Re: [Qemu-devel] [PATCH for-3.0] slirp: Correct size check in m_inc()

[Peter Maydell]

On 9 August 2018 at 12:12, Dr. David Alan Gilbert <address@hidden> wrote:
> * Peter Maydell (address@hidden) wrote:
>> diff --git a/slirp/mbuf.c b/slirp/mbuf.c
>> index 0c189e1a7bf..1b7868355a3 100644
>> --- a/slirp/mbuf.c
>> +++ b/slirp/mbuf.c
>> @@ -154,7 +154,7 @@ m_inc(struct mbuf *m, int size)
>>      int datasize;
>>
>>      /* some compilers throw up on gotos.  This one we can fake. */
>> -    if (m->m_size > size) {
>> +    if (M_ROOM(m) > size) {
>>          return;
>>      }
>
> I'm worried about a side effect of this change.
> A few lines below we have:
>
>         datasize = m->m_data - m->m_dat;
>         m->m_ext = g_malloc(size + datasize);
>         memcpy(m->m_ext, m->m_dat, m->m_size);
>         m->m_flags |= M_EXT;
>
> Question: What guarantees there's m_size room for that
> memcpy in the new m_ext?

It did take me a while to convince myself that that was true
when I was writing the patch... Here's the ASCII art:


   |--datasize---->|---m_len------->
   |----------m_size------------------------------>
                   |----M_ROOM-------------------->
                                    |-M_FREEROOM-->

   ^               ^                               ^
   m_dat           m_data                          end of buffer

("datasize" is a bit misnamed, as it's "size of the leading
gap between the start of the buffer and the data"; "gapsize"
would be more helpful.)

Anyway, we allocate size + datasize, and
m_size == datasize + M_ROOM. We know that size >= M_ROOM,
so the allocated buffer must be at least m_size big.

thanks
-- PMM