -66,9 +66,9 @@ NESTED(handle_sys64, PT_SIZE, sp)
 	sd	t0, PT_R7(sp)		# set error flag
 	beqz	t0, 1f
 
+	ld	t1, PT_R2(sp)		# syscall number
 	dnegu	v0			# error
-	sd	v0, PT_R0(sp)		# set flag for syscall
-					# restarting
+	sd	t1, PT_R0(sp)		# save it for syscall restarting
 1:	sd	v0, PT_R2(sp)		# result
 
 n64_syscall_exit:
@@ -109,8 +109,9 @@ syscall_trace_entry:
 	sd	t0, PT_R7(sp)		# set error flag
 	beqz	t0, 1f
 
+	ld	t1, PT_R2(sp)		# syscall number
 	dnegu	v0			# error
-	sd	v0, PT_R0(sp)		# set flag for syscall restarting
+	sd	t1, PT_R0(sp)		# save it for syscall restarting
 1:	sd	v0, PT_R2(sp)		# result
 
 	j	syscall_exit

arch/mips/kernel/scall64-n32.S

@@ -65,8 +65,9 @@ NESTED(handle_sysn32, PT_SIZE, sp)
 	sd	t0, PT_R7(sp)		# set error flag
 	beqz	t0, 1f
 
+	ld	t1, PT_R2(sp)		# syscall number
 	dnegu	v0			# error
-	sd	v0, PT_R0(sp)		# set flag for syscall restarting
+	sd	t1, PT_R0(sp)		# save it for syscall restarting
 1:	sd	v0, PT_R2(sp)		# result
 
 	local_irq_disable		# make sure need_resched and
@@ -106,8 +107,9 @@ n32_syscall_trace_entry:
 	sd	t0, PT_R7(sp)		# set error flag
 	beqz	t0, 1f
 
+	ld	t1, PT_R2(sp)		# syscall number
 	dnegu	v0			# error
-	sd	v0, PT_R0(sp)		# set flag for syscall restarting
+	sd	t1, PT_R0(sp)		# save it for syscall restarting
 1:	sd	v0, PT_R2(sp)		# result
 
 	j	syscall_exit

arch/mips/kernel/scall64-o32.S

@@ -93,8 +93,9 @@ NESTED(handle_sys, PT_SIZE, sp)
 	sd	t0, PT_R7(sp)		# set error flag
 	beqz	t0, 1f
 
+	ld	t1, PT_R2(sp)		# syscall number
 	dnegu	v0			# error
-	sd	v0, PT_R0(sp)		# flag for syscall restarting
+	sd	t1, PT_R0(sp)		# save it for syscall restarting
 1:	sd	v0, PT_R2(sp)		# result
 
 o32_syscall_exit:
@@ -142,8 +143,9 @@ trace_a_syscall:
 	sd	t0, PT_R7(sp)		# set error flag
 	beqz	t0, 1f
 
+	ld	t1, PT_R2(sp)		# syscall number
 	dnegu	v0			# error
-	sd	v0, PT_R0(sp)		# set flag for syscall restarting
+	sd	t1, PT_R0(sp)		# save it for syscall restarting
 1:	sd	v0, PT_R2(sp)		# result
 
 	j	syscall_exit
@@ -155,7 +157,6 @@ trace_a_syscall:
 	 */
 bad_stack:
 	dnegu	v0			# error
-	sd	v0, PT_R0(sp)
 	sd	v0, PT_R2(sp)
 	li	t0, 1			# set error flag
 	sd	t0, PT_R7(sp)

arch/mips/kernel/signal.c

@@ -550,23 +550,26 @@ static int handle_signal(unsigned long sig, siginfo_t *info,
 	struct mips_abi *abi = current->thread.abi;
 	void *vdso = current->mm->context.vdso;
 
-	switch(regs->regs[0]) {
-	case ERESTART_RESTARTBLOCK:
-	case ERESTARTNOHAND:
-		regs->regs[2] = EINTR;
-		break;
-	case ERESTARTSYS:
-		if (!(ka->sa.sa_flags & SA_RESTART)) {
+	if (regs->regs[0]) {
+		switch(regs->regs[2]) {
+		case ERESTART_RESTARTBLOCK:
+		case ERESTARTNOHAND:
 			regs->regs[2] = EINTR;
 			break;
+		case ERESTARTSYS:
+			if (!(ka->sa.sa_flags & SA_RESTART)) {
+				regs->regs[2] = EINTR;
+				break;
+			}
+		/* fallthrough */
+		case ERESTARTNOINTR:
+			regs->regs[7] = regs->regs[26];
+			regs->regs[2] = regs->regs[0];
+			regs->cp0_epc -= 4;
 		}
-	/* fallthrough */
-	case ERESTARTNOINTR:		/* Userland will reload $v0.  */
-		regs->regs[7] = regs->regs[26];
-		regs->cp0_epc -= 8;
-	}
 
-	regs->regs[0] = 0;		/* Don't deal with this again.  */
+		regs->regs[0] = 0;		/* Don't deal with this again.  */
+	}
 
 	if (sig_uses_siginfo(ka))
 		ret = abi->setup_rt_frame(vdso + abi->rt_signal_return_offset,
@@ -625,17 +628,13 @@ static void do_signal(struct pt_regs *regs)
 		return;
 	}
 
-	/*
-	 * Who's code doesn't conform to the restartable syscall convention
-	 * dies here!!!  The li instruction, a single machine instruction,
-	 * must directly be followed by the syscall instruction.
-	 */
 	if (regs->regs[0]) {
 		if (regs->regs[2] == ERESTARTNOHAND ||
 		    regs->regs[2] == ERESTARTSYS ||
 		    regs->regs[2] == ERESTARTNOINTR) {
+			regs->regs[2] = regs->regs[0];
 			regs->regs[7] = regs->regs[26];
-			regs->cp0_epc -= 8;
+			regs->cp0_epc -= 4;
 		}
 		if (regs->regs[2] == ERESTART_RESTARTBLOCK) {
 			regs->regs[2] = current->thread.abi->restart;

arch/mips/kernel/unaligned.c

@@ -109,8 +109,6 @@ static void emulate_load_store_insn(struct pt_regs *regs,
 	unsigned long value;
 	unsigned int res;
 
-	regs->regs[0] = 0;
-
 	/*
 	 * This load never faults.
 	 */