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Date:   Wed, 2 Dec 2020 10:55:47 +0100
From:   Christoph Hellwig <hch@....de>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     Christoph Hellwig <hch@....de>,
        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>,
        St├ęphane Graber <stgraber@...ntu.com>,
        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, fstests@...r.kernel.org,
        linux-security-module@...r.kernel.org, linux-api@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-integrity@...r.kernel.org,
        selinux@...r.kernel.org
Subject: Re: [PATCH v3 04/38] fs: add mount_setattr()

On Wed, Dec 02, 2020 at 10:47:51AM +0100, Christoph Hellwig wrote:
> On Wed, Dec 02, 2020 at 10:42:18AM +0100, Christian Brauner wrote:
> > > > +	if (upper_32_bits(attr->attr_set))
> > > > +		return -EINVAL;
> > > > +	if (build_attr_flags(lower_32_bits(attr->attr_set), &kattr->attr_set))
> > > > +		return -EINVAL;
> > > > +
> > > > +	if (upper_32_bits(attr->attr_clr))
> > > > +		return -EINVAL;
> > > > +	if (build_attr_flags(lower_32_bits(attr->attr_clr), &kattr->attr_clr))
> > > > +		return -EINVAL;
> > > 
> > > What is so magic about the upper and lower 32 bits?
> > 
> > Nothing apart from the fact that they arent't currently valid. I can
> > think about reworking these lines. Or do you already have a preferred
> > way of doing this in mind?
> 
> Just turn the attr_flags argument to build_attr_flags into a u64 and
> the first sanity check there will catch all invalid flags, no matter
> where they are places.  That should also generate more efficient code.

And while we're at it:  the check for valid flags in the current
code is a little weird, given that build_attr_flags checks for
them, and but sys_fsmount also has its own slightly narrower checks.

I think it might make sense to split the validity check out of
build_attr_flags. E.g. something like:

static unsigned int attr_flags_to_mnt_flags(u64 attr_flags)
{
	unsigned int mnt_flags = 0;

	if (attr_flags & MOUNT_ATTR_RDONLY)
		mnt_flags |= MNT_READONLY;
	if (attr_flags & MOUNT_ATTR_NOSUID)
		mnt_flags |= MNT_NOSUID;
	if (attr_flags & MOUNT_ATTR_NODEV)
		mnt_flags |= MNT_NODEV;
	if (attr_flags & MOUNT_ATTR_NOEXEC)
		mnt_flags |= MNT_NOEXEC;
	if (attr_flags & MOUNT_ATTR_NODIRATIME)
		mnt_flags |= MNT_NODIRATIME;

	return mnt_flags;
}

#define MOUNT_SETATTR_VALID_FLAGS \
	(MOUNT_ATTR_RDONLY | MOUNT_ATTR_NOSUID | MOUNT_ATTR_NODEV | \
	 MOUNT_ATTR_NOEXEC | MOUNT_ATTR__ATIME | MOUNT_ATTR_NODIRATIME | \
	 MOUNT_ATTR_IDMAP)

static int build_mount_kattr(const struct mount_attr *attr, size_t usize,
			     struct mount_kattr *kattr, unsigned int flags)
{
	...

	if ((attr->attr_set | attr->attr_clr) & ~MOUNT_SETATTR_VALID_FLAGS)
		return -EINVAL;
	kattr->attr_set = attr_flags_to_mnt_flags(attr->attr_set);
	kattr->attr_clr = attr_flags_to_mnt_flags(attr->attr_clr);
	...
}

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