On 19 Nov 2017, at 19:49, Catalin Salgau wrote:
I'm trying to address the limitation in (upstream) net/vblade that was
brought up in
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=205164
This is related to writes larger than hw.pagesize but smaller than the
configured MTU with BPF.
I traced this to sys/net/bpf.c where calls to bpfwrite() will use
bpf_movein() which in turn uses m_get2() to allocate a single mbuf.
This
will fail if the requested mbuf size is larger than MJUMPAGESIZE
(defined as PAGE_SIZE on x86). I believe this should use m_getm2() and
populate multiple mbufs.
Code in NetBSD explicitly notes that they omit mbuf chaining, but this
is not documentated behaviour in the man page.
Your analysis looks correct.
Any chance of having this fixed in a supported release, or getting a
usable/documented workaround?
Can you see if this works for you?
diff --git a/sys/net/bpf.c b/sys/net/bpf.c
index b176856cf35..b9ff40699bb 100644
--- a/sys/net/bpf.c
+++ b/sys/net/bpf.c
@@ -547,9 +547,11 @@ bpf_movein(struct uio *uio, int linktype, struct
ifnet *ifp, struct mbuf **mp,
if (len < hlen || len - hlen > ifp->if_mtu)
return (EMSGSIZE);
- m = m_get2(len, M_WAITOK, MT_DATA, M_PKTHDR);
+ m = m_getm2(NULL, len, M_WAITOK, MT_DATA, M_PKTHDR);
if (m == NULL)
return (EIO);
+ KASSERT(m->m_next == NULL, ("mbuf chains not supported here"));
+
m->m_pkthdr.len = m->m_len = len;
*mp = m;
It’s a little icky to trust that this will produce a single mbuf
rather than a chain, but it appears to be the case. Sadly the rest of
the bpf code (and especially bpf_filter()) really needs the mbuf to have
a single contiguous buffer.
Regards,
Kristof
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