linux-kernel - Re: [PATCH v3] proc: report open files as size in stat() for /proc/pid/fd

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Date:   Thu, 20 Oct 2022 17:52:21 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Brian Foster <bfoster@...hat.com>
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>,
        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 v3] proc: report open files as size in stat() for
 /proc/pid/fd

On Wed, 19 Oct 2022 07:28:45 -0400 Brian Foster <bfoster@...hat.com> wrote:

> On Tue, Oct 18, 2022 at 11:51:02AM -0700, Ivan Babrou wrote:
> > On Tue, Oct 18, 2022 at 11:16 AM Brian Foster <bfoster@...hat.com> wrote:
> > > > +static int proc_readfd_count(struct inode *inode)
> > > > +{
> > > > +     struct task_struct *p = get_proc_task(inode);
> > > > +     struct fdtable *fdt;
> > > > +     unsigned int open_fds = 0;
> > > > +
> > > > +     if (!p)
> > > > +             return -ENOENT;
> > >
> > > Maybe this shouldn't happen, but do you mean to assign the error code to
> > > stat->size in the caller? Otherwise this seems reasonable to me.
> > 
> > You are right. As unlikely as it is to happen, we shouldn't return
> > negative size.
> > 
> > What's the idiomatic way to make this work? My two options are:
> > 
> > 1. Pass &stat->size into proc_readfd_count:
> > 
> >   if (S_ISDIR(inode->i_mode)) {
> >     rv = proc_readfd_count(inode, &stat->size);
> >     if (rv < 0)
> >       goto out;
> >   }
> > 
> > out:
> >   return rv;
> > 
> > OR without a goto:
> > 
> >   if (S_ISDIR(inode->i_mode)) {
> >     rv = proc_readfd_count(inode, &stat->size));
> >     if (rv < 0)
> >       return rv;
> >   }
> > 
> >   return rv;
> > 
> > 2. Return negative count as error (as we don't expect negative amount
> > of files open):
> > 
> >   if (S_ISDIR(inode->i_mode)) {
> >     size = proc_readfd_count(inode);
> >     if (size < 0)
> >       return size;
> >     stat->size = size;
> >   }
> > 
> 
> I suppose the latter is less of a change to the original patch..? Either
> way seems reasonable to me. I have no strong preference FWIW.

If get_proc_task() failed then something has gone horridly wrong,
hasn't it?  Wouldn't it make sense in this situation to make the
.getattr() itself return an errno, in which case the data at *stat
dosen't matter - it's invalid anyway.

This seems to be the general approach in procfs when get_proc_task()
fails - return -ESRCH (or, seemingly randomly, -ENOENT) to the caller.