Date: Tue, 12 Dec 2006 05:57:56 -0800 From: Luigi Rizzo <rizzo@icir.org> To: ticso@cicely.de Cc: cvs-src@FreeBSD.org, Luigi Rizzo <luigi@FreeBSD.org>, src-committers@FreeBSD.org, imp@FreeBSD.org, cvs-all@FreeBSD.org Subject: Re: cvs commit: src/sys/net if_ethersubr.c Message-ID: <20061212055756.A61182@xorpc.icir.org> In-Reply-To: <20061212131333.GU54209@cicely12.cicely.de>; from ticso@cicely12.cicely.de on Tue, Dec 12, 2006 at 02:13:34PM %2B0100 References: <200612081036.kB8AakMD029277@repoman.freebsd.org> <20061212131333.GU54209@cicely12.cicely.de>
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On Tue, Dec 12, 2006 at 02:13:34PM +0100, Bernd Walter wrote:
> On Fri, Dec 08, 2006 at 10:36:46AM +0000, Luigi Rizzo wrote:
> > luigi 2006-12-08 10:36:45 UTC
> >
> > FreeBSD src repository
> >
> > Modified files:
> > sys/net if_ethersubr.c
> > Log:
> > Fix an oscure bug triggered by a recent change in kern_socket.c.
> > The symptoms were that outgoing DHCP requests for diskless kernels
> > had the IP header corrupt. After long investigations, the source of
> > the problem was found in ether_output() - for SIMPLEX interfaces
> > and broadcast traffic, a copy of the packet is passed back to the kernel
> > through if_simloop(). However if_simloop() modifies the mbuf, while
> > the copy obtained through m_copym() is a readonly one.
> >
> > The bug has been there forever, but it has been triggered only recently
> > by a change in sosend_dgram() which passed down mbufs with sufficient
> > space to prepend the header.
> >
> > This fix is trivial - use m_dup() instead of m_copy() to create
> > the copy. As an alternative, we could try and modify if_simloop()
> > to play safely with readonly mbufs, but i don't think it is worthwhile
> > because 1) this is a relatively infrequent code path so we do not need
> > to worry too much about performance, and 2) the cost of doing an
> > extra m_pullup in if_simloop() is probably the same as doing the
> > copy of the cluster, anyways.
>
> This change produces an alignment panic on arm.
> Reverting it gets my system back to live.
then i suppose the proper fix is to revert to m_copy() and
work on if_simloop() so that 1. it handles a readonly chain, and
2. when doing so, it passes up a properly aligned packet...
however note that there is already some code in net/if_loop.c::if_simloop(),
just that it uses this:
#if defined(__ia64__) || defined(__sparc64__)
/*
* Some archs do not like unaligned data, so
* we move data down in the first mbuf.
*/
if (mtod(m, vm_offset_t) & 3) {
KASSERT(hlen >= 3, ("if_simloop: hlen too small"));
bcopy(m->m_data,
(char *)(mtod(m, vm_offset_t)
- (mtod(m, vm_offset_t) & 3)),
m->m_len);
m->m_data -= (mtod(m,vm_offset_t) & 3);
}
#endif
to detect whether the architecture is alignment-sensitive.
Is there any other identifier that we can use to check ?
cheers
luigi
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