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

[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.