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Date:   Wed, 10 Oct 2018 22:51:28 +0200 (CEST)
From:   Julia Lawall <julia.lawall@lip6.fr>
To:     Joel Fernandes <joel@joelfernandes.org>
cc:     linux-kernel@vger.kernel.org,
        Gilles Muller <Gilles.Muller@lip6.fr>,
        Nicolas Palix <nicolas.palix@imag.fr>,
        Michal Marek <michal.lkml@markovi.net>, cocci@systeme.lip6.fr,
        Kees Cook <keescook@chromium.org>
Subject: Re: First coccinelle script, need some help.
In-Reply-To: <20181010204517.GA109962@joelaf.mtv.corp.google.com>
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References: <20181010193854.GA93016@joelaf.mtv.corp.google.com> <alpine.DEB.2.21.1810102217260.2479@hadrien> <20181010204517.GA109962@joelaf.mtv.corp.google.com>
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On Wed, 10 Oct 2018, Joel Fernandes wrote:

> On Wed, Oct 10, 2018 at 10:23:18PM +0200, Julia Lawall wrote:
> >
> >
> > On Wed, 10 Oct 2018, Joel Fernandes wrote:
> >
> > >
> > > Hi!
> > >
> > > I am trying to determine if a function argument is used across the whole
> > > kernel for a certain kernel function.
> > >
> > > I mustered up enough courage to write my first coccinelle script after a few
> > > late nights of reading up about it :)
> > >
> > > Here is .cocci script. I am trying to find if address is used at all in any
> > > possible definitions of pte_alloc():
> > >
> > > $ cat ~/pte_alloc.cocci
> > > virtual report
> > >
> > > @pte_args depends on report@
> > > identifier E1, E2;
> > > type T1, T2;
> > > position p;
> > > @@
> > >
> > >  pte_alloc@p(T1 E1, T2 E2)
> > >  {
> > > ...
> > > (
> > > ...
> > >  E2
> > > ...
> > > )
> > > ...
> > >  }
> >
> >
> > In report mode, by default, the pattern has to match on all paths.  Also
> > when you have ... before or after E2, there can be no occurrence of E2 in
> > the code matched by the ...  So your rule requires that on every possible
> > execution path through the function, there is exactly one occurrence of
> > E2.
> >
> > You can try the following instead:
> >
> > virtual report
> >
> > @pte_args depends on report exists@
> > identifier E1, E2;
> > type T1, T2;
> > position p;
> > @@
> >
> >  pte_alloc@p(T1 E1, T2 E2)
> >  {
> >  ... when any
> >  E2
> >  ... when any
> >  }
>
> Thanks for the quick reply.
> If I just add 'depends on report exists' to the rule, then my original
> example works fine now. I did not need to add the 'when any'. Do you mind
> taking my original simple test.c example and modify it and let me know under
> what situation would it not work?
>
> I even added address = 1 outside of the if block and it works fine, I see the
> warning as I expect without 'when any' in pront of the "...".
>
> struct page *pte_alloc(struct mm_struct *mm, unsigned long address)
> {
> 	 address = 1;
>          if (condition()) {
> 		 while (1) {
> 			address++;
> 		 }
>                 return NULL;
>         }
> }

This works, because there exists a path through the function that has only
one use of address, ie the path where condition() is false.  It should
break if you put address = 2; just under address = 1, for example.

julia