Re: [Qemu-devel] [PULL 10/14] nbd/client: Reject inaccessible tail of inconsistent server

From: Eric Blake <eblake@redhat.com>
To: qemu-devel@nongnu.org
Cc: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>,
	"open list:Network Block Dev..." <qemu-block@nongnu.org>
Subject: Re: [Qemu-devel] [PULL 10/14] nbd/client: Reject inaccessible tail of inconsistent server
Date: Tue, 2 Apr 2019 17:40:47 -0500	[thread overview]
Message-ID: <a7d73404-3b61-22f5-626d-6c4791c609b0@redhat.com> (raw)
In-Reply-To: <20190401140903.19186-11-eblake@redhat.com>

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On 4/1/19 9:08 AM, Eric Blake wrote:
> The NBD spec suggests that a server should never advertise a size
> inconsistent with its minimum block alignment, as that tail is
> effectively inaccessible to a compliant client obeying those block
> constraints. Since we have a habit of rounding up rather than
> truncating, to avoid losing the last few bytes of user input, and we
> cannot access the tail when the server advertises bogus block sizing,
> abort the connection to alert the server to fix their bug.  And
> rejecting such servers matches what we already did for a min_block
> that was not a power of 2 or which was larger than max_block.
> 
> Does not impact either qemu (which always sends properly aligned
> sizes) or nbdkit (which does not send minimum block requirements yet);
> so this is mostly aimed at new NBD server implementations, and ensures
> that the rest of our code can assume the size is aligned.
> 
> Signed-off-by: Eric Blake <eblake@redhat.com>
> Message-Id: <20190330155704.24191-1-eblake@redhat.com>
> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>  nbd/client.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/nbd/client.c b/nbd/client.c
> index de7da48246b..427980bdd22 100644
> --- a/nbd/client.c
> +++ b/nbd/client.c
> @@ -426,6 +426,14 @@ static int nbd_opt_info_or_go(QIOChannel *ioc, uint32_t opt,
>                  nbd_send_opt_abort(ioc);
>                  return -1;
>              }
> +            if (info->min_block &&
> +                !QEMU_IS_ALIGNED(info->size, info->min_block)) {
> +                error_setg(errp, "export size %" PRIu64 "is not multiple of "

Missing a space in the error message. If I have any other patches worthy
of an -rc3 pull request, I'll do the obvious fix then.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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