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

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Message-ID: <87frrsmclr.fsf@oldenburg.str.redhat.com>
Date: Mon, 29 Jul 2024 17:39:28 +0200
From: Florian Weimer <fweimer@...hat.com>
To: Jeff Layton <jlayton@...nel.org>
Cc: linux-fsdevel@...r.kernel.org,  linux-kernel@...r.kernel.org,
  linux-api@...r.kernel.org,  Dave Chinner <dchinner@...hat.com>
Subject: Re: Testing if two open descriptors refer to the same inode

* Jeff Layton:

> On Mon, 2024-07-29 at 08:55 +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.
>> Applications sometimes need to check if two (open) files are the
>> same.
>> For example, a program may want to use a temporary file if is invoked
>> with input and output files referring to the same file.
>> 
>> How can we check for this?  The POSIX way is to compare st_ino and
>> st_dev in stat output, but if inode numbers are not unique, that will
>> result in files falsely being reported as identical.  It's harmless
>> in
>> the temporary file case, but it in other scenarios, it may result in
>> data loss.
>> 
>
> I believe this is the problem that STATX_SUBVOL was intended to solve.
>
> Both bcachefs and btrfs will provide this attribute if requested. So,
> basically to uniquely ID an inode using statx, you need a tuple of:
>
> stx_dev_major/minor
> stx_subvol
> stx_ino
>
> If the filesystem doesn't provide STATX_SUBVOL, then one can (likely)
> conclude that stx_dev_* and stx_ino are enough.

Does this really work for the virtiofs case, though?  It has to pass
through all three *and* make things unique relative to the host, I
think.

Thanks,
Florian