[PATCH V5] x86: Add tls check in gas

[Cui, Lili]

> I've grabbed the newest tls.pdf I could find (still over 10 years old, from Ulrich
> Drepper's site as referenced by the Thread-Local Storage section in gcc14 doc),
> and that raises questions:
> 

Jan,  you can find the latest TLS in ABI chapter 10 (https://gitlab.com/x86-psABIs/x86-64-ABI) and some TLS doc linkers there (e.g. http://www.akkadia.org/drepper/tls.pdf)

> On 19.09.2024 08:38, Cui, Lili wrote:
> > @@ -6357,6 +6387,356 @@ static INLINE bool may_need_pass2 (const
> insn_template *t)
> >  	       && (t->base_opcode | 8) == 0x2c);  }
> >
> > +static enum x86_tls_error_type
> > +x86_check_tls_relocation (enum bfd_reloc_code_real r_type) {
> > +  switch (r_type)
> > +    {
> > +    case BFD_RELOC_386_TLS_GOTDESC:
> > +      /* Check GDesc access model:
> 
> What's "GDesc access model"? I can't find anything named like this, nor mention
> of ...
> > +	 leal x@tlsdesc(%ebx), %reg32 --> Memory reg must be %ebx and
> > +					  SIB is not supported.
> > +       */
> 
> ... @tlsdesc.
> 

It is defined in ABI doc.

> > +      if (i.tm.mnem_off != MN_lea)
> > +	return x86_tls_error_insn;
> > +      if (i.index_reg)
> > +	return x86_tls_error_sib;
> > +      if (!i.base_reg)
> > +	return x86_tls_error_no_base_reg;
> > +      if (i.base_reg->reg_type.bitfield.instance != RegB)
> > +	return x86_tls_error_ebx;
> > +      if (!i.op[1].regs->reg_type.bitfield.dword)
> > +	return x86_tls_error_dest_32bit_reg_size;
> > +      break;
> > +
> > +    case BFD_RELOC_386_TLS_GD:
> > +      /* Check GD access model:
> > +
> > +	 leal foo@tlsgd(,%ebx,1), %eax   --> Only this fixed format is supported.
> > +	 leal foo@tlsgd(%reg32), %eax    --> Dest reg must be '%eax'
> > +					     Memory reg can't be %eax.
> > +       */
> 
> Where's this 2nd form coming from? The doc says the SIB form must be used,
> and the converted-to code sequence would require one more byte than the non-
> SIB form provides space for.
> 
> The doc doesn't say that the index reg has to be %ebx, it merely mentions that in
> the example, for being the commonly used GOT reg.
> This is the same ...
> 

It is mentioned in 4.1 of tls.pdf.

> > +      if (i.tm.mnem_off != MN_lea)
> > +	return x86_tls_error_insn;
> > +      if (i.op[1].regs->reg_type.bitfield.instance != Accum)
> > +	return x86_tls_error_dest_eax;
> > +      if (!i.op[1].regs->reg_type.bitfield.dword)
> > +	return x86_tls_error_dest_32bit_reg_size;
> > +      if (i.index_reg)
> > +	{
> > +	  if (i.base_reg)
> > +	    return x86_tls_error_base_reg;
> > +	  if (i.index_reg->reg_type.bitfield.instance != RegB)
> > +	    return x86_tls_error_index_ebx;
> > +	  if (i.log2_scale_factor)
> > +	    return x86_tls_error_scale_factor;
> > +	}
> > +      else
> > +	{
> > +	  if (!i.base_reg)
> > +	    return x86_tls_error_no_base_reg;
> > +	  if (i.base_reg->reg_type.bitfield.instance == Accum)
> > +	    return x86_tls_error_eax;
> > +	}
> > +      break;
> > +
> > +    case BFD_RELOC_386_TLS_LDM:
> > +      /*  Check LDM access model:
> > +
> > +	  leal foo@tlsldm(%reg32), %eax --> Dest reg must be '%eax'
> > +				            Memory reg can't be %eax and SIB
> > +					    is not supported.
> > +       */
> 
> ... for this case, and surprisingly any register is permitted here.
> 

There are two types of sequence listings in linker comments:

          /* Check transition from LD access model.  Only
                leal foo@tlsldm(%ebx), %eax
                call ___tls_get_addr@PLT
             or
                leal foo@tlsldm(%reg), %eax
                call *___tls_get_addr@GOT(%reg)
                which may be converted to
                addr32 call ___tls_get_addr
             can transit to different access model.  */

They may be related to 4.2 of tls.pdf.

Thanks,
Lili.

> I'll stop here for now, as this is all unclear enough already. I'm getting the
> impression that gas is being made match ld without regard to whether ld is
> actually correct. Checks in gas - if we already have any - should be tied to what
> the spec demands, not how a particular linker is implemented. As said before -
> people may be using different linkers.
> 
> Jan