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Re: [Qemu-block] [Qemu-devel] [PATCH 4/4] nbd/client: Deal with unaligne

From: Eric Blake
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH 4/4] nbd/client: Deal with unaligned size from server
Date: Fri, 17 Aug 2018 10:01:10 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1

On 08/17/2018 08:57 AM, Vladimir Sementsov-Ogievskiy wrote:
02.08.2018 17:48, Eric Blake wrote:
When a server advertises an unaligned size but no block sizes,
the code was rounding up to a sector-aligned size (a known
limitation of bdrv_getlength()), then assuming a request_alignment
of 512 (the recommendation of the NBD spec for maximum portability).
However, this means that qemu will actually attempt to access the
padding bytes of the trailing partial sector.

An easy demonstration, using nbdkit as the server:
$ nbdkit -fv random size=1023
$ qemu-io -r -f raw -c 'r -v 0 1023' nbd://localhost:10809
read failed: Invalid argument

because the client rounded the request up to 1024 bytes, which
nbdkit then rejected as beyond the advertised size of 1023.

Note that qemu as the server refuses to send an unaligned size, as
it has already rounded the unaligned image up to sector size, and
then happily resizes the image on access (at least when serving a
POSIX file over NBD).

Reported-by: Richard W.M. Jones <address@hidden>
Signed-off-by: Eric Blake <address@hidden>

Reviewed-by: Vladimir Sementsov-Ogievskiy <address@hidden>

This patch is not a full solution. It fixes things so that a client accessing the first half of the final sector no longer rounds things up and chokes the server, but does not prevent a client from attempting to access the second half of the final sector (where that access still reaches the server). I probably need yet another patch, similar to Rich's 'nbdkit --filter truncate', where reads of the trailing hole created by qemu's rounding are padded to NUL without asking the server, and where writes are ignored if all zero or cause ENOSPACE if nonzero.

Or, a much bigger patch series to make qemu quit rounding size up :) (I'd like to get there someday, but it's faster to kick out the quick patch for just NBD than to audit the entire block stack)

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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