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Date:   Sun, 1 Oct 2023 09:45:41 +0200 (CEST)
From:   Julia Lawall <julia.lawall@inria.fr>
To:     Kees Cook <keescook@chromium.org>
cc:     Christophe JAILLET <christophe.jaillet@wanadoo.fr>,
        Ian Abbott <abbotti@mev.co.uk>,
        H Hartley Sweeten <hsweeten@visionengravers.com>,
        "Gustavo A. R. Silva" <gustavoars@kernel.org>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Tom Rix <trix@redhat.com>, linux-kernel@vger.kernel.org,
        kernel-janitors@vger.kernel.org, linux-hardening@vger.kernel.org,
        llvm@lists.linux.dev
Subject: Re: [PATCH] comedi: Annotate struct comedi_lrange with
 __counted_by
In-Reply-To: <202309301342.5B5BED40A1@keescook>
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References: <5c3b7459b820e22e2ac6ce892d4aadcc119cc919.1696065263.git.christophe.jaillet@wanadoo.fr> <202309301342.5B5BED40A1@keescook>
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On Sat, 30 Sep 2023, Kees Cook wrote:

> On Sat, Sep 30, 2023 at 11:14:47AM +0200, Christophe JAILLET wrote:
> > Prepare for the coming implementation by GCC and Clang of the __counted_by
> > attribute. Flexible array members annotated with __counted_by can have
> > their accesses bounds-checked at run-time checking via CONFIG_UBSAN_BOUNDS
> > (for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
> > functions).
> >
> > Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
> > ---
> > This patch is part of a work done in parallel of what is currently worked
> > on by Kees Cook.
> >
> > My patches are only related to corner cases that do NOT match the
> > semantic of his Coccinelle script[1].
>
> Nice!
>
> struct comedi_lrange {
>         int length;
>         struct comedi_krange range[];
> };
> ...
> static const struct comedi_lrange range_rti800_ai_10_bipolar = {
>         4, {
>                 BIP_RANGE(10),
>                 BIP_RANGE(1),
>                 BIP_RANGE(0.1),
>                 BIP_RANGE(0.02)
>         }
> };
>
> I'm struggling to come up with a way for Coccinelle to find this kind of
> thing in other places...

Kees, what exactly are you trying to match?  Static allocations?

julia

>
> > In this case, it is been spotted because of comedi_alloc_spriv().
> > All other usages of struct comedi_lrange seem to be static definition of
> > the structure that explicitly set the .length field.
>
> Ah-ha, I found it in drivers/comedi/drivers/das16.c das16_ai_range():
>
>                 lrange = comedi_alloc_spriv(s,
>                                             struct_size(lrange, range, 1));
>
> I was also able to find this:
>
> union jr3_pci_single_range {
>         struct comedi_lrange l;
>         char _reserved[offsetof(struct comedi_lrange, range[1])];
> };
>
> Which looks a lot like DEFINE_FLEX:
> https://lore.kernel.org/linux-hardening/20230912115937.1645707-2-przemyslaw.kitszel@intel.com/
> But that above for stack varaibles rather than globals. But I'm way off
> topic now. ;)
>
> Reviewed-by: Kees Cook <keescook@chromium.org>
>
> >
> > [1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci
> > ---
> >  include/linux/comedi/comedidev.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/include/linux/comedi/comedidev.h b/include/linux/comedi/comedidev.h
> > index 0a1150900ef3..c08416a7364b 100644
> > --- a/include/linux/comedi/comedidev.h
> > +++ b/include/linux/comedi/comedidev.h
> > @@ -633,7 +633,7 @@ extern const struct comedi_lrange range_unknown;
> >   */
> >  struct comedi_lrange {
> >  	int length;
> > -	struct comedi_krange range[];
> > +	struct comedi_krange range[] __counted_by(length);
> >  };
> >
> >  /**
> > --
> > 2.34.1
> >
>
> --
> Kees Cook
>