MIPS: Fix restart of indirect syscalls

When 32-bit MIPS userspace invokes a syscall indirectly via syscall(number,
arg1, ..., arg7), the kernel looks up the actual syscall based on the given
number, shifts the other arguments to the left, and jumps to the syscall.

If the syscall is interrupted by a signal and indicates it needs to be
restarted by the kernel (by returning ERESTARTNOINTR for example), the
syscall must be called directly, since the number is no longer the first
argument, and the other arguments are now staged for a direct call.

Before shifting the arguments, store the syscall number in pt_regs->regs[2].
This gets copied temporarily into pt_regs->regs[0] after the syscall returns.
If the syscall needs to be restarted, handle_signal()/do_signal() copies the
number back to pt_regs->reg[2], which ends up in $v0 once control returns to
userspace.

Signed-off-by: Ed Swierk <eswierk@skyportsystems.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/8929/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/scall32-o32.S

@@ -181,6 +181,7 @@ 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

arch/mips/kernel/scall64-o32.S

@@ -186,6 +186,7 @@ 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