[systemd-devel] 220 regression: path_is_mount_point() for non-directories

[Lennart Poettering]

On Mon, 25.05.15 21:06, Martin Pitt (martin.pitt at ubuntu.com) wrote:

> Hello all,
> 
> in 220, path_is_mount_point() now always fails with "-20 ENOTDIR" when
> calling it on files. This is problematic as it's perfectly valid to
> have bind-mounted files; in fact, systemd's machine_id_setup() itself
> creates a /run/machine-id → /etc/machine-id bind mount if /etc is
> read-only at that time (and systemd-machine-id-commit will write it
> once root fs becomes writable).
> 
> This was first introduced here:
> 
>   http://cgit.freedesktop.org/systemd/systemd/commit/?id=27cc6f166
>       path-util: fix path_is_mount_point() for symlinks
> 
> which added O_DIRECTORY, so that openat() fails with the above error.
> It also replaced path_get_parent() (which works fine for files) with
> name_to_handle_at(fd, "..", ...) which only works for directories.
> 
> In fd_is_mount_point() we obviously don't have access to the file name
> any more, and presumably we do want to keep it for efficiently
> implementing the "rm-rf: never cross mount points" commit f25afeb.
> 
> Since all possible checks in fd_is_mount_point() rely on *at(, "..")
> which doesn't work for files, one solution that I see
> is to add a fallback path_is_mountpoint() if fd_is_mount_point() fails
> with ENOTDIR; in that case, should we implement all fallbacks again
> (lots of duplicated code), or just one (st_dev comparison, which seems
> the most widely supported one)?
> 
> Alternatively we could pass a "parent_path" to fd_is_mount_point(),
> compute that in path_is_mountpoint(), and don't specify one (or maybe
> we even can specify it easily, I didn't check) in rm_rf_children() as
> file bind mounts don't seem relevant there. This would be simpler, but
> it would technically be an API break (unless we want to add
> fd_is_mount_point_with_parent).

Not an API break. Only calls prefixed with "sd_" are API, we never
export any others.

But yeah, path_is_mount_point() should be reworked to operate like
openat() and similar calls, i.e. taking an fd to a dir, plus a
filename below that dir, instead of directly and only an fd to the
file below that dir.

Lennart

-- 
Lennart Poettering, Red Hat