Re: [Qemu-devel] [PATCH v3 05/19] nbd/server: Favor [u]int64_t over off_t

[Eric Blake <eblake@redhat.com>]

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On 1/16/19 2:23 AM, Vladimir Sementsov-Ogievskiy wrote:

> 
> Interesting, decided to search a bit, about don't we have an overflow,
> when adding request.offset to exp.dev_offset and found in nbd_co_receive_request:
> 
>      if (request->from > client->exp->size ||
>          request->from + request->len > client->exp->size) {

uint64_t + uint32_t > uint64_t

> 
> shouldn't it be
> 
>      if (request->from > client->exp->size ||
>          request->len > client->exp->size - request->from) {
> 
> to avoid overflow?

You are correct that the canonical way to check for overflow is to
compare against subtraction, rather than do addition before compare.
But we got lucky: even though client->exp->size is uint64_t, in practice
it can only represent at most off_t (remember, off_t is signed), so the
first leg of the branch proves that request->from fits in 63 bits, and
thus the second is performing 63-bit + 32-bit which can be at most 64
bits, and therefore does not overflow.  Still, I might rewrite it.

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


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