[PATCH bpf-next 2/2] selftests/bpf: test bpf_path_d_path() on a file released at task exit

[Matt Bobrowski <matt@bobrowski.net>]

Add a fs_kfuncs subtest for the crash fixed by the previous commit. A
child opens a file and exits without closing it, so exit_files() drops
the last reference and the file is released from exit_task_work(),
after exit_fs() has cleared the task's fs. A file_release program calls
bpf_path_d_path() on that release, matched by the file's inode number,
and records what it returned.

Without the fix the subtest crashes the kernel in d_path(). With it,
bpf_path_d_path() returns -EINVAL.

Fixes: d08e2045ebf0 ("bpf: introduce new VFS based BPF kfuncs")
Cc: stable@vger.kernel.org
Signed-off-by: Matt Bobrowski <matt@bobrowski.net>
---
 .../selftests/bpf/prog_tests/fs_kfuncs.c      | 51 +++++++++++++++++++
 .../selftests/bpf/progs/test_path_d_path.c    | 35 +++++++++++++
 2 files changed, 86 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/progs/test_path_d_path.c

diff --git a/tools/testing/selftests/bpf/prog_tests/fs_kfuncs.c b/tools/testing/selftests/bpf/prog_tests/fs_kfuncs.c
index 43a26ec69a8e..510cf11c8bd8 100644
--- a/tools/testing/selftests/bpf/prog_tests/fs_kfuncs.c
+++ b/tools/testing/selftests/bpf/prog_tests/fs_kfuncs.c
@@ -4,12 +4,15 @@
 #include <stdlib.h>
 #include <sys/types.h>
 #include <sys/xattr.h>
+#include <sys/stat.h>
+#include <sys/wait.h>
 #include <linux/fsverity.h>
 #include <unistd.h>
 #include <test_progs.h>
 #include "test_get_xattr.skel.h"
 #include "test_set_remove_xattr.skel.h"
 #include "test_fsverity.skel.h"
+#include "test_path_d_path.skel.h"
 
 static const char testfile[] = "/tmp/test_progs_fs_kfuncs";
 
@@ -268,6 +271,51 @@ static void test_fsverity(void)
 	remove(testfile);
 }
 
+static void test_path_d_path_on_exit(void)
+{
+	struct test_path_d_path *skel = NULL;
+	struct stat st;
+	int fd, status;
+	pid_t pid;
+
+	fd = open(testfile, O_CREAT | O_RDONLY, 0644);
+	if (!ASSERT_GE(fd, 0, "create_file"))
+		return;
+	close(fd);
+
+	if (!ASSERT_OK(stat(testfile, &st), "stat"))
+		goto out;
+
+	skel = test_path_d_path__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "test_path_d_path__open_and_load"))
+		goto out;
+
+	skel->bss->monitored_ino = st.st_ino;
+
+	if (!ASSERT_OK(test_path_d_path__attach(skel), "test_path_d_path__attach"))
+		goto out;
+
+	/*
+	 * The child exits with testfile still open, so exit_files() drops
+	 * the last reference and the file is released from exit_task_work(),
+	 * after exit_fs().
+	 */
+	pid = fork();
+	if (!ASSERT_GE(pid, 0, "fork"))
+		goto out;
+	if (pid == 0)
+		_exit(open(testfile, O_RDONLY) < 0);
+
+	if (!ASSERT_EQ(waitpid(pid, &status, 0), pid, "waitpid"))
+		goto out;
+	ASSERT_TRUE(WIFEXITED(status) && !WEXITSTATUS(status), "child_open");
+	ASSERT_EQ(skel->bss->called, 1, "called");
+	ASSERT_EQ(skel->bss->ret, -EINVAL, "bpf_path_d_path");
+out:
+	test_path_d_path__destroy(skel);
+	remove(testfile);
+}
+
 void test_fs_kfuncs(void)
 {
 	/* Matches xattr_names in progs/test_get_xattr.c */
@@ -288,4 +336,7 @@ void test_fs_kfuncs(void)
 
 	if (test__start_subtest("fsverity"))
 		test_fsverity();
+
+	if (test__start_subtest("path_d_path_on_exit"))
+		test_path_d_path_on_exit();
 }
diff --git a/tools/testing/selftests/bpf/progs/test_path_d_path.c b/tools/testing/selftests/bpf/progs/test_path_d_path.c
new file mode 100644
index 000000000000..4ea50a067819
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/test_path_d_path.c
@@ -0,0 +1,35 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include "vmlinux.h"
+#include <errno.h>
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+#include "bpf_misc.h"
+#include "bpf_experimental.h"
+
+#define PF_EXITING 0x00000004
+
+char _license[] SEC("license") = "GPL";
+
+__u64 monitored_ino;
+int called;
+int ret;
+
+/*
+ * A task that exits with a file still open releases it from
+ * exit_task_work(), after exit_fs() has already cleared its fs.
+ */
+SEC("lsm/file_release")
+int BPF_PROG(path_d_path_on_exit, struct file *file)
+{
+	struct task_struct *task = bpf_get_current_task_btf();
+	char buf[64];
+
+	if (file->f_inode->i_ino != monitored_ino ||
+	    !(task->flags & PF_EXITING))
+		return 0;
+
+	ret = bpf_path_d_path(&file->f_path, buf, sizeof(buf));
+	called = 1;
+	return 0;
+}
-- 
2.47.3