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

[Florian Weimer <fweimer@redhat.com>]

* Mateusz Guzik:

> 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?

Sorry, it was an internal thread.  It's supposed to be common knowledge
among Linux file system developers.  Aleksa referenced LSF/MM
discussions.

> 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:

It's certainly much easier to use than name_to_handle_at, so it looks
like a useful option to have.

Could we return a three-way comparison result for sorting?  Or would
that expose too much about kernel pointer values?

Thanks,
Florian