powerpc: Don't negate error in syscall_set_return_value()

Currently the only caller of syscall_set_return_value() is seccomp
filter, which is not enabled on powerpc.

This means we have not noticed that our implementation of
syscall_set_return_value() negates error, even though the value passed
in is already negative.

So remove the negation in syscall_set_return_value(), and expect the
caller to do it like all other implementations do.

Also add a comment about the ccr handling.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Kees Cook <keescook@chromium.org>

arch/powerpc/include/asm/syscall.h

@@ -44,9 +44,15 @@ static inline void syscall_set_return_value(struct task_struct *task,
 					    struct pt_regs *regs,
 					    int error, long val)
 {
+	/*
+	 * In the general case it's not obvious that we must deal with CCR
+	 * here, as the syscall exit path will also do that for us. However
+	 * there are some places, eg. the signal code, which check ccr to
+	 * decide if the value in r3 is actually an error.
+	 */
 	if (error) {
 		regs->ccr |= 0x10000000L;
-		regs->gpr[3] = -error;
+		regs->gpr[3] = error;
 	} else {
 		regs->ccr &= ~0x10000000L;
 		regs->gpr[3] = val;