[PATCH v2 1/4] selftests/seccomp: Record syscall during ptrace entry

[Kees Cook]

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 <cascardo at canonical.com>
Link: https://lore.kernel.org/linux-kselftest/20200911181012.171027-1-cascardo@canonical.com/
Signed-off-by: Kees Cook <keescook at chromium.org>
---
 tools/testing/selftests/seccomp/seccomp_bpf.c | 40 +++++++++++++------
 1 file changed, 27 insertions(+), 13 deletions(-)

diff --git a/tools/testing/selftests/seccomp/seccomp_bpf.c b/tools/testing/selftests/seccomp/seccomp_bpf.c
index bc0fb463c709..c0311b4c736b 100644
--- a/tools/testing/selftests/seccomp/seccomp_bpf.c
+++ b/tools/testing/selftests/seccomp/seccomp_bpf.c
@@ -1949,12 +1949,19 @@ void tracer_seccomp(struct __test_metadata *_metadata, pid_t tracee,
 
 }
 
+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
@@ -1968,24 +1975,31 @@ void tracer_ptrace(struct __test_metadata *_metadata, pid_t tracee,
 	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) {
 	/*
 	 * All of the SECCOMP_RET_TRACE behaviors can be tested with either
@@ -2044,7 +2058,7 @@ FIXTURE_SETUP(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);
-- 
2.25.1