On Mon, Aug 18, 2025 at 02:16:04AM +1000, Aleksa Sarai wrote:

On 2025-08-17, Askar Safin [off-list ref] wrote:

quoted

I noticed that you changed docs for automounts. So I dig into
automounts implementation. And I found a bug in openat2. If
RESOLVE_NO_XDEV is specified, then name resolution doesn't cross
automount points (i. e. we get EXDEV), but automounts still happen! I
think this is a bug. Bug is reproduced in 6.17-rc1. In the end of this
mail you will find reproducer. And miniconfig.

Yes, this is a bug -- we check LOOKUP_NO_XDEV after traverse_mounts()
because we want to error out if we actually jumped to a different mount.
We should probably be erroring out in follow_automount() as well, and I
missed this when I wrote openat2().

openat2() also really needs RESOLVE_NO_AUTOMOUNT (and probably
RESOLVE_NO_DOTDOT as well as some other small features). I'll try to
send something soon.

quoted

Are automounts actually used? Is it possible to deprecate or
remove them? It seems for me automounts are rarely tested obscure
feature, which affects core namei code.

I use them for auto-mounting NFS shares on my laptop, and I'm sure there
are plenty of other users. They are little bit funky but I highly doubt
they are "unused". Howells probably disagrees in even stronger terms.
Most distributions provide autofs as a supported package (I think it
even comes pre-installed for some distros).

They are not tested by fstests AFAICS, but that's more of a flaw in
fstests (automount requires you to have a running autofs daemon, which
probably makes testing it in fstests or selftests impractical) not the
feature itself.

quoted

This reproducer is based on "tracing" automount, which
actually *IS* already deprecated. But automount mechanism
itself is not deprecated, as well as I know.

The automount behaviour of tracefs is different to the general automount
mechanism which is managed by userspace with the autofs daemon. I don't
know the history behind the deprecation, but I expect that it was
deprecated in favour of configuring it with autofs (or just enabling it
by default).

quoted

Also, I did read namei code, and I think that
options AT_NO_AUTOMOUNT, FSPICK_NO_AUTOMOUNT, etc affect
last component only, not all of them. I didn't test this yet.
I plan to test this within next days.

No, LOOKUP_AUTOMOUNT affects all components. I double-checked this with
Christian.

Hm? I was asking the question in the chat because I was unsure and not
in front of a computer you then said that it does affect all components. :)

You would think that it's only the last component (like O_DIRECTORY,
O_NOFOLLOW, AT_SYMLINK_{,NO}FOLLOW) but follow_automount() is called for
all components (i.e., as part of step_into()). It hooks into the regular
lookup flow for mountpoints.

Yes, it is quite funky that AT_NO_AUTOMOUNT is the only AT_* flag that
works this way -- hence why I went with a different RESOLVE_* namespace
for openat2() (which _always_ act on _all_ components).

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
https://www.cyphar.com/