linux-kernel - Re: LOOP_CONFIGURE ioctl doesn't work if lo_offset/lo

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Message-ID: <CAB0TPYHncOkecsmyypr2LACnbupfML7jn1kEdfR78rbA_w0EfQ@mail.gmail.com>
Date:   Tue, 25 Aug 2020 09:20:28 +0200
From:   Martijn Coenen <maco@...roid.com>
To:     Lennart Poettering <mzxreary@...inter.de>
Cc:     linux-block <linux-block@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Jens Axboe <axboe@...nel.dk>, Christoph Hellwig <hch@....de>,
        Yang Xu <xuyang2018.jy@...fujitsu.com>
Subject: Re: LOOP_CONFIGURE ioctl doesn't work if lo_offset/lo_sizelimit are set

Hi,

I just sent a patch to fix the issue. The loop device would have
respected the configuration, but indeed the size of the underlying
block device was not set correctly, so reading back the size would
give the wrong result.

Thanks,
Martijn

On Mon, Aug 24, 2020 at 8:24 PM Martijn Coenen <maco@...roid.com> wrote:
>
> 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@...inter.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