[PATCH bpf-next v1 06/11] selftests/bpf: exercise the path ancestor iterator

[Justin Suess <utilityemal77@gmail.com>]

Walk the ancestry of a mkdir's parent directory from a sleepable LSM
program and check that the iteration reaches the root: the temporary
directory, its parent and / are visited, no position comes back
flagged, and the flags read back after the loop show it ended at the
root rather than on a failed allocation.

Hold the first position across the rest of the walk and resolve its
pathname only once the loop is done and the iterator destroyed: the
acquired reference, not the walk's, is then all that keeps the
position alive.

Signed-off-by: Justin Suess <utilityemal77@gmail.com>
---
 .../selftests/bpf/prog_tests/path_ancestors.c | 57 ++++++++++++++
 .../selftests/bpf/progs/path_ancestors.c      | 74 +++++++++++++++++++
 2 files changed, 131 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/path_ancestors.c
 create mode 100644 tools/testing/selftests/bpf/progs/path_ancestors.c

diff --git a/tools/testing/selftests/bpf/prog_tests/path_ancestors.c b/tools/testing/selftests/bpf/prog_tests/path_ancestors.c
new file mode 100644
index 000000000000..d1bd26bce10b
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/path_ancestors.c
@@ -0,0 +1,57 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2026 Justin Suess */
+
+#include <sys/stat.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <test_progs.h>
+#include "path_ancestors.skel.h"
+
+void test_path_ancestors(void)
+{
+	char base[] = "/tmp/path_ancestors_XXXXXX";
+	struct path_ancestors *skel = NULL;
+	char suba[280], subb[280];
+
+	if (!ASSERT_OK_PTR(mkdtemp(base), "mkdtemp"))
+		return;
+	snprintf(suba, sizeof(suba), "%s/a", base);
+	snprintf(subb, sizeof(subb), "%s/a/b", base);
+	if (!ASSERT_OK(mkdir(suba, 0755), "mkdir_a"))
+		goto out_rm;
+
+	skel = path_ancestors__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "open_and_load"))
+		goto out_rm;
+	skel->bss->monitored_pid = getpid();
+	if (!ASSERT_OK(path_ancestors__attach(skel), "attach"))
+		goto out;
+
+	/* mkdir b: the hook sees dir == suba, whose ancestry is walked. */
+	if (!ASSERT_OK(mkdir(subb, 0755), "mkdir_b"))
+		goto out;
+
+	/* suba, base, /tmp, / at least. */
+	ASSERT_GE(skel->bss->ref_count, 4, "ref_count");
+	ASSERT_EQ(skel->bss->ref_flags, 0, "ref_flags");
+	/*
+	 * The referenced loop ended at the root, not on an allocation
+	 * failure.
+	 */
+	ASSERT_EQ(skel->bss->end_flags, 0, "end_flags");
+
+	/* The acquired second position, used after its step was taken. */
+	ASSERT_STREQ(skel->bss->second_path, base, "second_path");
+	ASSERT_EQ(skel->bss->second_len, strlen(base) + 1, "second_len");
+
+	/* The first position, used only after the walk itself was gone. */
+	ASSERT_STREQ(skel->bss->first_path, suba, "first_path");
+	ASSERT_EQ(skel->bss->first_len, strlen(suba) + 1, "first_len");
+
+out:
+	path_ancestors__destroy(skel);
+out_rm:
+	rmdir(subb);
+	rmdir(suba);
+	rmdir(base);
+}
diff --git a/tools/testing/selftests/bpf/progs/path_ancestors.c b/tools/testing/selftests/bpf/progs/path_ancestors.c
new file mode 100644
index 000000000000..6287f5099a18
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/path_ancestors.c
@@ -0,0 +1,74 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2026 Justin Suess */
+
+#include "vmlinux.h"
+#include <errno.h>
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+#include "bpf_kfuncs.h"
+
+char _license[] SEC("license") = "GPL";
+
+__u32 monitored_pid;
+
+int ref_count;		/* positions seen by the pure referenced walk */
+int ref_flags;		/* pos flags seen by the referenced walk */
+int end_flags;		/* flags after a referenced loop ended: 0 unless NOMEM */
+int second_len;		/* d_path length of the walk's second position */
+int first_len;		/* d_path length of the first position, after the walk */
+char second_path[256];
+char first_path[256];
+
+static bool monitored(void)
+{
+	return (bpf_get_current_pid_tgid() >> 32) == monitored_pid;
+}
+
+SEC("lsm.s/path_mkdir")
+int BPF_PROG(walk_modes, const struct path *dir, struct dentry *dentry,
+	     umode_t mode)
+{
+	struct bpf_iter_path_ancestors it;
+	struct path *pos, *first = NULL;
+
+	/*
+	 * Keep first's init opaque: gcc-bpf otherwise folds it into a
+	 * preceding loop's exit value, which the verifier cannot track.
+	 */
+	barrier_var(first);
+
+	if (!monitored())
+		return 0;
+
+	/*
+	 * Referenced walk: every position comes acquired, so it stays valid
+	 * past the step that yielded it - the first one is held across the
+	 * rest of the walk and used only once the walk is gone.
+	 */
+	bpf_iter_path_ancestors_new(&it, (struct path *)dir, 0);
+	while ((pos = bpf_iter_path_ancestors_next(&it))) {
+		ref_count++;
+		ref_flags |= bpf_path_ancestors_pos_flags(&it);
+		if (!first) {
+			first = pos;
+			continue;
+		}
+		if (ref_count == 2)
+			second_len = bpf_path_d_path(pos, second_path,
+						     sizeof(second_path));
+		bpf_path_put(pos);
+	}
+	/* NULL from an allocation failure, not the root, surfaces here. */
+	end_flags |= bpf_path_ancestors_pos_flags(&it);
+	bpf_iter_path_ancestors_destroy(&it);
+	/*
+	 * The walk's own references moved on and the iterator is destroyed:
+	 * the acquired reference is all that keeps the first position alive.
+	 */
+	if (first) {
+		first_len = bpf_path_d_path(first, first_path,
+					    sizeof(first_path));
+		bpf_path_put(first);
+	}
+	return 0;
+}
-- 
2.55.0