From: Fangrui Song <i@maskray.me>
To: "H.J. Lu" <hjl.tools@gmail.com>
Cc: GNU gABI gnu-gabi <gnu-gabi@sourceware.org>,
GCC Development <gcc@gcc.gnu.org>,
Binutils <binutils@sourceware.org>,
GNU C Library <libc-alpha@sourceware.org>,
llvm-dev@lists.llvm.org
Subject: Re: RFC: Add GNU_PROPERTY_UINT32_AND_XXX/GNU_PROPERTY_UINT32_OR_XXX
Date: Thu, 21 Jan 2021 13:42:42 -0800 [thread overview]
Message-ID: <20210121214242.3ha7fiecfbohxgyd@gmail.com> (raw)
In-Reply-To: <CAMe9rOoG6NtMFp=jdj9yKPqoWbQ0932OodVRVcVsCUxhi8dLnQ@mail.gmail.com>
On 2021-01-21, H.J. Lu via Gnu-gabi wrote:
>On Wed, Jan 13, 2021 at 9:06 AM H.J. Lu <hjl.tools@gmail.com> wrote:
>>
>> 1. GNU_PROPERTY_UINT32_AND_LO..GNU_PROPERTY_UINT32_AND_HI
>>
>> #define GNU_PROPERTY_UINT32_AND_LO 0xb0000000
>> #define GNU_PROPERTY_UINT32_AND_HI 0xb0007fff
>>
>> A bit in the output pr_data field is set only if it is set in all
>> relocatable input pr_data fields. If all bits in the the output
>> pr_data field are zero, this property should be removed from output.
>>
>> If the bit is 1, all input relocatables have the feature. If the
>> bit is 0 or the property is missing, the info is unknown.
>>
>> 2. GNU_PROPERTY_UINT32_OR_LO..GNU_PROPERTY_UINT32_OR_HI
>>
>> #define GNU_PROPERTY_UINT32_OR_LO 0xb0008000
>> #define GNU_PROPERTY_UINT32_OR_HI 0xb000ffff
>>
>> A bit in the output pr_data field is set if it is set in any
>> relocatable input pr_data fields. If all bits in the the output
>> pr_data field are zero, this property should be removed from output.
>>
>> If the bit is 1, some input relocatables have the feature. If the
>> bit is 0 or the property is missing, the info is unknown.
>>
>> The PDF is at
>>
>> https://gitlab.com/x86-psABIs/Linux-ABI/-/wikis/uploads/0690db0a3b7e5d8a44e0271a4be54aa7/linux-gABI-and-or-2021-01-13.pdf
>>
>> --
>> H.J.
>
>Here is the binutils patch to implement it.
>
>--
>H.J.
Hi, H.J.
Thank you for CCing llvm-dev:) In the past various GNU ABI proposals
went unnoticed by LLVM folks who don't happen to subscribe to GNU lists.
(A lot! I personally subscribe to some lists and check the discussion
just in case I miss something important:) )
I have researched a bit and observed that the following GNU_PROPERTY
values are currently used by compilers/linkers:
Bitwise OR for relocatable links. Bitwise AND for executable/shared
object links.
* GNU_PROPERTY_X86_FEATURE_1_AND = GNU_PROPERTY_X86_UINT32_AND_LO + 0,
* used by Intel Indirect branch tracking and Shadow Stack
* GNU_PROPERTY_AARCH64_FEATURE_1_AND, used by AArch64 Branch Target
* Identification and Pointer Authentication
Bitwise OR for all links.
* GNU_PROPERTY_X86_ISA_1_NEEDED = GNU_PROPERTY_X86_UINT32_OR_LO + 2,
* used by GCC -mneeded (for -march=x86-64-v[234])
There appear to be another type of AND/OR bits which are not defined in
ABIs (AFAICT):
* GNU_PROPERTY_X86_ISA_1_USED = GNU_PROPERTY_X86_UINT32_OR_AND_LO + 2
* GNU_PROPERTY_X86_FEATURE_2_USED = GNU_PROPERTY_X86_UINT32_OR_AND_LO +
* 1
I think generalizing the AND/OR idea to all architectures probably
requires us to think about these questions:
* What's the impending usage of the generic AND/OR ranges? ifunc? :)
* Does the concept generalize well to other architectures? If we
* consider AArch64/x86 FEATURE_1_AND to be the same thing, the current
* usage is purely x86 specific.
* Is AND/OR encoding expressive enough to represent the required states?
* I've asked two folks and they expressed concerns. I think the three
* AND/OR usage above speak for themselves.
* Szabolcs Nagy mentioned that GNU_PROPERTY is an OS-specific mechanism
* (GNU), but the features are oftentimes arch specific which make sense
* to other OSes or bare-metal.
* Szabolcs: Do we need any versioning mechanism?
The feature selection and compatibility checking mechanism has some
overlap with GNU/arch-specific attributes (e.g .ARM.attributes,
.riscv.attributes). If I understand correctly, GNU_PROPERTY has an
associated program header so it can be checked by loaders
(kernel/ld.so/emulator) while Attributes don't have program headers so
they are largely assembler/linker protocols. In an inflexible way that
such feature bits can affect observable states to loaders as well, e.g.
.ARM.attributes can affect e_flags (soft/hard float). .MIPS.abiflags
has an associated program header PT_MIPS_ABIFLAGS (I know nearly nothing
about mips) Some thoughts from mips folks would be useful.
Last, I think a feature selection and compatibility checking mechanism
is assuredly useful, but whether the current AND/OR scheme can perfectly
satisfy that goal I am unsure. Having the proposal is a very good start,
though:) Thanks a lot for driving the discussion:)
next prev parent reply other threads:[~2021-01-21 21:42 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-13 17:06 H.J. Lu
2021-01-21 15:02 ` H.J. Lu
2021-01-21 21:42 ` Fangrui Song [this message]
2021-04-17 12:48 ` H.J. Lu
2021-04-17 18:25 ` Fangrui Song
2021-04-17 19:05 ` H.J. Lu
2021-06-17 18:59 ` H.J. Lu
2021-06-17 19:38 ` [llvm-dev] " Fangrui Song
2021-06-17 19:45 ` H.J. Lu
2021-06-17 20:25 ` Fāng-ruì Sòng
2021-06-17 23:01 ` H.J. Lu
2021-06-18 0:06 ` Fāng-ruì Sòng
2021-06-18 0:24 ` H.J. Lu
2021-06-18 0:49 ` Fāng-ruì Sòng
2021-06-18 2:40 ` H.J. Lu
2021-06-21 14:35 ` Michael Matz
2021-06-22 14:30 ` H.J. Lu
2021-06-22 14:54 ` Michael Matz
2021-06-18 2:45 ` H.J. Lu
2021-06-18 15:38 ` RFC: Add GNU_PROPERTY_1_NEEDED H.J. Lu
2021-06-18 21:34 ` [llvm-dev] " Fangrui Song
2021-06-19 1:09 ` H.J. Lu
2021-09-27 14:22 RFC: Add GNU_PROPERTY_UINT32_AND_XXX/GNU_PROPERTY_UINT32_OR_XXX GNU gABI gnu-gabi
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