linux-kernel - Re: [PATCH v4] proc: report open files as size in stat() for /proc/pid/fd

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Date:   Mon, 21 Nov 2022 06:42:24 -0500
From:   Brian Foster <bfoster@...hat.com>
To:     Christian Brauner <brauner@...nel.org>
Cc:     Ivan Babrou <ivan@...udflare.com>, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-team@...udflare.com,
        Alexey Dobriyan <adobriyan@...il.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Theodore Ts'o <tytso@....edu>,
        David Laight <David.Laight@...lab.com>,
        Jonathan Corbet <corbet@....net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Christoph Anton Mitterer <mail@...istoph.anton.mitterer.name>,
        Mike Rapoport <rppt@...nel.org>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Kalesh Singh <kaleshsingh@...gle.com>
Subject: Re: [PATCH v4] proc: report open files as size in stat() for
 /proc/pid/fd

On Sat, Nov 19, 2022 at 01:01:11PM +0100, Christian Brauner wrote:
> On Fri, Nov 18, 2022 at 02:33:27PM -0500, Brian Foster wrote:
> > On Fri, Nov 18, 2022 at 11:18:36AM -0800, Ivan Babrou wrote:
> > > On Fri, Nov 18, 2022 at 11:10 AM Brian Foster <bfoster@...hat.com> wrote:
> > > > > +static int proc_fd_getattr(struct user_namespace *mnt_userns,
> > > > > +                     const struct path *path, struct kstat *stat,
> > > > > +                     u32 request_mask, unsigned int query_flags)
> > > > > +{
> > > > > +     struct inode *inode = d_inode(path->dentry);
> > > > > +     int rv = 0;
> > > > > +
> > > > > +     generic_fillattr(&init_user_ns, inode, stat);
> > > > > +
> > > >
> > > > Sorry I missed this on v3, but shouldn't this pass through the
> > > > mnt_userns parameter?
> > > 
> > > The mnt_userns parameter was added in 549c729 (fs: make helpers idmap
> > > mount aware), and it's not passed anywhere in fs/proc.
> > > 
> > > Looking at other uses of generic_fillattr, all of them use "init_user_ns":
> > > 
> > 
> > Interesting. It looks like this would have used mnt_userns from
> > vfs_getattr_nosec() before proc_fd_getattr() is wired up, right? I'm not
> > familiar enough with that change to say whether /proc should use one
> > value or the other, or perhaps it just doesn't matter.?
> > 
> > Christian?
> 
> Hey Brian,
> 
> This should pass init_user_ns. So it is correct the way it is done now.
> The init_user_ns is used to indicate that no idmappings are used and
> since procfs doesn't support the creation of idmapped mounts and doesn't
> need to, passing it here makes the most sense. Technically passing down
> mnt_userns would work too but that would make it look like procfs could
> support idmapped mounts which isn't the case and so we don't do it this
> way.
> 

Got it, thanks for the context.

Ivan,

Sorry for the noise. FWIW, for this version of the patch:

Reviewed-by: Brian Foster <bfoster@...hat.com>

> Starting soon this will be a lot clearer too since we're about to
> introduce struct mnt_idmap and replace passing around userns here.
> That'll make things also safer as the helpers that currently could be
> passed a mnt_userns - which could be any userns - will now only be able
> to take mnt_idmap which is a different type.
> 
> Long story short, the way your patch does it is correct.
> 
> Thanks!
> Christian
>