Re: [Minios-devel] [UNIKRAFT PATCH 03/22] plat/kvm: Use the same declaration for _text and _end for Arm64

[Julien Grall <julien.grall@xxxxxxx>]

On 09/11/2018 06:52, Wei Chen (Arm Technology China) wrote:
> Hi Julien,
Hi,

> > -----Original Message-----
> > From: Julien Grall <julien.grall@xxxxxxx>
> > Sent: 2018年11月8日 20:02
> > To: Wei Chen (Arm Technology China) <Wei.Chen@xxxxxxx>; minios-
> > devel@xxxxxxxxxxxxxxxxxxxx; simon.kuenzer@xxxxxxxxx
> > Cc: Kaly Xin (Arm Technology China) <Kaly.Xin@xxxxxxx>; nd <nd@xxxxxxx>
> > Subject: Re: [Minios-devel] [UNIKRAFT PATCH 03/22] plat/kvm: Use the same
> > declaration for _text and _end for Arm64
> >
> > Hi,
> >
> > On 11/8/18 9:51 AM, Wei Chen wrote:
> > > We get a warning that _end and _text are differently defined within
> > > setup.c and memory.c:
> > >
> > > warning: type of ‘_end’ does not match original declaration
> > >         extern char _end[];
> > >                     ^
> > > note: previously declared here
> > >              _ctors, _ectors, __bss_start, _end;
> > >
> > > We should use the same definition  in both files and then this warning
> > > goes away.
> > That's a call to have the _end and _text defined in an header.
> Yes, it would be better to have a clean-up patch to do this.
>
> > Also, keep in mind that you can't compare two pointers that points to a
> I don't quite understand this comment. We treat all these pointers as char
> pointers. Why did you think they are different?
Whether the pointers are treated as char * or anything else does not matter 
here. The problem is the C spec states that comparing 2 pointers belonging to 
distinct object is undefined (see [1] and [2]).
So check such as "mem_base > (uint64_t)&_text" is undefined because the compiler 
could see-through the cast. So you have to hide it from the compiler (see 
__pa_symbol in Linux).
> > different object. So you may want to find a different way to access _end
> > & co.
> Sorry, what's "co" here?
"and company" by that I meant all the other cases similar to _end.

Cheers,

[1] 
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
[2] 6.5.8 http://www.dii.uchile.cl/~daespino/files/Iso_C_1999_definition.pdf

--
Julien Grall

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