[Virtio-fs] [PATCH 0/1] fuse: acl: Send file mode updates using SETATTR

[Luis Henriques]

On Wed, Mar 17, 2021 at 11:18:57AM -0400, Vivek Goyal wrote:
> On Wed, Mar 17, 2021 at 02:29:03PM +0000, Luis Henriques wrote:
> > On Tue, Mar 16, 2021 at 12:01:46PM -0400, Vivek Goyal wrote:
> > > Hi Miklos,
> > > 
> > > Please find attached a patch to fix the SGID clearing issue upon 
> > > ACL change. 
> > > 
> > > Luis reported that currently fstests generic/375 fails on virtiofs. And
> > > reason being that we don't clear SGID when it should be.
> > > 
> > > Setting ACL can lead to file mode change. And this in-turn also can
> > > lead to clearing SGID bit if.
> > > 
> > > - None of caller's groups match file owner group.
> > > AND
> > > - Caller does not have CAP_FSETID.
> > > 
> > > Current implementation relies on server updating the mode. But file
> > > server does not have enough information to do so. 
> > > 
> > > Initially I thought of sending CAP_FSETID information to server but
> > > then I realized, it is just one of the pieces. What about all the
> > > groups caller is a member of. If this has to work correctly, then
> > > all the information will have to be sent to virtiofsd somehow. Just
> > > sending CAP_FSETID information required adding V2 of fuse_setxattr_in
> > > because we don't have any space for sending extra information.
> > > 
> > > https://github.com/rhvgoyal/linux/commit/681cf5bdbba9c965c3dbd4337c16e9b17f27debe
> > > 
> > > Also this approach will not work with idmapped mounts because server
> > > does not have information about idmapped mappings.
> > > 
> > > So I started to look at the approach of sending file mode updates
> > > using SETATTR. As filesystems like 9pfs and ceph are doing. This
> > > seems simpler approach. Though it has its issues too.
> > > 
> > > - File mode update and setxattr(system.posix_acl_access) are not atomic.
> > 
> > After reviewing (and testing) the patch, the only comment I have is that
> > we should at least pr_warn() an eventual failure in setxattr().  But f
> > that operation fails at that point, probably something went wrong on the
> > other side
> 
> Hi Luis,
> 
> If setxattr failed, user will get the error. 
> 
> I guess pr_warn() could help with figuring out that there was a side affect
> of failed failed setxattr operation. (mode changed). I will add something.
> 
> > and the kernel is unlikely to be able to revert the mode
> > changes anyway.
> 
> Interestingly ceph code seems to revert mode changes if setxattr fails.
> I think for now I am happy with just a pr_warn().

Yeah, ceph does that.  And I *should* know it ;-)

Anyway, to mimic ceph's behaviour should be easy enough, although I guess
it's just a best-effort thing.

Cheers,
--
Luís


> > 
> > (And a nit: your patch seems to require some whitespaces clean-up.)
> 
> Will check it and fix it and post V2.
> 
> Thanks
> Vivek
> 
> > 
> > Cheers,
> > --
> > Luís
> > 
> > 
> > > None of the approaches seem very clean to me. But sending SETATTR
> > > explicitly seems to be lesser of two evils to me at this point of time.
> > > Hence I am proposing this patch. 
> > > 
> > > I have run fstests acl tests and they pass. (./check -g acl).
> > > 
> > > Corresponding virtiofsd patches are here.
> > > 
> > > https://github.com/rhvgoyal/qemu/commits/acl-sgid-setattr
> > > 
> > > What do you think.
> > > 
> > > Vivek Goyal (1):
> > >   fuse: Add a mode where fuse client sends mode changes on ACL change
> > > 
> > >  fs/fuse/acl.c             | 54 ++++++++++++++++++++++++++++++++++++---
> > >  fs/fuse/dir.c             | 11 ++++----
> > >  fs/fuse/fuse_i.h          |  9 ++++++-
> > >  fs/fuse/inode.c           |  4 ++-
> > >  include/uapi/linux/fuse.h |  5 ++++
> > >  5 files changed, 71 insertions(+), 12 deletions(-)
> > > 
> > > -- 
> > > 2.25.4
> > > 
> > 
>