Re: RISC-V ABI issue with ULEB128

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On Tue, Jan 2, 2024 at 1:09 PM Richard W.M. Jones <rjones@xxxxxxxxxx> wrote:
>
>
> I'm not sure exactly the effect on RISC-V binaries, but I wanted to
> raise it here to get the attention of the Fedora toolchain team ...
>
> Here's the bug:
>
> https://sourceware.org/bugzilla/show_bug.cgi?id=31179
> RISC-V: The SET/ADD/SUB fix breaks ABI compatibility with 2.41 objects
>
> It refers to this change in binutils 2.41:
>
> https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=73d931e560059a87d76f528fafbb4270a98746bc
>
> As far as I understand the issue (which is not too far) this mainly
> affects shipped *.o and *.a files (ie. static libraries and similar)
> which were compiled with binutils < 2.41, which either won't link
> correctly or will give a linker error when using GNU ld from binutils 2.41.

Correction. This is broken in <= 2.41 (incl. the current stable
binutils release).

>
> Unclear if it also affects *.so files (which would be a much more
> serious ABI break), and also if it affects most binaries or just a
> few.  I initially thought this only affected programs using 128 bit
> ints, so didn't think it was too important, but after reading the
> commit I'm not sure that is really true.

Nelson Chu committed (5 days ago) a change:

https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=73d931e560059a87d76f528fafbb4270a98746bc

This will accept whatever is produced by <= 2.41 and the final binary
will be correct. There is also a new ld flag (--check-uleb128) which
can produce a warning if it detects an issue.

I plan/hope to use binutils 2.42 in Fedora/RISCV 40. That would happen
before I start mass rebuilding, and that should fix this.

>
> There's been discussion about adding an ELF tag, but that seems very
> problematic to me:
> https://github.com/riscv-non-isa/riscv-elf-psabi-doc/pull/414

Yeah. I doubt we need a tag. I don't think it solves anything. Nelson
commit (i.e. workaround) + mass rebuild should resolve any
inconveniences AFAIK.

Cheers,
david

>
> Rich.
>
> --
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