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

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Message-ID: <a13bff5812cb36adf3fed80093cbe1de601ec506.camel@kernel.org>
Date: Mon, 29 Jul 2024 11:24:41 -0400
From: Jeff Layton <jlayton@...nel.org>
To: Florian Weimer <fweimer@...hat.com>, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-api@...r.kernel.org
Cc: Dave Chinner <dchinner@...hat.com>
Subject: Re: Testing if two open descriptors refer to the same inode

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.
-- 
Jeff Layton <jlayton@...nel.org>