On 9 August 2018 at 12:12, Dr. David Alan Gilbert <[email protected]> wrote:
> * Peter Maydell ([email protected]) 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
