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

[H.J. Lu]

On Fri, Feb 7, 2025 at 2:32 PM Jan Beulich <jbeulich@suse.com> wrote:
>
> 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.

For x86-64, GOT is:

extern Elf64_Addr _GLOBAL_OFFSET_TABLE_ [];

"foo@GOTPCREL(%rip)" allocates a 64-bit GOT slot for foo.   Memory
access of "foo@GOTPCREL(%rip)" beyond its GOT slot is invalid.

> >> 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?
>

Show me an example.

-- 
H.J.