Re: [Xen-devel] [PATCH v3 3/4] xen: introduce SYMBOL

[Julien Grall <julien.grall@xxxxxxx>]

Hi Stefano,

On 06/11/2018 22:05, Stefano Stabellini wrote:
> Introduce a macro, SYMBOL, which is a simple wrapper around RELOC_HIDE
> to be used everywhere symbols such as _stext and _etext are used in the
> code.
>
> RELOC_HIDE is needed when accessing symbols such as _stext and _etext
> because the C standard forbids for both comparisons and substraction
> (see C Standard, 6.5.6 [ISO/IEC 9899:2011] and [1]).
>
> forbids comparisons between pointers pointing to
> different objects. _stext, _etext, etc. are all pointers to different
> objects from ANCI C point of view.
>
> To work around potential C compiler issues (which have actually
> been found, see the comment on top of RELOC_HIDE in Linux), and to help
> with certifications, let's introduce some syntactic sugar to be used in
> following patches.
>
> [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
>
> Signed-off-by: Stefano Stabellini <stefanos@xxxxxxxxxx>
> CC: JBeulich@xxxxxxxx
> CC: andrew.cooper3@xxxxxxxxxx
> CC: wei.liu2@xxxxxxxxxx
> ---
> Changes in v3:
> - improve commit message
> - rename __symbol to SYMBOL to avoid name space violations
>
> Changes in v2:
> - do not cast return to char*
> - move to common header
> ---
>   xen/include/xen/compiler.h | 6 ++++++
>   1 file changed, 6 insertions(+)
>
> diff --git a/xen/include/xen/compiler.h b/xen/include/xen/compiler.h
> index ff6c0f5..b3375f6 100644
> --- a/xen/include/xen/compiler.h
> +++ b/xen/include/xen/compiler.h
> @@ -99,6 +99,12 @@
>       __asm__ ("" : "=r"(__ptr) : "0"(ptr));      \
>       (typeof(ptr)) (__ptr + (off)); })
>   
> +/*
> + * Use RELOC_HIDE with symbols such as _stext and _etext to avoid errors
It is not clear what "errors" means here. I think it would be better to use 
"undefined behavior" instead.
With that:

Acked-by: Julien Grall <julien.grall@xxxxxxx>

Cheers,

> + * on comparing pointers to different objects
> + */
> +#define SYMBOL(x)         (RELOC_HIDE((unsigned long)(x), 0))
> +
>   #ifdef __GCC_ASM_FLAG_OUTPUTS__
>   # define ASM_FLAG_OUT(yes, no) yes
>   #else
--
Julien Grall

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel