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

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Message-ID: <ghqndyn4x7ujxvybbwet5vxiahus4zey6nkfsv6he3d4en6ehu@bq5s23lstzor>
Date: Mon, 29 Jul 2024 12:18:15 +0200
From: Mateusz Guzik <mjguzik@...il.com>
To: Florian Weimer <fweimer@...hat.com>
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

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'.

I know the 32-bit inode allocation code can result in unintentional
duplicates after wrap around (see get_next_ino), but that's for
in-memory stuff only(?) like pipes, so perhaps tolerable.

Anyhow, the kernel recently got F_DUPFD_QUERY which tests if the *file*
object is the same.

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?)