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 <keescook@chromium.org>
Tested-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/kernel/ptrace.c

@@ -908,7 +908,7 @@ enum ptrace_syscall_dir {
 	PTRACE_SYSCALL_EXIT,
 };
 
-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;
@@ -926,7 +926,6 @@ static int tracehook_report_syscall(struct pt_regs *regs,
 		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)
@@ -938,7 +937,9 @@ asmlinkage int syscall_trace_enter(struct pt_regs *regs, int scno)
 		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);