In security use cases, it is common to apply rules to VFS subtrees. However, filtering files in a subtree is not straightforward [1]. One solution to this problem is to start from a path and walk up the VFS tree (towards the root). Among in-tree LSMs, Landlock uses this solution. BPF LSM solutions, such like Tetragon [2], also use similar approaches. However, due to lack of proper helper/kfunc support, BPF LSM solutions usually do the path walk with probe read, which is racy. This patchset introduces a new helper path_walk_parent, which walks path to its VFS parent. The helper is used in Landlock. A new BPF iterator, path iterator, is introduced to do the path walking. The BPF path iterator uses the new path_walk_parent help to walk the VFS tree. Changes v3 => v4: 1. Rewrite path_walk_parent based on suggestion from Neil Brown. 2. When path_walk_parent() returns false, it call path_put on @path and then zeros @path. v3: https://lore.kernel.org/bpf/20250606213015.255134-1-song@xxxxxxxxxx/ Changes v2 => v3: 1. Fix an issue with path_walk_parent. 2. Move bpf path iterator to fs/bpf_fs_kfuncs.c 3. Optimize bpf path iterator (less memory). 4. Add more selftests. 5. Add more comments. v2: https://lore.kernel.org/bpf/20250603065920.3404510-1-song@xxxxxxxxxx/ Changes v1 => v2: 1. Rename path_parent => path_walk_parent. 2. Remove path_connected check in path_walk_parent. 3. Fix is_access_to_paths_allowed(). 4. Remove mode for path iterator, add a flag instead. v1: https://lore.kernel.org/bpf/20250528222623.1373000-1-song@xxxxxxxxxx/ [1] https://lpc.events/event/18/contributions/1940/ [2] https://github.com/cilium/tetragon/ Song Liu (5): namei: Introduce new helper function path_walk_parent() landlock: Use path_walk_parent() bpf: Introduce path iterator selftests/bpf: Add tests for bpf path iterator selftests/bpf: Path walk test fs/bpf_fs_kfuncs.c | 72 +++++++++ fs/namei.c | 99 ++++++++++-- include/linux/namei.h | 2 + kernel/bpf/verifier.c | 5 + security/landlock/fs.c | 28 +--- .../testing/selftests/bpf/bpf_experimental.h | 6 + .../selftests/bpf/prog_tests/path_iter.c | 111 ++++++++++++++ tools/testing/selftests/bpf/progs/path_iter.c | 145 ++++++++++++++++++ tools/testing/selftests/bpf/progs/path_walk.c | 59 +++++++ 9 files changed, 493 insertions(+), 34 deletions(-) create mode 100644 tools/testing/selftests/bpf/prog_tests/path_iter.c create mode 100644 tools/testing/selftests/bpf/progs/path_iter.c create mode 100644 tools/testing/selftests/bpf/progs/path_walk.c -- 2.47.1
