linux-kernel - Re: [PATCH 2/4] configfs: Fix writing at a non-zero offset

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Date:   Mon, 26 Jul 2021 20:17:33 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Bodo Stroesser <bostroesser@...il.com>,
        Christoph Hellwig <hch@....de>
Cc:     Joel Becker <jlbec@...lplan.org>, linux-kernel@...r.kernel.org,
        "Martin K . Petersen" <martin.petersen@...cle.com>,
        Yanko Kaneti <yaneti@...lera.com>,
        Brendan Higgins <brendanhiggins@...gle.com>
Subject: Re: [PATCH 2/4] configfs: Fix writing at a non-zero offset

On 7/26/21 5:54 PM, Bodo Stroesser wrote:
> The new behavior can also cause trouble with existing store handlers.
> Example:
> The tcmu attribute files cmd_time_out and qfull_time_out just take a
> string containing the decimal formatted number of seconds of the
> timeout. Each number up to now had to be transferred in a single write.
> Assume the old value is 30 and we want to change to 19. If userspace
> writes byte by byte, you end up calling
> store(item, "1\0", 1) and then
> store(item, "19\9", 2).
> If these quick changes do not cause trouble in tcmu's scsi cmd handling,
> then think what happens, if userspace is interrupted between the two
> writes. Allowing to split the writes cause a loss of "atomicity".

>From Documentation/filesystems/configfs.rst, for normal attributes:
"Configfs expects write(2) to store the entire buffer at once." In other
words, the behavior for partial writes is undocumented. My changes
preserve the behavior if a buffer is written in its entirety. I do not
agree that my changes can cause trouble for existing store handlers.

Bart.