selftests/seccomp: Record syscall during ptrace entry

In preparation for performing actions during ptrace syscall exit, save
the syscall number during ptrace syscall entry. Some architectures do
no have the syscall number available during ptrace syscall exit.

Suggested-by: Thadeu Lima de Souza Cascardo <[e-mail]>
Link: https://lore.kernel.org/linux-kselftest/[e-mail]/
Acked-by: Christian Brauner <[e-mail]>
Link: https://lore.kernel.org/lkml/20200921074354.6shkt2e5yhzhj3sn@wittgenstein/
Signed-off-by: Kees Cook <[e-mail]>

tools/testing/selftests/seccomp/seccomp_bpf.c

···
 
 }
 
+FIXTURE(TRACE_syscall) {
+	struct sock_fprog prog;
+	pid_t tracer, mytid, mypid, parent;
+	long syscall_nr;
+};
+
 void tracer_ptrace(struct __test_metadata *_metadata, pid_t tracee,
 		   int status, void *args)
 {
-	int ret, nr;
+	int ret;
 	unsigned long msg;
 	static bool entry;
+	FIXTURE_DATA(TRACE_syscall) *self = args;
 
 	/*
 	 * The traditional way to tell PTRACE_SYSCALL entry/exit
···
 	EXPECT_EQ(entry ? PTRACE_EVENTMSG_SYSCALL_ENTRY
 			: PTRACE_EVENTMSG_SYSCALL_EXIT, msg);
 
-	if (!entry)
+	/*
+	 * Some architectures only support setting return values during
+	 * syscall exit under ptrace, and on exit the syscall number may
+	 * no longer be available. Therefore, save the initial sycall
+	 * number here, so it can be examined during both entry and exit
+	 * phases.
+	 */
+	if (entry)
+		self->syscall_nr = get_syscall(_metadata, tracee);
+	else
 		return;
 
-	nr = get_syscall(_metadata, tracee);
-
-	if (nr == __NR_getpid)
+	switch (self->syscall_nr) {
+	case __NR_getpid:
 		change_syscall(_metadata, tracee, __NR_getppid, 0);
-	if (nr == __NR_gettid)
+		break;
+	case __NR_gettid:
 		change_syscall(_metadata, tracee, -1, 45000);
-	if (nr == __NR_openat)
+		break;
+	case __NR_openat:
 		change_syscall(_metadata, tracee, -1, -ESRCH);
+		break;
+	}
 }
-
-FIXTURE(TRACE_syscall) {
-	struct sock_fprog prog;
-	pid_t tracer, mytid, mypid, parent;
-};
 
 FIXTURE_VARIANT(TRACE_syscall) {
 	/*
···
 	self->tracer = setup_trace_fixture(_metadata,
 					   variant->use_ptrace ? tracer_ptrace
 							       : tracer_seccomp,
-					   NULL, variant->use_ptrace);
+					   self, variant->use_ptrace);
 
 	ret = prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);
 	ASSERT_EQ(0, ret);