[PATCH 04/11] elf: Adjust DT_EXTRATAGIDX to avoid undefined shifts

[Florian Weimer]

* Adhemerval Zanella Netto:

> On 20/05/25 09:48, Florian Weimer wrote:
>> * Adhemerval Zanella:
>> 
>>> From: Richard Henderson <rth@twiddle.net>
>>>
>>> When building with --enable-ubsan, the relocation code triggers:
>>>
>>> UBSAN: Undefined behaviour in get-dynamic-info.h:56:30 left shift of 1879047925 by 1 cannot be represented in type 'int'
>>>
>>> Originally from
>>> https://sourceware.org/pipermail/libc-alpha/2015-August/063015.html.
>>> ---
>>>  elf/elf.h | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/elf/elf.h b/elf/elf.h
>>> index 1e1a59c14d..d48cf47b9a 100644
>>> --- a/elf/elf.h
>>> +++ b/elf/elf.h
>>> @@ -995,7 +995,7 @@ typedef struct
>>>     range.  Be compatible.  */
>>>  #define DT_AUXILIARY    0x7ffffffd      /* Shared object to load before self */
>>>  #define DT_FILTER       0x7fffffff      /* Shared object to get values from */
>>> -#define DT_EXTRATAGIDX(tag)	((Elf32_Word)-((Elf32_Sword) (tag) <<1>>1)-1)
>>> +#define DT_EXTRATAGIDX(tag)  (-((Elf32_Sword)((Elf32_Word)(tag) * 2) / 2 + 1))
>>>  #define DT_EXTRANUM	3
>>>  
>>>  /* Values of `d_un.d_val' in the DT_FLAGS entry.  */
>> 
>> Doesn't this change the type of the constant?  And rely on a GCC
>> extension?
>
> Indeed, I think an extra cast to Elf32_Word should keep the same type.
> Wrt to GCC extension, this definition is already gated through _GNU_SOURCE
> and afaik only used internally on glibc (I couldn't find any other project
> that actually uses it), so I think it should be fine to rely on it.

If it's an internal definition, should we move it into elf/include.h
instead (with a NEWS entry)?

Thanks,
Florian