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

[Phil Sutter <phil@nwl.cc>]

On Thu, Oct 12, 2023 at 10:14:31PM +0200, Pablo Neira Ayuso wrote:
> 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.

Sounds reasonable. My efforts at documenting the current behaviour
probably show how unintuitive this currently is.

Thanks for the suggestion!

Cheers, Phil