From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id ABDEC154C0C; Mon, 29 Jul 2024 15:24:44 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1722266684; cv=none; b=Ax5r0NgDRWzDA60z8WMiRTtbbf2cxYYHwKMG49/UymgVeRagz32F5a2cXgN05KDxPXJWxUJIjZ2Ro0YkJ7pgcdrdKL2KaI3L2FqhUoGAa6qzoYq3aj6KvNHHj8D6B0HsYEv1nZDThuO9eIJuiKX3lhMeQ5PDzK3dVvEk5vO3Y4A= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1722266684; c=relaxed/simple; bh=IV6VxQVQk69smlu9QxmbULLkxhEzAF8V7DvqmyX572Q=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=SGIZ8c2SM3vIk3s7wnhCuV+0eDuTBvZNWdhRJJqsoWsM32LHlcP5HlYKY5p8WllwFMd6E+HcSo9GEr6tuuRQtd5POo1VY7f51DVegQwmFV3aZJiRSUQdPK+An3uCUjY8S97ZM6+jSQAAAWyt2d5yqVaLFNRyWUvOB/wGCS5kjro= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=UL+GnERz; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="UL+GnERz" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 86633C32786; Mon, 29 Jul 2024 15:24:43 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1722266684; bh=IV6VxQVQk69smlu9QxmbULLkxhEzAF8V7DvqmyX572Q=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=UL+GnERz173cSufLEisaQT3icbwJoxwGungiPtZC1NN1YSQ2vx2Ln9/uA1e7FSzL6 zMBwIpw4i3nAWc89xMxcn+ZqXGKf16J9BrFUwQYfXCt1OT9inUf+ZVBCGijYvkqiYs MqK36+EqEZ8Y9NODYlfvR58jhbqdppx4DJNrOUR08NOMRpffldamW/3luwBPnvuYov SA28THmPqB4kc2cbY9GctqhhDl+yJX1CC6Zb9PGkvdfllmQUoQs8RJU1GtpNjAD6it qiTI1xuTPP2Sp/HlkTwWZwFfq0ru0pe876qR5tOoaTh+E8cfCFejdy9dSxzhmJcAVp 5fcEpTWm/RdZg== Message-ID: Subject: Re: Testing if two open descriptors refer to the same inode From: Jeff Layton To: Florian Weimer , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-api@vger.kernel.org Cc: Dave Chinner Date: Mon, 29 Jul 2024 11:24:41 -0400 In-Reply-To: <874j88sn4d.fsf@oldenburg.str.redhat.com> References: <874j88sn4d.fsf@oldenburg.str.redhat.com> Autocrypt: addr=jlayton@kernel.org; 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charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.50.4 (3.50.4-1.fc39) Precedence: bulk X-Mailing-List: linux-api@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 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. >=20 > How can we check for this?=C2=A0 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.=C2=A0 It's harmless > in > the temporary file case, but it in other scenarios, it may result in > data loss. >=20 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. --=20 Jeff Layton