selftests/seccomp: fix syscall_restart test for arm compat

The inconsistencies in the systcall ABI between arm and arm-compat can
can cause a failure in the syscall_restart test due to the logic
attempting to work around the differences. The 'machine' field for an
ARM64 device running in compat mode can report 'armv8l' or 'armv8b'
which matches with the string 'arm' when only examining the first three
characters of the string.

This change adds additional validation to the workaround logic to make
sure we only take the arm path when running natively, not in arm-compat.

Fixes: 256d0af ("selftests/seccomp: build and pass on arm64")
Signed-off-by: Neill Kapron <nkapron@google.com>
Link: https://lore.kernel.org/r/20250427094103.3488304-2-nkapron@google.com
Signed-off-by: Kees Cook <kees@kernel.org>

Author: nkapron

tools/testing/selftests/seccomp/seccomp_bpf.c

@@ -3166,12 +3166,15 @@ TEST(syscall_restart)
 	ret = get_syscall(_metadata, child_pid);
 #if defined(__arm__)
 	/*
-	 * FIXME:
 	 * - native ARM registers do NOT expose true syscall.
 	 * - compat ARM registers on ARM64 DO expose true syscall.
+	 * - values of utsbuf.machine include 'armv8l' or 'armb8b'
+	 *   for ARM64 running in compat mode.
 	 */
 	ASSERT_EQ(0, uname(&utsbuf));
-	if (strncmp(utsbuf.machine, "arm", 3) == 0) {
+	if ((strncmp(utsbuf.machine, "arm", 3) == 0) &&
+	    (strncmp(utsbuf.machine, "armv8l", 6) != 0) &&
+	    (strncmp(utsbuf.machine, "armv8b", 6) != 0)) {
 		EXPECT_EQ(__NR_nanosleep, ret);
 	} else
 #endif