linux-kernel - [PATCH v6 04/10] selftests/harness: Fix interleaved scheduling leading to race conditions

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Date: Mon,  6 May 2024 18:55:12 +0200
From: Mickaël Salaün <mic@...ikod.net>
To: Christian Brauner <brauner@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jakub Kicinski <kuba@...nel.org>,
	Kees Cook <keescook@...omium.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Mark Brown <broonie@...nel.org>,
	Sasha Levin <sashal@...nel.org>,
	Sean Christopherson <seanjc@...gle.com>,
	Shengyu Li <shengyu.li.evgeny@...il.com>,
	Shuah Khan <shuah@...nel.org>,
	Shuah Khan <skhan@...uxfoundation.org>
Cc: Mickaël Salaün <mic@...ikod.net>,
	Bagas Sanjaya <bagasdotme@...il.com>,
	Brendan Higgins <brendanhiggins@...gle.com>,
	David Gow <davidgow@...gle.com>,
	"David S . Miller" <davem@...emloft.net>,
	Florian Fainelli <florian.fainelli@...adcom.com>,
	Günther Noack <gnoack@...gle.com>,
	Jon Hunter <jonathanh@...dia.com>,
	Ron Economos <re@...z.net>,
	Ronald Warsow <rwarsow@....de>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	Will Drewry <wad@...omium.org>,
	kernel test robot <oliver.sang@...el.com>,
	kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	linux-kselftest@...r.kernel.org,
	netdev@...r.kernel.org,
	stable@...r.kernel.org
Subject: [PATCH v6 04/10] selftests/harness: Fix interleaved scheduling leading to race conditions

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 <brauner@...nel.org>
Cc: David S. Miller <davem@...emloft.net>
Cc: Günther Noack <gnoack@...gle.com>
Cc: Jakub Kicinski <kuba@...nel.org>
Cc: Mark Brown <broonie@...nel.org>
Cc: Shuah Khan <shuah@...nel.org>
Cc: Will Drewry <wad@...omium.org>
Fixes: 41cca0542d7c ("selftests/harness: Fix TEST_F()'s vfork handling")
Reviewed-by: Kees Cook <keescook@...omium.org>
Signed-off-by: Mickaël Salaün <mic@...ikod.net>
Link: https://lore.kernel.org/r/20240506165518.474504-5-mic@digikod.net
---

Changes since v2:
* Replace __attribute__((__unused__)) with inline for clone3_vfork()
  (suggested by Kees and Jakub)
---
 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..9d7178a71c2c 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 inline pid_t 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;
-- 
2.45.0