ARM: 8087/1: ptrace: reload syscall number after secure_computing() check

On the syscall tracing path, we call out to secure_computing() to allow
seccomp to check the syscall number being attempted. As part of this, a
SIGTRAP may be sent to the tracer and the syscall could be re-written by
a subsequent SET_SYSCALL ptrace request. Unfortunately, this new syscall
is ignored by the current code unless TIF_SYSCALL_TRACE is also set on
the current thread.

This patch slightly reworks the enter path of the syscall tracing code
so that we always reload the syscall number from
current_thread_info()->syscall after the potential ptrace traps.

Acked-by: Kees Cook <>
Tested-by: Kees Cook <>
Signed-off-by: Will Deacon <>
Signed-off-by: Russell King <>
diff --git a/arch/arm/kernel/ptrace.c b/arch/arm/kernel/ptrace.c
index 03deeff..394424b 100644
--- a/arch/arm/kernel/ptrace.c
+++ b/arch/arm/kernel/ptrace.c
@@ -916,7 +916,7 @@
-static int tracehook_report_syscall(struct pt_regs *regs,
+static void tracehook_report_syscall(struct pt_regs *regs,
 				    enum ptrace_syscall_dir dir)
 	unsigned long ip;
@@ -934,7 +934,6 @@
 		current_thread_info()->syscall = -1;
 	regs->ARM_ip = ip;
-	return current_thread_info()->syscall;
 asmlinkage int syscall_trace_enter(struct pt_regs *regs, int scno)
@@ -946,7 +945,9 @@
 		return -1;
 	if (test_thread_flag(TIF_SYSCALL_TRACE))
-		scno = tracehook_report_syscall(regs, PTRACE_SYSCALL_ENTER);
+		tracehook_report_syscall(regs, PTRACE_SYSCALL_ENTER);
+	scno = current_thread_info()->syscall;
 	if (test_thread_flag(TIF_SYSCALL_TRACEPOINT))
 		trace_sys_enter(regs, scno);