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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Thu, 19 Dec 2019 09:03:09 +0100
Message-ID: <CAK8P3a2eT=bHkUamyp-P3Y2adNq1KBk7UknCYBY5_aR4zJmYaQ@mail.gmail.com>
Subject: Re: [PATCH v4 2/5] pid: Add PIDFD_IOCTL_GETFD to fetch file
 descriptors from processes
To:     Sargun Dhillon <sargun@sargun.me>
Cc:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Linux Containers <containers@lists.linux-foundation.org>,
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        Christian Brauner <christian.brauner@ubuntu.com>,
        Oleg Nesterov <oleg@redhat.com>,
        Andy Lutomirski <luto@amacapital.net>,
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On Thu, Dec 19, 2019 at 12:55 AM Sargun Dhillon <sargun@sargun.me> wrote:

> +#define PIDFD_IOCTL_GETFD      _IOWR('p', 0xb0, __u32)

This describes an ioctl command that reads and writes a __u32 variable
using a pointer passed as the argument, which doesn't match the
implementation:

> +static long pidfd_getfd(struct pid *pid, u32 fd)
> +{
...
> +       return retfd;

This function passes an fd as the argument and returns a new
fd, so the command number would be

#define PIDFD_IOCTL_GETFD      _IO('p', 0xb0)

While this implementation looks easy enough, and it is roughly what
I would do in case of a system call, I would recommend for an ioctl
implementation to use the __u32 pointer instead:

static long pidfd_getfd_ioctl(struct pid *pid, u32 __user *arg)
{
         int their_fd, new_fd;
         int ret;

         ret = get_user(their_fd, arg);
         if (ret)
              return ret;

        new_fd = pidfd_getfd(pid, their_fd);
        if (new_fd < 0)
                return new_fd;

         return put_user(new_fd, arg);
}

Direct argument passing in ioctls may confuse readers because it
is fairly unusual, and it doesn't work with this:

>  const struct file_operations pidfd_fops = {
>         .release = pidfd_release,
>         .poll = pidfd_poll,
> +       .unlocked_ioctl = pidfd_ioctl,
> +       .compat_ioctl = compat_ptr_ioctl,

compat_ptr_ioctl() only works if the argument is a pointer, as it
mangles the argument to turn it from a 32-bit pointer value into
a 64-bit pointer value. These are almost always the same
(arch/s390 being the sole exception), but you should not rely
on it. For now it would be find to do '.compat_ioctl = pidfd_ioctl',
but that in turn is wrong if you ever add another ioctl command
that does pass a pointer.

       Arnd