Date: Tue, 10 Oct 2000 18:14:00 -0700 (PDT) From: Archie Cobbs <archie@whistle.com> To: Bosko Milekic <bmilekic@dsuper.net> Cc: freebsd-net@freebsd.org Subject: Re: ip_input.c patch Message-ID: <200010110114.e9B1E0J40614@bubba.whistle.com> In-Reply-To: <Pine.BSF.4.21.0010101850530.8597-100000@jehovah.technokratis.com> "from Bosko Milekic at Oct 10, 2000 07:11:05 pm"
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Bosko Milekic writes:
> > /*
> > - * Convert fields to host representation.
> > + * Convert fields to host representation. But first make
> > + * sure we don't write into a multiply-referenced mbuf.
> > */
> > + if ((m->m_flags & M_EXT) != 0 && MEXT_IS_REF(m)
> > + && (m = m_pullup(m, sizeof(*ip))) == NULL) {
>
> Assuming that you only want to attempt to pullup into a "multiply"
> referenced mbuf, this check is OK.
Right.. we only need to pullup if (a) we're going to write into
the mbuf and (b) it's multiply referenced.
> > - if (m->m_flags & M_EXT) { /* XXX */
> > + if ((m->m_flags & M_EXT) != 0 && MEXT_IS_REF(m)) {
> > if ((m = m_pullup(m, hlen)) == 0) {
> > ipstat.ips_toosmall++;
>
> How about collapsing that m_pullup into the same if() statement, to
> remain consistent with the above? The reason I'm suggesting you be picky
> about this is that those relatively repetetive extensive checks on the
> "readability" of the mbuf will likely soon be replaced ... as soon as I
> merge a few diffs ( :-) ) and it will be simpler to search and replace
> this way.
I think I'll wait for your diffs.
Uh oh.. I just thought of a plan.. :-)
1. Add a new macro to make an mbuf writable:
#define M_MKWRITABLE(m, len)
do {
if ((m)->m_len < (len)
|| (((m)->m_flags & M_EXT) != 0 && MEXT_IS_REF(m))) {
(m) = m_pullup((m), (len));
}
} while (0)
2. Temporarily change the definition of mtod() as follows:
BEFORE
------
#define mtod(m, t) ((t)((m)->m_data))
AFTER
-----
#define mtod(m, t) ((const t)((m)->m_data))
3. Compile LINT and find and fix every place that generates
an error from the const-cast in mtod():
(A) If the code doesn't need to modify the mbuf (probably
99% of the time), then change it like so:
BEFORE
------
struct ip *ip;
ip = mtod(m, struct ip *);
AFTER
------
const struct ip *ip;
ip = mtod(m, struct ip *);
(B) If the code does modify the mbuf, insert M_MKWRITABLE()
at the appropriate point.
4. Put mtod() back the way it was
-Archie
___________________________________________________________________________
Archie Cobbs * Whistle Communications, Inc. * http://www.whistle.com
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