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

["Wei Chen (Arm Technology China)" <Wei.Chen@xxxxxxx>]

Hi Julien,

> -----Original Message-----
> From: Julien Grall <julien.grall@xxxxxxx>
> Sent: 2018年12月14日 19:14
> To: Wei Chen (Arm Technology China) <Wei.Chen@xxxxxxx>; minios-
> devel@xxxxxxxxxxxxxxxxxxxx; simon.kuenzer@xxxxxxxxx; 
> florian.schmidt@xxxxxxxxx;
> yuri.volchkov@xxxxxxxxx; Sharan.Santhanam@xxxxxxxxx; Felipe.Huici@xxxxxxxxx
> Cc: Kaly Xin (Arm Technology China) <Kaly.Xin@xxxxxxx>; nd <nd@xxxxxxx>;
> Jianyong Wu (Arm Technology China) <Jianyong.Wu@xxxxxxx>; Justin He (Arm
> Technology China) <Justin.He@xxxxxxx>
> Subject: Re: [Minios-devel] [UNIKRAFT PATCHv3 04/25] plat/common: Do
> obfuscates arithmetic for kernel image symbols
> 
> 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?

We just hide the relationship of the pointer and the object using this MACRO.  

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

I remember I had checked with Simon, in this stage, we just need to focus on
GCC.

> 
> Cheers,
> 
> --
> Julien Grall
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