[PATCH v2] powerpc/ptrace: Do not return ENOSYS if invalid syscall

[Thadeu Lima de Souza Cascardo <cascardo@canonical.com>]

If a tracer sets the syscall number to an invalid one, allow the return
value set by the tracer to be returned the tracee.

The test for NR_syscalls is already at entry_64.S, and it's at
do_syscall_trace_enter only to skip audit and trace.

After this, two failures from seccomp_bpf selftests complete just fine,
as the failing test was using ptrace to change the syscall to return an
error or a fake value, but were failing as it was always returning
-ENOSYS.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
---
 arch/powerpc/kernel/ptrace.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/powerpc/kernel/ptrace.c b/arch/powerpc/kernel/ptrace.c
index 25c0424e8868..557ae4bc2331 100644
--- a/arch/powerpc/kernel/ptrace.c
+++ b/arch/powerpc/kernel/ptrace.c
@@ -3314,7 +3314,7 @@ long do_syscall_trace_enter(struct pt_regs *regs)
 
 	/* Avoid trace and audit when syscall is invalid. */
 	if (regs->gpr[0] >= NR_syscalls)
-		goto skip;
+		return regs->gpr[0];
 
 	if (unlikely(test_thread_flag(TIF_SYSCALL_TRACEPOINT)))
 		trace_sys_enter(regs, regs->gpr[0]);
-- 
2.17.1