linux-kernel - Re: [PATCH v3 00/12] man2: document "new" mount API

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Message-ID: <2025-08-19.1755593370-twitchy-liquid-houses-wink-kqgdBL@cyphar.com>
Date: Tue, 19 Aug 2025 18:50:31 +1000
From: Aleksa Sarai <cyphar@...har.com>
To: Christian Brauner <brauner@...nel.org>
Cc: Askar Safin <safinaskar@...omail.com>, alx@...nel.org, 
	dhowells@...hat.com, g.branden.robinson@...il.com, jack@...e.cz, 
	linux-api@...r.kernel.org, linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-man@...r.kernel.org, mtk.manpages@...il.com, viro@...iv.linux.org.uk, 
	Ian Kent <raven@...maw.net>, autofs mailing list <autofs@...r.kernel.org>
Subject: Re: [PATCH v3 00/12] man2: document "new" mount API

On 2025-08-19, Christian Brauner <brauner@...nel.org> wrote:
> On Mon, Aug 18, 2025 at 02:16:04AM +1000, Aleksa Sarai wrote:
> > On 2025-08-17, Askar Safin <safinaskar@...omail.com> wrote:
> > > 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.
> > 
> > > 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.
> > 
> > > 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).
> > 
> > > 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. :)

Yeah I misunderstood what you said -- didn't mean to throw you under the
bus, sorry about that!

> > 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/
> 
> 

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

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