Re: [PATCH] x86: Don't remove empty x86 properties

[Cary Coutant <ccoutant at gmail dot com>]

> Here are what I have on my machine:
>
> [hjl@gnu-cfl-2 compilers]$ readelf -n /lib64/ld-linux-x86-64.so.2  | head
>
> Displaying notes found in: .note.gnu.property
>   Owner                 Data size Description
>   GNU                  0x00000030 NT_GNU_PROPERTY_TYPE_0
>       Properties: x86 feature: IBT, SHSTK
> x86 ISA used: CMOV, SSE, SSE2, AVX, AVX512F
> x86 feature used: x86, x87, XMM, YMM, ZMM, FXSR, XSAVE, XSAVEC
>
> x86 feature: IBT, SHSTK
>
> is AND/GNU_PROPERTY_X86_FEATURE_1_AND.  It is generated
> by GCC with -fcf-protection.
>
> x86 ISA used: CMOV, SSE, SSE2, AVX, AVX512F
>
> is OR_AND/GNU_PROPERTY_X86_ISA_1_USED.  It is generated by
> assembler with -mx86-used-note=yes.
>
> x86 feature used: x86, x87, XMM, YMM, ZMM, FXSR, XSAVE, XSAVEC
>
> is OR_AND/GNU_PROPERTY_X86_FEATURE_2_USED.  It is generated
> by assembler with -mx86-used-note=yes.
>
> They tell
>
> 1.  The binary supports IBT and SHSTK.
> 2.  The binary only uses base ISA + CMOV, SSE, SSE2, AVX, AVX512F.
> 3   The binary only uses x86, x87, XMM, YMM, ZMM, FXSR, XSAVE, XSAVEC.
>
> If someone wants to create a set of binaries for AVX2 and above.  He/she
> can set AVX2 in OR/GNU_PROPERTY_X86_ISA_1_NEEDED and XMM/YMM
> in OR/GNU_PROPERTY_X86_FEATURE_2_NEEDED.
>
> How can I achieve the above with your suggestion?

Before I answer that, let's talk about "NEEDED" vs. "USED".

In the psABI, you have described both ISA_1_USED and FEATURE_2_USED as
"their support in the hardware is optional". Yet:

(a) You've assigned values for both of these in the OR_AND range,
which you've described earlier in this thread as meaning "It is not OK
if run-time doesn't have the feature the program requires."

(b) Your example from ld-linux-x86-64.so.2 shows ISA_1_USED and
FEATURE_2_USED, but does not use either "NEEDED" property. Based on
everything you've said so far, I'd expect you to be using ISA_1_NEEDED
and FEATURE_2_NEEDED instead, and these properties should be in the
OR_AND range.

Why do CMOV, SSE, SSE2, AVX, and AVX512F go in ISA_1_USED, but AVX2
goes in ISA_1_NEEDED?

Why do x86, x87, XMM, YMM, ZMM, FXSR, XSAVE, and XSAVEC go in
FEATURE_2_USED, but XMM and YMM go in FEATURE_2_NEEDED?

How exactly are you expecting these properties to be treated by the loader?

For USED (but optional) features, who is the intended consumer? A
loader can't use them to reject the program on reduced-capability
hardware, since those features are described as optional. Code tagged
with a used-but-optional feature presumably has been written to
dynamically test for the presence of a capability, and use it if it's
present, falling back to a less-preferred mechanism if it's not. Such
a property might be useful for link-time selection of variant
implementations of a routine, or even for load-time selection, but
only if the property remains attached to the specific entry point. As
a binary-wide property, I don't see how it would be of much use. If
the intended consumer is nothing more than a human reader of objdump
output, that's fine, but you're going to an awful lot of work if
that's it.

For NEEDED features, the intent is clearly that the loader can
determine whether the hardware is capable of executing a particular
binary, and rejecting it gracefully if it's not (as opposed to letting
it core dump when it tries to use an absent feature -- or worse!). For
these, it seems silly to discard the whole set of NEEDED features when
even a single input file is missing the property -- after all, even if
that missing input file might need some unique features, it's still
the case that the ones we do know about from other files are still
needed, and it's still desirable to let the loader reject the binary
if it can. This is why I don't really understand your motivation for
the OR_AND rule.

-cary