[PATCH V5] x86: Add tls check in gas

[Jan Beulich]

On 27.09.2024 09:53, H.J. Lu wrote:
> On Fri, Sep 27, 2024, 3:25 PM Jan Beulich <jbeulich@suse.com> wrote:
> 
>> On 27.09.2024 01:12, H.J. Lu wrote:
>>> 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.
>>>
>>> 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.
>>
>> Here we go again - you apply personal preferences. For those APX insn
>> promotions you've insisted on us sticking to the spec. Here you say you
>> want to keep the linker as is, no matter whether it matches the spec
>> (wherever the missing parts thereof are). I'm afraid I'd like to ask
>> that you make up your mind in a consistent way: Do specs matter, or can
>> we take arbitrary liberties? My view is that in both cases we could
>> permit sensible extensions, as long as those are sufficiently clear to
>> be in line with the original intentions of the spec. In the case here
>> that of course would first mean to properly identify what the extension
>> parts are. And then any extensions of course want to be consistent with
>> the original spec as well as self-consistent.
>>
> 
> Linker and assembler are different.

Well. Of course they're different, yet that doesn't mean one needs to adhere
to specs and the other can do whatever it wants.

> Linker check was added for a reason.
> You can ask the author of such linker
> check.  He or she may remember the reason.

Have I not been doing this all the time? You're the author of 6eaa7fb59b32
("Support i386 TLS code sequences without PLT"), which switched R_386_TLS_GD
handling from any-reg to %ebx-only.

Jan