Re: [Minios-devel] [UNIKRAFT PATCHv3 04/25] plat/common: Do obfuscates arithmetic for kernel image symbols

[Julien Grall <julien.grall@xxxxxxx>]

Hi,

On 13/12/2018 09:15, Wei Chen wrote:
> As we discussed in [1], the C spec states that comparing 2 pointers
> belonging to distinct object is undefined [2]. In this case, we defined
> macros to wrap kernel image symbols. they  will make GCC can not
> recognize varirable's original type, and do further assumptions for it.
>
> [1] https://lists.xen.org/archives/html/minios-devel/2018-11/msg00054.html
> [2] 
> https://wiki.sei.cmu.edu/confluence/display/c/ARR36-C.+Do+not+subtract+or+compare+two+pointers+that+do+not+refer+to+the+same+array
>
> Signed-off-by: Wei Chen <wei.chen@xxxxxxx>
> ---
>   plat/common/include/sections.h | 32 ++++++++++++++++++++++++++++++++
>   1 file changed, 32 insertions(+)
>
> diff --git a/plat/common/include/sections.h b/plat/common/include/sections.h
> index 42f41d2..b32389c 100644
> --- a/plat/common/include/sections.h
> +++ b/plat/common/include/sections.h
> @@ -62,4 +62,36 @@ extern char __bss_start[];
>   /* _end: end of kernel image */
>   extern char _end[];
>   
> +
> +/* The C spec states that comparing 2 pointers belonging to distinct object is 
> undefined:
> + * 
> https://wiki.sei.cmu.edu/confluence/display/c/ARR36-C.+Do+not+subtract+or+compare+two+pointers+that+do+not+refer+to+the+same+array
> + *
> + * In this case, we define following macro to access above symbols. This will
> + * help us to avoid subtracting or comparing these symbols as distinct 
> pointers.
> + *
> + * This macro hide the original variable for a variable address, it will make
> + * GCC couldn't recognize varirable's original type, and do further assumptions
s/varirable's/variable/

> + * for it.
> + */
> +#define HIDE_VAR_FOR_GCC(var, ofs)     \
> +({                                     \
> +       unsigned long __var;            \
> +       __var = (unsigned long) (var);  \
> +       (typeof(var))(__var + (ofs));   \
> +})
I don't understand how this avoids GCC to "see-through". Can you explain it?

Also, why is it GCC specific? How about compiler like Clang?

Cheers,

--
Julien Grall

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