Re: [Qemu-devel] [PATCH v3] slirp: fix ICMP handling on macOS hosts

[Andrew Oates]

On Wed, Aug 15, 2018 at 7:03 AM Samuel Thibault <address@hidden>
wrote:

> Andrew Oates, le mar. 14 août 2018 22:35:21 -0400, a ecrit:
> > On Linux, SOCK_DGRAM+IPPROTO_ICMP sockets give only the ICMP packet when
> > read from.  On macOS, however, the socket acts like a SOCK_RAW socket
> > and includes the IP header as well.
> >
> > This change strips the extra IP header from the received packet on macOS
> > before sending it to the guest.  SOCK_DGRAM ICMP sockets aren't
> > supported on other BSDs, but we enable this behavior for them as well to
> > treat the sockets the same as raw sockets.
> >
> > Signed-off-by: Andrew Oates <address@hidden>
> > ---
> > v2: check validity of inner_hlen and update len appropriately
> > v3: CONFIG_DARWIN -> CONFIG_BSD; add comment explaining #ifdef
> >
> >  slirp/ip_icmp.c | 24 +++++++++++++++++++++++-
> >  1 file changed, 23 insertions(+), 1 deletion(-)
> >
> > diff --git a/slirp/ip_icmp.c b/slirp/ip_icmp.c
> > index 0b667a429a..0e289fd9d9 100644
> > --- a/slirp/ip_icmp.c
> > +++ b/slirp/ip_icmp.c
> > @@ -420,7 +420,29 @@ void icmp_receive(struct socket *so)
> >      icp = mtod(m, struct icmp *);
> >
> >      id = icp->icmp_id;
> > -    len = qemu_recv(so->s, icp, m->m_len, 0);
> > +    len = qemu_recv(so->s, icp, M_ROOM(m), 0);
> > +    /*
> > +     * The behavior of reading SOCK_DGRAM+IPPROTO_ICMP sockets is
> inconsistent
> > +     * between host OSes.  On Linux, only the ICMP header and payload is
> > +     * included.  On macOS/Darwin, the socket acts like a raw socket and
> > +     * includes the IP header as well.  On other BSDs,
> SOCK_DGRAM+IPPROTO_ICMP
> > +     * sockets aren't supported at all, so we treat them like raw
> sockets.  It
> > +     * isn't possible to detect this difference at runtime, so we must
> use an
> > +     * #ifdef to determine if we need to remove the IP header.
> > +     */
> > +#ifdef CONFIG_BSD
> > +    if (len > 0) {
>
> Looking at it again, this used to be
>
>   if (len >= sizeof(struct ip)) {
>
> shouldn't be it that way so that
>
> > +        struct ip *inner_ip = mtod(m, struct ip *);
> > +        int inner_hlen = inner_ip->ip_hl << 2;
>
> Reading in the header doesn't give uninitialized values?
>
> I guess that to be on the safe side and trigger explicit warnings, we
> should reject with EINVAL packets which are smaller than an IP header?
>

Hmm, yeah, sorry.  That got reverted in my patch and I didn't notice.  Will
re-send.


>
> > +        if (inner_hlen > len) {
> > +            len = -1;
> > +            errno = -EINVAL;
> > +        } else {
> > +            len -= inner_hlen;
> > +            memmove(icp, (unsigned char *)icp + inner_hlen, len);
> > +        }
> > +    }
> > +#endif
> >      icp->icmp_id = id;
> >
> >      m->m_data -= hlen;
> > --
> > 2.18.0.865.gffc8e1a3cd6-goog
> >
>
> --
> Samuel
> <N> (* If you have a precise idea of the intended use of the following
> code, please
> <N>    write to address@hidden and ask for the prize :-)
> <N>    -- Eduardo (11/8/97) *)
>  -+- N sur #ens-mim - et c'était un des développeurs -+-
>