From: Günther Noack Date: Wed, 1 Apr 2026 15:09:10 +0000 (+0200) Subject: landlock: Document fallocate(2) as another truncation corner case X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=3457a5ccacd34fdd5ebd3a4745e721b5a1239690;p=thirdparty%2Fkernel%2Flinux.git 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 Link: https://lore.kernel.org/r/20260401150911.1038072-1-gnoack@google.com Signed-off-by: Mickaël Salaün --- diff --git a/Documentation/userspace-api/landlock.rst b/Documentation/userspace-api/landlock.rst index b83665ff4b55f..fd8b78c31f2fe 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