[PATCH v4 bpf-next 0/5] bpf path iterator

[Song Liu]

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

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2.47.1