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:

quoted

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