[PATCH RESEND] selftests/seccomp: fix syscall_restart test for arm compat

[PATCH RESEND] selftests/seccomp: fix syscall_restart test for arm compat

[Neill Kapron]

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: 256d0afb11d6 ("selftests/seccomp: build and pass on arm64")
Signed-off-by: Neill Kapron <nkapron@google.com>
---
 tools/testing/selftests/seccomp/seccomp_bpf.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/seccomp/seccomp_bpf.c b/tools/testing/selftests/seccomp/seccomp_bpf.c
index b2f76a52215a..53bf6a9c801f 100644
--- a/tools/testing/selftests/seccomp/seccomp_bpf.c
+++ b/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
-- 
2.49.0.850.g28803427d3-goog

Re: [PATCH RESEND] selftests/seccomp: fix syscall_restart test for arm compat

[Kees Cook]

On Sun, 27 Apr 2025 09:40:58 +0000, Neill Kapron wrote:
> 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.
> 
> [...]

Applied to for-next/seccomp, thanks!

[1/1] selftests/seccomp: fix syscall_restart test for arm compat
      https://git.kernel.org/kees/c/797002deed03

Take care,

-- 
Kees Cook