On 2017-10-13 5:10 PM, Gleb Smirnoff wrote:
On Fri, Oct 13, 2017 at 12:59:47AM -0700, Adrian Chadd wrote:
A> >>>> When doing so m_move_pkthdr is called to copy the current PKTHDR fields
A> >>>> (tags and flags) to the mbuf that was prepended. The function also does:
A> >>>>
A> >>>> to->m_pkthdr = from->m_pkthdr;
A> >>>>
A> >>>> This, for the case I am interested in, essentially leaves the 'from'
A> >>>> mbuf
A> >>>> with a dangling pointer m_nextpkt pointing to the next fragment. While
A> >>>> this
A> >>>> is mostly harmless because only mbufs of pkthdr types are supposed to
A> >>>> have
A> >>>> m_nextpkt it triggers some panics when running with INVARIANTS in
A> >>>> NetGraph
A> >>>> (see ng_base.c :: CHECK_DATA_MBUF(m)):
A> >>>>
A> >>>> ...
A> >>>> if (n->m_nextpkt != NULL)
A> >>>> \
A> >>>> panic("%s: m_nextpkt", __func__);
A> >>>> \
A> >>>> }
A> >>>> ...
A> >>>>
A> >>>> So I would like to propose the following patch:
A> >>>>
A> >>>> @@ -442,10 +442,11 @@ m_move_pkthdr(struct mbuf *to, struct mbuf *from)
A> >>>> if ((to->m_flags & M_EXT) == 0)
A> >>>> to->m_data = to->m_pktdat;
A> >>>> to->m_pkthdr = from->m_pkthdr; /* especially tags */
A> >>>> SLIST_INIT(&from->m_pkthdr.tags); /* purge tags from src
A> >>>> */
A> >>>> from->m_flags &= ~M_PKTHDR;
A> >>>> + from->m_nextpkt = NULL;
A> >>>> }
Not only mbufs of M_PKTHDR may have m_nextpkt set. However, I tend to agree
with the patch. But shouldn't we first copy the m_nextpkt to the new mbuf:
+ to->m_nextpkt = from->m_nextpkt;
+ from->m_nextpkt = NULL;
Same way as we deal with tags.
Hi Gleb,
I think you are correct. If we look at the 'spirit' of m_move_pkthdr();
In my mind, it is to deep copy all fields related to a packet header and
since m_nextpkt should only be carried by packet headers, it makes sense
to copy it within m_move_pkthdr().
This also raises the question (my apologies in advance from bringing
this up...) of weather or not m_nextpkt belongs in struct m_hdr and not
in struct pkthdr.
In our case we are copying it explicitly outside the function as most of
users of m_move_pkthdr() do.
Thanks for looking in to this.
Karim.
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