[PATCH v2 4/9] selftests/harness: Fix interleaved scheduling leading to race conditions

[Mickaël Salaün]

Fix a race condition when running several FIXTURE_TEARDOWN() managing
the same resource.  This fixes a race condition in the Landlock file
system tests when creating or unmounting the same directory.

Using clone3() with CLONE_VFORK guarantees that the child and grandchild
test processes are sequentially scheduled.  This is implemented with a
new clone3_vfork() helper replacing the fork() call.

This avoids triggering this error in __wait_for_test():
  Test ended in some other way [127]

Cc: Christian Brauner <brau...@kernel.org>
Cc: David S. Miller <da...@davemloft.net>
Cc: Günther Noack <gno...@google.com>
Cc: Jakub Kicinski <k...@kernel.org>
Cc: Kees Cook <keesc...@chromium.org>
Cc: Mark Brown <broo...@kernel.org>
Cc: Shuah Khan <sh...@kernel.org>
Cc: Will Drewry <w...@chromium.org>
Fixes: 41cca0542d7c ("selftests/harness: Fix TEST_F()'s vfork handling")
Signed-off-by: Mickaël Salaün <m...@digikod.net>
Link: https://lore.kernel.org/r/20240429130931.2394118-5-...@digikod.net
---
 tools/testing/selftests/kselftest_harness.h | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/kselftest_harness.h 
b/tools/testing/selftests/kselftest_harness.h
index 55699a762c45..9f04638707ae 100644
--- a/tools/testing/selftests/kselftest_harness.h
+++ b/tools/testing/selftests/kselftest_harness.h
@@ -66,6 +66,8 @@
 #include <sys/wait.h>
 #include <unistd.h>
 #include <setjmp.h>
+#include <syscall.h>
+#include <linux/sched.h>
 
 #include "kselftest.h"
 
@@ -80,6 +82,17 @@
 #  define TH_LOG_ENABLED 1
 #endif
 
+/* Wait for the child process to end but without sharing memory mapping. */
+static pid_t __attribute__((__unused__)) clone3_vfork(void)
+{
+       struct clone_args args = {
+               .flags = CLONE_VFORK,
+               .exit_signal = SIGCHLD,
+       };
+
+       return syscall(__NR_clone3, &args, sizeof(args));
+}
+
 /**
  * TH_LOG()
  *
@@ -1183,7 +1196,7 @@ void __run_test(struct __fixture_metadata *f,
        fflush(stdout);
        fflush(stderr);
 
-       t->pid = fork();
+       t->pid = clone3_vfork();
        if (t->pid < 0) {
                ksft_print_msg("ERROR SPAWNING TEST CHILD\n");
                t->exit_code = KSFT_FAIL;
-- 
2.44.0