linux-ext4 - Re: [PATCH v5 37/42] xfs: support idmapped mounts

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Date:   Thu, 14 Jan 2021 23:10:48 +0100
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Dave Chinner <david@...morbit.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        linux-fsdevel@...r.kernel.org,
        John Johansen <john.johansen@...onical.com>,
        James Morris <jmorris@...ei.org>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        Dmitry Kasatkin <dmitry.kasatkin@...il.com>,
        Stephen Smalley <stephen.smalley.work@...il.com>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Arnd Bergmann <arnd@...db.de>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>,
        Geoffrey Thomas <geofft@...reload.com>,
        Mrunal Patel <mpatel@...hat.com>,
        Josh Triplett <josh@...htriplett.org>,
        Andy Lutomirski <luto@...nel.org>,
        Theodore Tso <tytso@....edu>, Alban Crequy <alban@...volk.io>,
        Tycho Andersen <tycho@...ho.ws>,
        David Howells <dhowells@...hat.com>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        Seth Forshee <seth.forshee@...onical.com>,
        Stéphane Graber <stgraber@...ntu.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Aleksa Sarai <cyphar@...har.com>,
        Lennart Poettering <lennart@...ttering.net>,
        "Eric W. Biederman" <ebiederm@...ssion.com>, smbarber@...omium.org,
        Phil Estes <estesp@...il.com>, Serge Hallyn <serge@...lyn.com>,
        Kees Cook <keescook@...omium.org>,
        Todd Kjos <tkjos@...gle.com>, Paul Moore <paul@...l-moore.com>,
        Jonathan Corbet <corbet@....net>,
        containers@...ts.linux-foundation.org,
        linux-security-module@...r.kernel.org, linux-api@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-integrity@...r.kernel.org, selinux@...r.kernel.org,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v5 37/42] xfs: support idmapped mounts

On Fri, Jan 15, 2021 at 07:51:54AM +1100, Dave Chinner wrote:
> On Tue, Jan 12, 2021 at 11:01:19PM +0100, Christian Brauner wrote:
> > From: Christoph Hellwig <hch@....de>
> > 
> > Enable idmapped mounts for xfs. This basically just means passing down
> > the user_namespace argument from the VFS methods down to where it is
> > passed to helper.
> > 
> > Signed-off-by: Christoph Hellwig <hch@....de>
> ....
> > @@ -654,6 +658,7 @@ xfs_vn_change_ok(
> >   */
> >  static int
> >  xfs_setattr_nonsize(
> > +	struct user_namespace	*mnt_userns,
> >  	struct xfs_inode	*ip,
> >  	struct iattr		*iattr)
> >  {
> > @@ -813,7 +818,7 @@ xfs_setattr_nonsize(
> >  	 * 	     Posix ACL code seems to care about this issue either.
> >  	 */
> >  	if (mask & ATTR_MODE) {
> > -		error = posix_acl_chmod(&init_user_ns, inode, inode->i_mode);
> > +		error = posix_acl_chmod(mnt_userns, inode, inode->i_mode);
> >  		if (error)
> >  			return error;
> >  	}
> > @@ -868,7 +873,7 @@ xfs_setattr_size(
> >  		 * Use the regular setattr path to update the timestamps.
> >  		 */
> >  		iattr->ia_valid &= ~ATTR_SIZE;
> > -		return xfs_setattr_nonsize(ip, iattr);
> > +		return xfs_setattr_nonsize(&init_user_ns, ip, iattr);
> 
> Shouldn't that be passing mnt_userns?

Hey Dave,

Thanks for taking a look.

This is the time updating codepath.

xfs_setattr_size();
-> xfs_setattr_nonsize(&init_user_ns);

The xfs_setattr_size() helper will assert:

ASSERT((iattr->ia_valid & (ATTR_UID|ATTR_GID|ATTR_ATIME|ATTR_ATIME_SET|
	ATTR_MTIME_SET|ATTR_KILL_PRIV|ATTR_TIMES_SET)) == 0);

While the

xfs_setattr_nonsize() helper will further assert:

ASSERT((mask & ATTR_SIZE) == 0);

so xfs_setattr_nonsize() in this callpath is only used to update the

if (!(iattr->ia_valid & (ATTR_CTIME|ATTR_MTIME)))
	return 0;

so there's no interactions with idmappings in any way. Simply passing
mnt_userns might be clearer though.

But if this would be using the wrong idmapping the xfstest suite I added
would've immediately caught that and failed.

But this specific codepath can also be reliably hit from userspace by
doing ftruncate(fd, 0) so just to be extra sure I added truncate tests
to the xfstests now for both the idmapped and non-idmapped case.

Christian