Re: [PATCH RFC 0/3] landlock: Add access rights on created files

["Günther Noack" <gnoack3000@gmail.com>]

On Sun, Oct 25, 2026 at 12:07:10PM +0100, Günther Noack wrote:
> Hello Farid!
> 
> On Sat, Oct 24, 2026 at 01:39:16PM -0700, Farid Zakaria wrote:
> > This is a proposal for an idea to add support for landlock to work
> > better with build systems such as Nix where they want to apply
> > restrictions but the paths may not yet exist.
> > 
> > TL;DR;
> > ========
> > 
> > Landlock rules attach to inodes that exist when a ruleset is built.
> > That makes a directory shared between trusted and sandboxed tasks hard
> > to confine: the sandbox may create entries there with
> > LANDLOCK_ACCESS_FS_MAKE_*, but no rule can name those entries ahead of
> > time, and granting read access on the directory to cover them exposes
> > every other entry as well. This series adds a rule flag that grants
> > access rights on the entries a domain creates in a directory, instead
> > of on the directory.
> > 
> > Motivation
> > ==========
> > 
> > The motivating user is the Nix build sandbox. Every Nix build runs in
> > its own sandbox, and a build may only read its declared inputs in
> > /nix/store and must be able to write and read back the outputs it
> > creates there. Nix enforces this with a private mount namespace: it
> > creates an empty store directory and bind-mounts each input path into
> > it, one mount per input. Nix has paid this per-input cost since the
> > sandbox was introduced [1].
> > 
> > The cost is paid per build and grows with the size of the input
> > closure. Nix is moving towards much finer-grained builds (many small
> > derivations instead of a few large ones), so the number of sandboxes
> > grows while each build's own work shrinks, and the sandbox setup is
> > becoming the bottleneck. A typical build has hundreds of inputs; large
> > ones have thousands.
> > 
> > A microbenchmark sets up and tears down a Nix-like sandbox (user,
> > mount, PID, IPC, UTS and network namespaces, pivot_root into a chroot,
> > removing the chroot afterwards). It compares one bind mount per input
> > with a single bind mount of the whole store plus one Landlock rule per
> > input. Median latency per sandbox, one sandbox at a time, Linux 7.1,
> > ext4, 16 cores:
> > 
> >   inputs   bind mount per input   one bind mount + Landlock   speedup
> >       10         3.2 ms                    2.3 ms               1.4x
> >      100        11.6 ms                    2.8 ms               4.1x
> >     1000        79.3 ms                    6.4 ms              12.4x
> >     5000       400.1 ms                   24.4 ms              16.4x
> > 
> > With Landlock, an input costs an O_PATH open and a landlock_add_rule()
> > call: there is a single mount, and no mountpoint directory to create
> > before the build and remove after it (removing the 1000 empty
> > mountpoints alone took 17 ms).
> > 
> > The single bind mount is only sound if the store listing and the
> > undeclared store paths stay unreadable while the build can still use
> > its outputs. Without this series, covering the outputs requires a rule
> > on /nix/store itself, which makes every store path readable again.
> > 
> > [1] https://github.com/NixOS/nix/issues/179
> 
> I have not done a thorough code review yet, and will focus on the
> general approach.
> 
> I am generally in favor of this approach where we keep track of the
> directory inode plus the file name.

Apologies, I think I was jumping to conclusions here when I read this
too superficially - I realize that you have implemented something
different;

If I understand correctly, your proposed patch set is granting the
access rights on every file that you are creating freshly, but not on
files that were created by other processes in the same directory.  The
semantics, from what I can see, seem like a Landlock variant to what
the "sticky bit" does in /tmp -- you have access to what you create
yourself, but not to the files created by other processes.


## Alternative suggestion

I have been discussing the other approach with Mickaël a while back.
This would also work in use cases where we want to express a rule on a
path whose underlying file inode might change, with examples being
/etc/resolv.conf (recreated by a DHCP client) or Pathname Unix Domain
Sockets (commonly recreated when services restart).

In that approach, we'd add a rule like

/* pseudocode; name and exact layout TBD */
struct landlock_path_beneath2_attr {
	__u64 allowed_access;
        __s32 parent_fd;
        char  *basename;
}

During the path walk when checking for an access right, it would match
if the parent_fd matches an inode and the name of the next path
component is basename.  For example, to access a (potentially
changing) /etc/resolv.conf:

struct landlock_path_beneath_attr2 attr = {
	.allowed_access = READ_ONLY_ACCESSES,
        .parent_fd      = open("/etc", O_PATH|O_CLOEXEC),
        .basename       = "resolv.conf",
}

Or to allow the creation of an output file:

struct landlock_path_beneath_attr2 attr = {
	.allowed_access = READ_WRITE_ACCESSES | LANDLOCK_ACCESS_FS_MAKE_REG,
        .parent_fd      = open("/output/directory", O_PATH|O_CLOEXEC),
        .basename       = "mymodule.o",
}

Would that be a model that would work for your Nix use case as well?
I'm imagining that you are declaring the exact list of output files,
is that correct?

–Günther