[PATCH] MIPS: ptrace: Fix syscall skipping via PTRACE_SYSCALL

[Thomas Bogendoerfer]

If tracer wanted to skip a syscall return value was always
overwritten with -ENOSYS. Fix this by checking against original
syscall number and only return -ENOSYS, if it is negative.

Fixes: b6318a903d06 ("MIPS/ptrace: Pick up ptrace/seccomp changed syscalls")
Signed-off-by: Thomas Bogendoerfer <tsbogend@xxxxxxxxxxxxxxxx>
---
 arch/mips/kernel/ptrace.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/mips/kernel/ptrace.c b/arch/mips/kernel/ptrace.c
index 3f4c94c88124..87102a03b6ea 100644
--- a/arch/mips/kernel/ptrace.c
+++ b/arch/mips/kernel/ptrace.c
@@ -1321,8 +1321,12 @@ long arch_ptrace(struct task_struct *child, long request,
  */
 asmlinkage long syscall_trace_enter(struct pt_regs *regs)
 {
+	long syscall;
+
 	user_exit();
 
+	syscall = current_thread_info()->syscall;
+
 	if (test_thread_flag(TIF_SYSCALL_TRACE)) {
 		if (ptrace_report_syscall_entry(regs))
 			return -1;
@@ -1342,7 +1346,7 @@ asmlinkage long syscall_trace_enter(struct pt_regs *regs)
 	 * Negative syscall numbers are mistaken for rejected syscalls, but
 	 * won't have had the return value set appropriately, so we do so now.
 	 */
-	if (current_thread_info()->syscall < 0)
+	if (syscall < 0)
 		syscall_set_return_value(current, regs, -ENOSYS, 0);
 	return current_thread_info()->syscall;
 }
-- 
2.51.0