Re: [Xen-devel] [PATCH v2 3/4] xen: introduce __symbol

[Julien Grall <julien.grall@xxxxxxx>]

On 17/10/2018 15:31, 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 comparisons between pointers pointing to
> different objects.
The C standard forbids for both comparisons and substraction (see C 
Standard, 6.5.6 [ISO/IEC 9899:2011] and [1]). Also, it is probably worth 
to give a pointer to the paragraph in the standard to help finding more 
details.
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
> _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.
>
> Signed-off-by: Stefano Stabellini <stefanos@xxxxxxxxxx>
> CC: JBeulich@xxxxxxxx
> CC: andrew.cooper3@xxxxxxxxxx
> CC: wei.liu2@xxxxxxxxxx
> ---
> 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..850b0d0 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
> + * 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