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| author | Günther Noack <gnoack@google.com> | 2026-04-01 17:09:10 +0200 |
|---|---|---|
| committer | Mickaël Salaün <mic@digikod.net> | 2026-04-07 18:51:11 +0200 |
| commit | 3457a5ccacd34fdd5ebd3a4745e721b5a1239690 (patch) | |
| tree | 21edc37e6b8af438f9f9dc908824494182dcfcd5 /Documentation/userspace-api | |
| parent | d1b2ab221d37f32cf1a796fc0ba3b8f9fc8458fa (diff) | |
landlock: Document fallocate(2) as another truncation corner case
Reinforce the already stated policy that LANDLOCK_ACCESS_FS_TRUNCATE should
always go hand in hand with LANDLOCK_ACCESS_FS_WRITE_FILE, as their
meanings and enforcement overlap in counterintuitive ways.
On many common file systems, fallocate(2) offers a way to shorten files as
long as the file is opened for writing, side-stepping the
LANDLOCK_ACCESS_FS_TRUNCATE right.
Assisted-by: Gemini-CLI:gemini-3.1
Signed-off-by: Günther Noack <gnoack@google.com>
Link: https://lore.kernel.org/r/20260401150911.1038072-1-gnoack@google.com
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Diffstat (limited to 'Documentation/userspace-api')
| -rw-r--r-- | Documentation/userspace-api/landlock.rst | 8 |
1 files changed, 6 insertions, 2 deletions
diff --git a/Documentation/userspace-api/landlock.rst b/Documentation/userspace-api/landlock.rst index b83665ff4b55..fd8b78c31f2f 100644 --- a/Documentation/userspace-api/landlock.rst +++ b/Documentation/userspace-api/landlock.rst @@ -383,8 +383,8 @@ Truncating files The operations covered by ``LANDLOCK_ACCESS_FS_WRITE_FILE`` and ``LANDLOCK_ACCESS_FS_TRUNCATE`` both change the contents of a file and sometimes -overlap in non-intuitive ways. It is recommended to always specify both of -these together. +overlap in non-intuitive ways. It is strongly recommended to always specify +both of these together (either granting both, or granting none). A particularly surprising example is :manpage:`creat(2)`. The name suggests that this system call requires the rights to create and write files. However, @@ -396,6 +396,10 @@ It should also be noted that truncating files does not require the system call, this can also be done through :manpage:`open(2)` with the flags ``O_RDONLY | O_TRUNC``. +At the same time, on some filesystems, :manpage:`fallocate(2)` offers a way to +shorten file contents with ``FALLOC_FL_COLLAPSE_RANGE`` when the file is opened +for writing, sidestepping the ``LANDLOCK_ACCESS_FS_TRUNCATE`` right. + The truncate right is associated with the opened file (see below). Rights associated with file descriptors |