Re: [Qemu-devel] [PATCH v3 04/19] nbd/server: Hoist length check to qemp_nbd_server_add

[Eric Blake <eblake@redhat.com>]

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On 1/15/19 10:26 AM, Vladimir Sementsov-Ogievskiy wrote:

>>> @size is not size of the image, but size of the export, so it may be less than dev_offset
>>> (qemu-nbd.c do "fd_size -= dev_offset" before "nbd_export_new(bs, dev_offset, fd_size, "
>>
>> But the assert is fine because patch 3/19 fixed qemu-nbd.c to never pass
>> in dev_offset larger than size (it fails up front if dev_offset is out
>> of bounds, whether from the -o command line option or from what it read
>> from the partition header with the -P command line option).
>>
> 
> Don't follow =(
> 
> Assume, image size 3M, and we have offset 2M, i.e. -o 2M.
> 
> than in qemu-nbd.c, we have
> 
> fd_size = blk_getlength(blk); # 3M
> ...
> fd_size -= dev_offset; # 1M
> ...
> export = nbd_export_new(bs, dev_offset, fd_size # bs, 2M, 1M
> 
> in nbd_export_new:
> 
> assert(dev_offset <= size); # 2M <= 1M
> 
> fail.

Ouch, you are right. I don't need the assertion in server.c at all;
because all callers pass in a validated size, but the validated size has
no comparable relation to dev_offset.

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


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