x86: ptrace vs -ENOSYS

When we're stopped at syscall entry tracing, ptrace can change the %rax
value from -ENOSYS to something else.  If no system call is actually made
because the syscall number (now in orig_rax) is bad, then we now always
reset %rax to -ENOSYS again.

This changes it to leave the return value alone after entry tracing.
That way, the %rax value set by ptrace is there to be seen in user mode
(or in syscall exit tracing).  This is consistent with what the 32-bit
kernel does.
Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/entry_64.S

@@ -319,19 +319,17 @@ badsys:
 	/* Do syscall tracing */
 tracesys:			 
 	SAVE_REST
-	movq $-ENOSYS,RAX(%rsp)
+	movq $-ENOSYS,RAX(%rsp) /* ptrace can change this for a bad syscall */
 	FIXUP_TOP_OF_STACK %rdi
 	movq %rsp,%rdi
 	call syscall_trace_enter
 	LOAD_ARGS ARGOFFSET  /* reload args from stack in case ptrace changed it */
 	RESTORE_REST
 	cmpq $__NR_syscall_max,%rax
-	movq $-ENOSYS,%rcx
-	cmova %rcx,%rax
-	ja  1f
+	ja   int_ret_from_sys_call	/* RAX(%rsp) set to -ENOSYS above */
 	movq %r10,%rcx	/* fixup for C */
 	call *sys_call_table(,%rax,8)
-1:	movq %rax,RAX-ARGOFFSET(%rsp)
+	movq %rax,RAX-ARGOFFSET(%rsp)
 	/* Use IRET because user could have changed frame */
 		
 /*