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From:   Ivan Babrou <ivan@cloudflare.com>
Date:   Wed, 21 Sep 2022 11:08:37 -0700
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Subject: Re: [PATCH] proc: report open files as size in stat() for /proc/pid/fd
To:     David Laight <David.Laight@aculab.com>
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        Alexey Dobriyan <adobriyan@gmail.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        "Theodore Ts'o" <tytso@mit.edu>, Jonathan Corbet <corbet@lwn.net>,
        Andrew Morton <akpm@linux-foundation.org>,
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        Christoph Anton Mitterer <mail@christoph.anton.mitterer.name>,
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        Mike Rapoport <rppt@kernel.org>,
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        Kalesh Singh <kaleshsingh@google.com>
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On Wed, Sep 21, 2022 at 3:21 AM David Laight <David.Laight@aculab.com> wrote:
>
> From: Ivan Babrou
> > Sent: 20 September 2022 20:06
> ...
> >
> > +static int proc_readfd_count(struct inode *inode)
> > +{
> > +     struct task_struct *p = get_proc_task(inode);
> > +     struct fdtable *fdt;
> > +     unsigned int i, size, open_fds = 0;
> > +
> > +     if (!p)
> > +             return -ENOENT;
> > +
> > +     if (p->files) {
> > +             fdt = files_fdtable(p->files);
> > +             size = fdt->max_fds;
> > +
> > +             for (i = size / BITS_PER_LONG; i > 0;)
> > +                     open_fds += hweight64(fdt->open_fds[--i]);
> > +     }

I'm missing put_task_struct(p) here.

> > +
> > +     return open_fds;
> > +}
> > +
>
> Doesn't that need (at least) rcu protection?

Should I enclose the "if" in rcu_read_lock() / rcu_read_unlock()?

files_fdtable() is this:

* https://elixir.bootlin.com/linux/v6.0-rc6/source/include/linux/fdtable.h#L77

#define files_fdtable(files) \
    rcu_dereference_check_fdtable((files), (files)->fdt)

And rcu_dereference_check_fdtable() is

#define rcu_dereference_check_fdtable(files, fdtfd) \
  rcu_dereference_check((fdtfd), lockdep_is_held(&(files)->file_lock))

I definitely need some help with locking here.

> There might also be issues reading p->files twice.

This block for reading twice:

if (p->files) {
    fdt = files_fdtable(p->files);

Already exists in fs/proc/array.c in task_state():

* https://elixir.bootlin.com/linux/v6.0-rc6/source/fs/proc/array.c#L173

There's task_lock(p) there, so maybe that's why it's allowed.

Should I run files_fdtable(p->files) unconditionally and then check
the result instead? I'm happy to change it to something else if you
can tell me what it should be.

If there are kernel options I should enable for testing this, I'd be
happy to hear them. So far we've tried running with KASAN with no
issues.