From: Florian Weimer <fweimer@redhat.com>
To: Vivek Goyal <vgoyal@redhat.com>
Cc: virtio-fs@redhat.com, qemu-devel@nongnu.org,
Misono Tomohiro <misono.tomohiro@jp.fujitsu.com>,
"Dr. David Alan Gilbert" <dgilbert@redhat.com>,
Miklos Szeredi <mszeredi@redhat.com>,
Stefan Hajnoczi <stefanha@redhat.com>,
German Maglione <gmaglione@redhat.com>,
Sergio Lopez <slp@redhat.com>
Subject: Re: [Virtio-fs] Use of unshare(CLONE_FS) in virtiofsd
Date: Fri, 04 Nov 2022 18:44:39 +0100 [thread overview]
Message-ID: <875yfuzkag.fsf@oldenburg.str.redhat.com> (raw)
In-Reply-To: <Y2UM/C3aYtQwf40M@redhat.com> (Vivek Goyal's message of "Fri, 4 Nov 2022 09:00:44 -0400")
* Vivek Goyal:
>> The usual
>> recommendation for emulating it is to use openat with O_PATH, and then
>> use getxattr on the virtual /proc/self/fd path. This needs an
>> additional system call (openat, getxattr, close instead of fchdir,
>> getxattr),
>
> openat(O_PATH) + getxattr(/proc/self/fd) + close() sounds reasonable
> too. Not sure why did we not take that path. May be due to that extra
> syscall or something else.
Thanks.
>> but it avoids the unshare(CLONE_FS) call behind libc's back.
>
> Hmm.., did not know that libc does not like threads calling
> unshare(CLONE_FS). Not sure why that is a problem.
Here's a corner case: We plan to add chroot detection to NSS module
loading (so that we do not load NSS modules after chroot), as a form of
security hardening. If the application calls unshare(CLONE_FS) and then
chroot, which NSS modules are loaded depends on which threads call NSS
functions.
One could argue that chdir/chroot are problematic, not
unshare(CLONE_FS). 8-)
> BTW, we need separate umask per thread as well. During file creation
> we might be switching to umask provide in fuse protocol message
> and then switch back. Given multiple therads might be doing this
> creation in parallel, so we ofcourse need this to be per thread
> property.
That's a good point. That's not something that can be worked around
with *at functions.
> So if your patches for pthread_create() with per thread filesystem
> attributes finally goes upstream, I guess we should be able to
> make use of it and drop unshare(CLONE_FS).
Thank you for the feedback.
Florian
prev parent reply other threads:[~2022-11-04 17:44 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-11-04 7:50 [Virtio-fs] Use of unshare(CLONE_FS) in virtiofsd Florian Weimer
2022-11-04 13:00 ` Vivek Goyal
2022-11-04 17:44 ` Florian Weimer [this message]
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