[Virtio-fs] Use of unshare(CLONE_FS) in virtiofsd

[Florian Weimer]

* 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