linux-kernel - Re: API break, sysfs "capability" file

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Message-ID: <54e3c969-3ee8-40d8-91d9-9b9402001d27@leemhuis.info>
Date: Mon, 8 Apr 2024 19:43:04 +0200
From: "Linux regression tracking (Thorsten Leemhuis)"
 <regressions@...mhuis.info>
To: Christoph Hellwig <hch@....de>
Cc: Lennart Poettering <lennart@...ttering.net>, linux-block@...r.kernel.org,
 LKML <linux-kernel@...r.kernel.org>,
 Linux kernel regressions list <regressions@...ts.linux.dev>,
 Jens Axboe <axboe@...nel.dk>
Subject: Re: API break, sysfs "capability" file

[adding the culprit's author to the loop; also CCing everyone else in
the Signed-off-by chain and a few lists that should be in the loop, too]

On 08.04.24 17:13, Lennart Poettering wrote:
> 
> So this broke systemd:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=e81cd5a983bb35dabd38ee472cf3fea1c63e0f23

FWIW, that is e81cd5a983bb35 ("block: stub out and deprecated the
capability attribute on the gendisk") [v6.3-rc1]

> We use the "capability" sysfs attr to figure out if a block device has
> part scanning enabled or not. There seems to be no other API for
> this. (We also use it in our test suite to see if devices match are
> expectations, and older systemd/udev versions used to match agains it
> from udev rules.)
> 
> The interface was part of sysfs, and documented:
> 
> https://www.kernel.org/doc/html/v5.5/block/capability.html
> 
> While it doesn't list the partscan bit it actually does document that
> one is supposed to look into include/linux/genhd.h for the various
> bits and their meanings. I'd argue that makes them API to some level.
> 
> Could this please be reverted? Just keeping the relevant bits (i.e. at
> least the media change feature bit, and the part scanning bit) is
> enough for retaining userspace compat.
> 
> (Please consider googling or a github code search or so before removing
> a public API like this. This compat breakage was very much avoidable
> with a tiny bit of googling.)

Thanks for the report. To be sure the issue doesn't fall through the
cracks unnoticed, I'm adding it to regzbot, the Linux kernel regression
tracking bot:

#regzbot ^introduced e81cd5a983bb3
#regzbot title block: sysfs "capability" file broke systemd's checking
for part scanning
#regzbot ignore-activity

Ciao, Thorsten (wearing his 'the Linux kernel's regression tracker' hat)
--
Everything you wanna know about Linux kernel regression tracking:
https://linux-regtracking.leemhuis.info/about/#tldr
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