linux-kernel - Re: [RFC PATCH 3/3] add listmnt(2) syscall

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Message-ID: <CAJfpeguE97q=esmS6zE4OaZBwkBBWykwH1MTnUvLeHcfb7NeTw@mail.gmail.com>
Date:   Thu, 14 Sep 2023 10:50:04 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Amir Goldstein <amir73il@...il.com>
Cc:     Miklos Szeredi <mszeredi@...hat.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org, linux-man@...r.kernel.org,
        linux-security-module@...r.kernel.org, Karel Zak <kzak@...hat.com>,
        Ian Kent <raven@...maw.net>,
        David Howells <dhowells@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <christian@...uner.io>
Subject: Re: [RFC PATCH 3/3] add listmnt(2) syscall

On Thu, 14 Sept 2023 at 08:00, Amir Goldstein <amir73il@...il.com> wrote:

> > +               if (ctr >= bufsize)
> > +                       return -EOVERFLOW;
> > +               if (put_user(r->mnt_id_unique, buf + ctr))
> > +                       return -EFAULT;
> > +               ctr++;
> > +               if (ctr < 0)
> > +                       return -ERANGE;
>
> I think it'd be good for userspace to be able to query required
> bufsize with NULL buf, listattr style, rather than having to
> guess and re-guess on EOVERFLOW.

The getxattr/listxattr style encourages the following code:

  size = get(NULL, 0);
  buf = alloc(size);
  err = get(buf, size);
  if (err)
      /* failure */

Which is wrong, since the needed buffer size could change between the two calls.

Doing it iteratively is the only correct way, and then adding
complexity to both userspace and the kernel for *optimizing* the
iteration is not really worth it, IMO.

Thanks,
Miklos