BPF packet pagesize limit

[Kristof Provost]

On 21 Nov 2017, at 17:14, Catalin Salgau wrote:
> Actually m_getm2() will always produce a chain for a size larger than
> the page size, due to m_getjcl() being called with MJUMPAGESIZE every
> time a large buffer is requested. The function could probably be 
> called
> with MJUM9BYTES in this case, but this should be dependant on backing
> interface configuration(?).

I’d be tempted to just always allocate MJUM9BYTES, but that’s 
wasteful of memory.
I believe the most common use case for this code is the DHCP client, 
where large packets are not a requirement.

There doesn’t seem to be an obvious way to allocate a contiguous mbuf, 
other than allocating the memory yourself, and creating an M_EXT mbuf.
Some care must be taken to ensure the memory is correctly freed, but at 
first glance that looks possible.

> On the other hand, as you pointed out, bpf_filter really needs a 
> single
> mbuf, and so does the call to uiomove(). The filter call, as it 
> stands,
> will overread due to being passed the larger len value, instead of the
> mbuf's len.
> As a note, to avoid the overruns and related panics, I'd suggest 
> anyone
> else trying this replace the assertion with an explicit
> 	if (m->m_next != NULL) {
> 		error = EIO;
> 		goto bad;
> 	}
>
Yes, that would be better.

Regards,
Kristof