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landlock: Document fallocate(2) as another truncation corner case

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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>
This commit is contained in:
Günther Noack
2026-04-01 17:09:10 +02:00
committed by Mickaël Salaün
parent d1b2ab221d
commit 3457a5ccac
1 changed files with 6 additions and 2 deletions
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8
Documentation/userspace-api/landlock.rst
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@@ -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