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From:   Martijn Coenen <maco@android.com>
Date:   Mon, 24 Aug 2020 20:24:05 +0200
Message-ID: <CAB0TPYEPWALD1iOP_5Rq0NKusJEvc_eOVVkoXRxjPtPTZU2GgA@mail.gmail.com>
Subject: Re: LOOP_CONFIGURE ioctl doesn't work if lo_offset/lo_sizelimit are set
To:     Lennart Poettering <mzxreary@0pointer.de>
Cc:     linux-block <linux-block@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Jens Axboe <axboe@kernel.dk>, Christoph Hellwig <hch@lst.de>,
        Yang Xu <xuyang2018.jy@cn.fujitsu.com>
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Hi Lennart,

Thanks for the report, I'll look into it. FWIW, we've been using
LOOP_CONFIGURE on Android with lo_offset/lo_sizelimit without issues,
but it may be a particular configuration that's causing issues.

Thanks,
Martijn

On Mon, Aug 24, 2020 at 5:44 PM Lennart Poettering <mzxreary@0pointer.de> wrote:
>
> Hi!
>
> Even with fe6a8fc5ed2f0081f17375ae2005718522c392c6 the LOOP_CONFIGURE
> ioctl doesn't work correctly. It gets confused if the
> lo_offset/lo_sizelimit fields are set to non-zero.
>
> In a quick test I ran (on Linux 5.8.3) I call LOOP_CONFIGURE with
> .lo_offset=3221204992 and .lo_sizelimit=50331648 and immediately
> verify the size of the block device with BLKGETSIZE64. It should of
> course return 50331648, but actually returns 3271557120. (the precise
> values have no particular relevance, it's just what I happened to use
> in my test.) If I instead use LOOP_SET_STATUS64 with the exact same
> parameters, everything works correctly. In either case, if I use
> LOOP_GET_STATUS64 insted of BLKGETSIZE64 to verify things, everything
> looks great.
>
> My guess is that the new ioctl simply doesn't properly propagate the
> size limit into the underlying block device like it should. I didn't
> have the time to investigate further though.
>
> Lennart