powerpc: Use orig_gpr3 in syscall_get_arguments()

Currently syscall_get_arguments() is used by syscall tracepoints, and
collect_syscall() which is used in some debugging as well as
/proc/pid/syscall.

The current implementation just copies regs->gpr[3 .. 5] out, which is
fine for all the current use cases.

When we enable seccomp filter, that will also start using
syscall_get_arguments(). However for seccomp filter we want to use r3
as the return value of the syscall, and orig_gpr3 as the first
parameter. This will allow seccomp to modify the return value in r3.

To support this we need to modify syscall_get_arguments() to return
orig_gpr3 instead of r3. This is safe for all uses because orig_gpr3
always contains the r3 value that was passed to the syscall. We store it
in the syscall entry path and never modify it.

Update syscall_set_arguments() while we're here, even though it's never
used.

Signed-off-by: Michael Ellerman <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>

arch/powerpc/include/asm/syscall.h

···
 					 unsigned int i, unsigned int n,
 					 unsigned long *args)
 {
-	unsigned long mask = -1UL;
+	unsigned long val, mask = -1UL;
 
 	BUG_ON(i + n > 6);
 
···
 	if (test_tsk_thread_flag(task, TIF_32BIT))
 		mask = 0xffffffff;
 #endif
-	while (n--)
-		args[n] = regs->gpr[3 + i + n] & mask;
+	while (n--) {
+		if (n == 0 && i == 0)
+			val = regs->orig_gpr3;
+		else
+			val = regs->gpr[3 + i + n];
+
+		args[n] = val & mask;
+	}
 }
 
 static inline void syscall_set_arguments(struct task_struct *task,
···
 {
 	BUG_ON(i + n > 6);
 	memcpy(&regs->gpr[3 + i], args, n * sizeof(args[0]));
+
+	/* Also copy the first argument into orig_gpr3 */
+	if (i == 0 && n > 0)
+		regs->orig_gpr3 = args[0];
 }
 
 static inline int syscall_get_arch(void)