linux-kernel - Re: [PATCH v3 0/9] Add a new LOOP_SET_FD_AND

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Date:   Tue, 28 Apr 2020 16:57:21 +0200
From:   Martijn Coenen <maco@...roid.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Jens Axboe <axboe@...nel.dk>, Ming Lei <ming.lei@...hat.com>,
        Narayan Kamath <narayan@...gle.com>,
        Zimuzo Ezeozue <zezeozue@...gle.com>, kernel-team@...roid.com,
        linux-block <linux-block@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Martijn Coenen <maco@...gle.com>,
        Bart Van Assche <bvanassche@....org>,
        Chaitanya Kulkarni <Chaitanya.Kulkarni@....com>,
        Jaegeuk Kim <jaegeuk@...nel.org>
Subject: Re: [PATCH v3 0/9] Add a new LOOP_SET_FD_AND_STATUS ioctl

On Tue, Apr 28, 2020 at 9:02 AM Christoph Hellwig <hch@....de> wrote:
> I think reusing LO_FLAGS_DIRECT_IO makes sense to me - we have 32
> flags in the existing flags field (at least for loop_info64), so
> we might as well use the field and the flags.  Then we need flags
> validation in that we don't accept new flags through the old
> interface, and the new one validates that no unknown flags are passed.
>
> E.g. in the LOOP_SET_STATUS / LOOP_SET_STATUS64 handler do:
>
>         lo->lo_flags &= ~(LO_LEGACY_FLAGS);
>

mmm, I thought lo_flags was read-only in LOOP_SET_STATUS(64):

     __u32              lo_flags;                    /* ioctl r/o */

but it looks LO_FLAGS_AUTOCLEAR is writable:

 if ((lo->lo_flags & LO_FLAGS_AUTOCLEAR) !=
               (info->lo_flags & LO_FLAGS_AUTOCLEAR))
                  lo->lo_flags ^= LO_FLAGS_AUTOCLEAR;

and it allows requesting a partition scan. It makes sense to maintain
that behavior, but what about LO_FLAGS_DIRECT_IO? I think you're
proposing LOOP_SET_STATUS(64) should keep ignoring that like it used
to?

Thanks,
Martijn

> and then in the main function reject anything not known.
>
> And then maybe add something like 64 bytes of padding to the end of the
> new structure, so that we can use flags to expand to it.