Re: [PATCH] comedi: Annotate struct comedi_lrange with __counted_by

[Kees Cook]

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@xxxxxxxxxx>
> ---
> 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...

> 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@xxxxxxxxx/
But that above for stack varaibles rather than globals. But I'm way off
topic now. ;)

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

> 
> [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