MIPS: Remove pt_regs adjustments in indirect syscall handler

If a restartable syscall is called using the indirect o32 syscall
handler - eg: syscall(__NR_waitid, ...), then it is possible for the
incorrect arguments to be passed to the syscall after it has been
restarted. This is because the syscall handler tries to shift all the
registers down one place in pt_regs so that when the syscall is restarted,
the "real" syscall is called instead. Unfortunately it only shifts the
arguments passed in registers, not the arguments on the user stack. This
causes the 4th argument to be duplicated when the syscall is restarted.

Fix by removing all the pt_regs shifting so that the indirect syscall
handler is called again when the syscall is restarted. The comment "some
syscalls like execve get their arguments from struct pt_regs" is long
out of date so this should now be safe.

Signed-off-by: James Cowgill <James.Cowgill@imgtec.com>
Reviewed-by: James Hogan <james.hogan@imgtec.com>
Tested-by: James Hogan <james.hogan@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/15856/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

Author: jcowgill

arch/mips/kernel/scall32-o32.S

@@ -190,12 +190,6 @@ illegal_syscall:
 	sll	t1, t0, 2
 	beqz	v0, einval
 	lw	t2, sys_call_table(t1)		# syscall routine
-	sw	a0, PT_R2(sp)			# call routine directly on restart
-
-	/* Some syscalls like execve get their arguments from struct pt_regs
-	   and claim zero arguments in the syscall table. Thus we have to
-	   assume the worst case and shuffle around all potential arguments.
-	   If you want performance, don't use indirect syscalls. */
 
 	move	a0, a1				# shift argument registers
 	move	a1, a2
@@ -207,11 +201,6 @@ illegal_syscall:
 	sw	t4, 16(sp)
 	sw	t5, 20(sp)
 	sw	t6, 24(sp)
-	sw	a0, PT_R4(sp)			# .. and push back a0 - a3, some
-	sw	a1, PT_R5(sp)			# syscalls expect them there
-	sw	a2, PT_R6(sp)
-	sw	a3, PT_R7(sp)
-	sw	a3, PT_R26(sp)			# update a3 for syscall restarting
 	jr	t2
 	/* Unreached */
 

arch/mips/kernel/scall64-o32.S

@@ -198,7 +198,6 @@ LEAF(sys32_syscall)
 	dsll	t1, t0, 3
 	beqz	v0, einval
 	ld	t2, sys32_call_table(t1)		# syscall routine
-	sd	a0, PT_R2(sp)		# call routine directly on restart
 
 	move	a0, a1			# shift argument registers
 	move	a1, a2
@@ -207,11 +206,6 @@ LEAF(sys32_syscall)
 	move	a4, a5
 	move	a5, a6
 	move	a6, a7
-	sd	a0, PT_R4(sp)		# ... and push back a0 - a3, some
-	sd	a1, PT_R5(sp)		# syscalls expect them there
-	sd	a2, PT_R6(sp)
-	sd	a3, PT_R7(sp)
-	sd	a3, PT_R26(sp)		# update a3 for syscall restarting
 	jr	t2
 	/* Unreached */