Re: Testing if two open descriptors refer to the same inode

[Mateusz Guzik <mjguzik@gmail.com>]

On Mon, Jul 29, 2024 at 12:50 PM Mateusz Guzik <mjguzik@gmail.com> wrote:
>
> On Mon, Jul 29, 2024 at 12:40:35PM +0200, Florian Weimer wrote:
> > * Mateusz Guzik:
> >
> > > On Mon, Jul 29, 2024 at 08:55:46AM +0200, Florian Weimer wrote:
> > >> It was pointed out to me that inode numbers on Linux are no longer
> > >> expected to be unique per file system, even for local file systems.
> > >
> > > I don't know if I'm parsing this correctly.
> > >
> > > Are you claiming on-disk inode numbers are not guaranteed unique per
> > > filesystem? It sounds like utter breakage, with capital 'f'.
> >
> > Yes, POSIX semantics and traditional Linux semantics for POSIX-like
> > local file systems are different.
> >
>
> Can you link me some threads about this?
>
> > > While the above is not what's needed here, I guess it sets a precedent
> > > for F_DUPINODE_QUERY (or whatever other name) to be added to handily
> > > compare inode pointers. It may be worthwhile regardless of the above.
> > > (or maybe kcmp could be extended?)
> >
> > I looked at kcmp as well, but I think it's dependent on
> > checkpoint/restore.  File sameness checks are much more basic than that.
> >
>
> I had this in mind (untested modulo compilation):
>
> diff --git a/fs/fcntl.c b/fs/fcntl.c
> index 300e5d9ad913..5723c3e82eac 100644
> --- a/fs/fcntl.c
> +++ b/fs/fcntl.c
> @@ -343,6 +343,13 @@ static long f_dupfd_query(int fd, struct file *filp)
>         return f.file == filp;
>  }
>
> +static long f_dupfd_query_inode(int fd, struct file *filp)
> +{
> +       CLASS(fd_raw, f)(fd);
> +
> +       return f.file->f_inode == filp->f_inode;
> +}
> +
>  static long do_fcntl(int fd, unsigned int cmd, unsigned long arg,
>                 struct file *filp)
>  {
> @@ -361,6 +368,9 @@ static long do_fcntl(int fd, unsigned int cmd, unsigned long arg,
>         case F_DUPFD_QUERY:
>                 err = f_dupfd_query(argi, filp);
>                 break;
> +       case F_DUPFD_QUERY_INODE:
> +               err = f_dupfd_query_inode(argi, filp);
> +               break;
>         case F_GETFD:
>                 err = get_close_on_exec(fd) ? FD_CLOEXEC : 0;
>                 break;
> diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
> index c0bcc185fa48..2e93dbdd8fd2 100644
> --- a/include/uapi/linux/fcntl.h
> +++ b/include/uapi/linux/fcntl.h
> @@ -16,6 +16,8 @@
>
>  #define F_DUPFD_QUERY  (F_LINUX_SPECIFIC_BASE + 3)
>
> +#define F_DUPFD_QUERY_INODE (F_LINUX_SPECIFIC_BASE + 4)
> +
>  /*
>   * Cancel a blocking posix lock; internal use only until we expose an
>   * asynchronous lock api to userspace:

To clarify, if indeed the dev + ino combo is no longer sufficient to
conclude it's the same thing, an explicit & handy to use way should be
provided.

For a case where you got both inodes open something like the above
will do the trick, unless I'm missing something. I can do a proper
patch posting later after runtime testing and whatnot if you confirm
this sorts out your problem.

-- 
Mateusz Guzik <mjguzik gmail.com>