[PATCH 5/6] aix: implement R_TOCU and R_TOCL relocations

CHIGOT, CLEMENT clement.chigot@atos.net
Mon Mar 1 09:20:26 GMT 2021


> +  if (rel->r_type == R_TOCU)
> +    *relocation = (*relocation & 0xffff0000) >> 16 ;
> 
> Please double-check this.  The doc might say "the high-order 16 bits
> of the displacement" but I suspect that isn't 100% correct.  If the
> code in your testcases is proper usage, and I expect it is, then the
> high 16 bits needs to be adjusted for the low 16 bits being signed.
> 
> You probably want:
>    *relocation = ((*relocation + 0x8000) >> 16) & 0xffff;

Yes, you're right. I wasn't aware of that. 

> +#ifdef OBJ_XCOFF
> +  /* AIX often generates addis instructions using "addis RT, D(RA)"
> +     format instead of the classic "addis RT, RA, SI" one.
> +     Restore it to the default format as it's the one encoded
> +     in ppc opcodes.  */
> +  if (!strcmp (opcode->name, "addis") && strchr (str, '(') != NULL)
> +    ppc_xcoff_fixup_addis (&str);
> +#endif
> +
> 
> Yikes, can't you tell people to fix their code?  I see so many ways
> for this to go wrong, for example, if an expression calculating an
> offset happened to use parentheses.  Also, ppc_xcoff_fixup_addis does
> no sanity checking and can easily cause buffer overflows.

Sadly no... AIX assembler wants the 3rd argument of an addis to be absolute. 
Thus, it's using a different syntax when it's not. 
To be precise, "addis RT, RA, SI" is allowed only if SI is an absolute and
"addis RT, D(RA)" is allowed only if D is a label/symbol having a relocation. 
Thus, I need to find a way to allow both in GNU as. 
I agree that my solution isn't the best one, but I didn't find any way to 
add an opcode in ppc-opc.c. 
If you have any idea how to do it properly, I would be glad. Otherwise, 
I will have to use this approach.. 

Clément  


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