[PATCH v2] x86: Disallow GOT memory as vector memory operand

[Jan Beulich]

On 07.02.2025 06:22, H.J. Lu wrote:
> On Thu, Feb 6, 2025 at 9:02 PM Jan Beulich <jbeulich@suse.com> wrote:
>> On 06.02.2025 04:32, H.J. Lu wrote:
>>> GOT memory is 4-byte for i386 and 8-byte for x86-64.  Disallow GOT memory
>>> as vector memory operand for vector instructions which don't support 32-bit
>>> nor 64-bit memory.
>>>
>>> PR gas/32624
>>> * config/tc-i386.c (output_disp): Disallow GOT memory as vector
>>> memory operand.
>>> * testsuite/gas/i386/got.s: Add vcvtph2qq with GOT memory.
>>> * testsuite/gas/i386/x86-64-gotpcrel-2.s: Add vcvtph2qq and
>>> vcvtps2pd with GOT memory.
>>> * testsuite/gas/i386/got-no-relax.d: Updated.
>>> * testsuite/gas/i386/got.d: Likewise.
>>> * testsuite/gas/i386/x86-64-gotpcrel-2.d: Likewise.
>>> * testsuite/gas/i386/i386.exp: Run inval-got.
>>> * testsuite/gas/i386/inval-got.l: New file.
>>> * testsuite/gas/i386/inval-got.s: Likewise.
>>> * testsuite/gas/i386/x86-64-inval-got.l: Likewise.
>>> * testsuite/gas/i386/x86-64-inval-got.l: Likewise.
>>> * testsuite/gas/i386/x86-64.exp: Run x86-64-inval-got.
>>
>> Hmm, I'm further puzzled: On what basis would VCVTPS2DQ and VCVTPH2QQ be
>> okay to use with an operand in GOT?
>>
>> And then, if I'm reading the added logic right,
>>
>>         VADDPS foo@GOTPCREL(%rip), %zmm0, %zmm1
>>
>> would be permitted despite having a 512-bit memory operand, just because
>> it also permits embedded broadcast (which isn't in use here). Whereas
> 
> Only for broadcast.

Hmm, perhaps I'm reading the code change incorrectly then. By checking
the template rather than i.types[] (as I had asked you to do) I would
assume templates permitting for broadcast would allow all forms of their
insns through.

>>         VADDPS foo@GOTPCREL(%rip), %ymm0, %ymm1
>>
>> wouldn't be, unless prefixed by {evex}. How in the world are people to
>> infer what is or is not allowed by the assembler?
> 
> Only 8-bit, 16-bit, 32-bit, 64-bit (in 64-bit mode) is allowed.
> 
>> As before, my requirements are:
>> 1) underlying criteria being clearly / unambiguously spelled out in the
>>    psABI, and/or
> 
> GOT is specified in the psABI.

GOT is, but I'm pretty sure there's nothing there about how GOT is to
be accessed.

>> 2) such extra checking being optional, default-off (to some degree
>> depending on how things would be worded in the psABI).
> 
> GOT size check should be on always.

Which is because you know better than any programmer what may make
sense in their specific case?

Jan