Re: [iptables PATCH] extensions: string: Clarify description of --to

[Pablo Neira Ayuso <pablo@netfilter.org>]

On Thu, Oct 12, 2023 at 09:47:38PM +0200, Pablo Neira Ayuso wrote:
> On Thu, Oct 12, 2023 at 06:08:42PM +0200, Phil Sutter wrote:
> > String match indeed returns a match as long as the given pattern starts
> > in the range of --from and --to, update the text accordingly.
> > Also add a note regarding fragment boundaries.
> >
> > Closes: https://bugzilla.netfilter.org/show_bug.cgi?id=1707
> > Signed-off-by: Phil Sutter <phil@nwl.cc>
> > ---
> >  extensions/libxt_string.man | 8 ++++++--
> >  1 file changed, 6 insertions(+), 2 deletions(-)
> > 
> > diff --git a/extensions/libxt_string.man b/extensions/libxt_string.man
> > index 2a470ece19c9d..efdda492ae78d 100644
> > --- a/extensions/libxt_string.man
> > +++ b/extensions/libxt_string.man
> > @@ -7,9 +7,13 @@ Select the pattern matching strategy. (bm = Boyer-Moore, kmp = Knuth-Pratt-Morri
> >  Set the offset from which it starts looking for any matching. If not passed, default is 0.
> >  .TP
> >  \fB\-\-to\fP \fIoffset\fP
> > -Set the offset up to which should be scanned. That is, byte \fIoffset\fP-1
> > -(counting from 0) is the last one that is scanned.
> > +Set the offset up to which should be scanned. If the pattern does not start
> > +within this offset, it is not considered a match.
> >  If not passed, default is the packet size.
> > +A second function of this parameter is instructing the kernel how much data
> > +from the packet should be provided. With non-linear skbuffs (e.g. due to
> > +fragmentation), a pattern extending past this offset may not be found. Also see
> > +the related note below about Boyer-Moore algorithm in these cases.
> 
> Then, matching on:
> 
> - linear skbuff: if the pattern falls within from-to, all good.
> - non-linear skbuff: if pattern falls within from-to, but remaining
>   patter
> 
> This is clearly broken, the fix is just to document this?
> 
> No attempt to fix it this from the kernel?

I would align linear skbuff behaviour to how non-linear skbuff works,
that is, pattern matching stops at to. This requires a small patch for
xt_string.