[PATCH V5] x86: Add tls check in gas

[H.J. Lu]

On Wed, Sep 25, 2024 at 10:29 PM Jan Beulich <jbeulich@suse.com> wrote:
>
> On 25.09.2024 14:45, Cui, Lili wrote:
> >> Subject: Re: [PATCH V5] x86: Add tls check in gas
> >>
> >> On 25.09.2024 05:32, Cui, Lili wrote:
> >>>> 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)
> >>
> >> So far all my questions were on the i386 side of things, which isn't covered there.
> >>
> >>> and some TLS doc linkers there (e.g.
> >>> http://www.akkadia.org/drepper/tls.pdf)
> >>
> >> This is the doc I was referring to.
> >>
> >> As an aside - of course it shouldn't really take two docs to get a complete picture.
> >
> > At least you can find " GDesc access model " in ABI doc.
>
> But not what exactly it means on i386. Are we here dealing with private,
> undocumented extensions to the TLS spec?
>
> >>>> 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.
> >>
> >> As per above - where? I can see the 64-bit counterpart of this being described in
> >> the x86-64 psABI, but that's not covering the case here.
> >>
> >>>>> +      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.
> >>
> >> Where? Please quote the sentence. I can't spot anything saying that it has to
> >> be %ebx. And similarly I can't spot anything indicating the non-SIB form would
> >> be okay to use; to the contrary it says "Note the form of the first operand of leal
> >> which forces the use of the SIB form ...". While that leaves open the use of a NOP
> >> (or address size override on the associated CALL, as per the checking the linker
> >> does) together with the non-SIB form,
> >
> > This is the place, tls.pdf is encrypted and cannot be copied. I typed a sentence.
> >
> > "To signal that this instruction is for the GNU variant of the access model the syntax @tlsgd(%ebx) is used."
> >
> > It mentioned @tlsgd(%ebx),
>
> As you say - mentions. It doesn't say that's the only valid form. Instead ...
>
> > but I think the details are not clear enough, after all, the documentation has not been updated for 10 years.
>
> ... it seems pretty obvious that if any reg other than %eax is fine for the
> non-SIB form, the same is true for the SIB one. The two forms are solely
> different encodings of the same EA.
>
> > This place needs to be confirmed with the author.
>
> First of all I'd like to understand the origin of the non-SIB form, and
> thus (hopefully) why that permits more registers than the SIB one.
>
> Jan

These assembler checks are the same as the linker checks.
Unless there is a real workload to show the linker check is
incorrect, I don't believe we should check the linker.

-- 
H.J.