On Tue, Aug 17, 2021 at 2:40 AM Niels Möller nisse@lysator.liu.se wrote:

Maamoun TK maamoun.tk@googlemail.com writes:

...

Forcing ELFv2 abi doesn't work for big-endian mode as this mode has no support for ELFv2. ppc64 linux big-endian is deprecated, it' not unexpected to get such issues. Dropping big-endian support for powerpc could be an option to solve this issue but that will be a drawback for AIX (BE) systems.

The configuration where it didn't work was powerpc64-openwrt-linux-musl. I'd like Nettle to work on embedded systems whenever practical. But support depends on assistance from users of those systems.

Musl Libc does not support ELFv1, so I don't understand how this configuration is possible.

As I understood it, this system needs to use the v2 ABI. I would hope it's easy to detect the abi used by the configured C compiler, and then select the same prologue sequence as is currently used for little-endian. I.e., one more configure test, and changing the "ifelse(WORDS_BIGENDIAN,no," condition in powerpc64/machine.m4 to check a different configure variable.

I don't know how the linker detected abi incompatibility (ld error message like "gcm-hash.o: ABI version 1 is not compatible with ABI version 2 output"), if that's based just on the presence of the special ".opd" section, or if there are other attributes in the ELF file, and if so, how the assembler decides which attributes to attach.

The linker looks for the special ELF attribute.

Thanks, David

I believe that some OS (FreeBSD? NetBSD?) is trying to use ELFv2 with PPC64 BE. That is a separate issue and I do not believe that Musl Libc plans to support that configuration.

AIX continues to use its ABI, which essentially is closely related to ELFv1 and uses the XCOFF file format and syntax.

Thanks, David

On Tue, Aug 17, 2021 at 1:27 PM Niels Möller nisse@lysator.liu.se wrote:

David Edelsohn dje.gcc@gmail.com writes:

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Musl Libc does not support ELFv1, so I don't understand how this configuration is possible.

If I understood the original report, musl always uses ELFv2 abi, for both little and big endian configurations. Which for big endian is incompatible with the way powerpc64 assembly is configured in nettle.

Nettle assembly files currently use ELFv2 on little endian, but always uses ELFv1 on big endian.

Regards, /Niels

-- Niels Möller. PGP-encrypted email is preferred. Keyid 368C6677. Internet email is subject to wholesale government surveillance.

On Thu, Aug 19, 2021 at 4:22 PM Niels Möller nisse@lysator.liu.se wrote:

David Edelsohn dje.gcc@gmail.com writes:

...

AIX continues to use its ABI, which essentially is closely related to ELFv1 and uses the XCOFF file format and syntax.

When you say "closely related" is it close enough that the same assembly can be used as for ELFv1? I don't remember if Maamoun or someone else tested Nettle's powerpc64 assembly on AIX?

Is there some established common name for this calling convention, for both ELFv1 and XCOFF? Otherwise, I'd refer to it as ELFv1, even though it works on non-ELF AIX too.

The AIX ABI and the ELFv1 ABI are not identical. AIX uses XCOFF file format and syntax. PPC64 BE Linux uses ELF file format and syntax.

The ELFv1 ABI was derived from the AIX ABI and both use function descriptors (ELFv1 calls OPD or Official Procedure Descriptors). Function pointers are pointers to the descriptors, not to code. ELFv1 uses ELF section names and decorations.

AIX uses the TOC for global data managed by the compiler. ELFv1 can use a TOC or a more standard GOT managed by the linker.

The assembly language for the instructions is the same (AIX prefers registers as only numbers, while ELF allows letters to distinguish the different register sets (GPRs, FPRs, VSRs).

One can translate between AIX and ELFv1 fairly easily, and possibly use a single assembly file with some m4 macros to hide the differences, but one cannot use identical assembly language files for both.

Thanks David

On Tue, Aug 17, 2021 at 9:40 AM Niels Möller nisse@lysator.liu.se wrote:

The configuration where it didn't work was powerpc64-openwrt-linux-musl. I'd like Nettle to work on embedded systems whenever practical. But support depends on assistance from users of those systems.

As I understood it, this system needs to use the v2 ABI. I would hope it's easy to detect the abi used by the configured C compiler, and then select the same prologue sequence as is currently used for little-endian. I.e., one more configure test, and changing the "ifelse(WORDS_BIGENDIAN,no," condition in powerpc64/machine.m4 to check a different configure variable.

I skipped processing the assembly files with a different approach, I made the configuration check for musl and endianness variant to trigger assembly processing. You can check the fix in this branch https://git.lysator.liu.se/mamonet/nettle/-/tree/ppc64_musl_fix Apparently, the bug reporter uses a cross-compiler for powerpc arch. It would be great to run this fix at the same bug environment since I tested the patch in different circumstances.

regards, Mamone