The output of ld.lld -v includes a message "compatiblewith GNU linkers" to address detectionmechanism used by GNU Libtool. This problem is described by Softwarecompatibility and our own "User-Agent" problem.

The latest m4/libtool.m4 continues to rely on aGNU check.

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[AC_CACHE_CHECK([if the linker ($LD) is GNU ld], lt_cv_prog_gnu_ld,
[# I'd rather use --version here, but apparently some GNU lds only accept -v.
case `$LD -v 2>&1 </dev/null` in
*GNU* | *'with BFD'*)
  lt_cv_prog_gnu_ld=yes
  ;;
*)
  lt_cv_prog_gnu_ld=no
  ;;
esac])

Check-basedconfiguration can be a valuable tool, ensuring software remainsfunctional in the future. However, this example highlights how overlyspecific checks can lead to unintended consequences.

This blog post explores more forms of the "User-Agent" problemexposed by an LLD patch changing the version message format.

LLD supports many object file formats. It mimicks GNU ld for ELF andMSVC link.exe for PE/COFF. Previously, the ELF port displays the versioninformation like this:

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% /tmp/out/custom2/bin/ld.lld --version
LLD 19.0.0 (compatible with GNU linkers)

A recent patch (llvm-project#97323)changed it to one of the following formats, depending on the build-timevariable LLVM_APPEND_VC_REV:

With LLVM_APPEND_VC_REV=on:

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% /tmp/out/custom2/bin/ld.lld --version
LLD 19.0.0 (git@github.com:llvm/llvm-project.git 0f9fbbb63cfcd2069441aa2ebef622c9716f8dbb), compatible with GNU linkers

With LLVM_APPEND_VC_REV=off:

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% /tmp/out/custom2/bin/ld.lld --version
LLD 19.0.0, compatible with GNU linkers

Meson

In Meson, mesonbuild/linkers/detect.py:guess_win_linkerchecks the --version output to determine whether the LLDinvocation is for ELF or PE/COFF. It performed an overly strict check"(compatible with GNU linkers)", which failed when the parentheses werestripped by #97323.

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# mesonbuild/linkers/detect.py
    if 'LLD' in o.split('\n', maxsplit=1)[0]:
        if '(compatible with GNU linkers)' in o:
            return linkers.LLVMDynamicLinker(
                compiler, for_machine, comp_class.LINKER_PREFIX,
                override, version=search_version(o))
        elif not invoked_directly:
            return linkers.ClangClDynamicLinker(
                for_machine, override, exelist=compiler, prefix=comp_class.LINKER_PREFIX,
                version=search_version(o), direct=False, machine=None)

The latest Meson has loosened the check (meson#13383).

Linux kernel

The Linux kernel's scripts/ld-version.sh script detectslinker versions. Introduced in 2014, it initially checked for GNU ldcompatibility with GCC LTO (though LTO support remains unmerged). It waslater revamped to handle LLD versions as well. While it can handlesuffixes like 2.34-4.fc32, it struggles with versionscontaining with comma suffix (19.0.0,).

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% scripts/ld-version.sh /tmp/out/custom2/bin/ld.lld
scripts/ld-version.sh: line 19: 10000 * 19 + 100 * 0 + 0,: syntax error: operand expected (error token is ",")

The script extracts the version string from the--version output and parses it as major.minor.patch.

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# Get the first line of the --version output.
IFS='
'
set -- $(LC_ALL=C "$@" --version)

# Split the line on spaces.
IFS=' '
set -- $1

...

# Some distributions append a package release number, as in 2.34-4.fc32
# Trim the hyphen and any characters that follow.
version=${version%-*}

To support suffixes starting with either - or,, the script willemploy a POSIX shell trick utilizing the "Remove Largest SuffixPattern" feature:

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version=${version%%[!0-9.]*}

More fun with versions

Ever wondered what the subtle differences are between-v, -V, and --version when usingGNU ld? Let's break it down:

• --version skips linker input processing and displaysbrief copyright information.
• -v and -V keep processing inputfiles.
• -V goes a step further than -v byincluding a list of supported BFD emulations alongside the versioninformation.

Prior to September 2022, -V in ld.lld used to an aliasfor --version. This caused issues when usinggcc -v -fuse-ld=lld on certain targets like*-freebsd and powerpc-*: gcc passes -V to thelinker, expecting it to process the input files and complete the linkingstep. However, ld.lld's behavior with -V skipped thisprocess.

I made an adjustment by making-V an alias for -v instead. This ensuresthat gcc -v -fuse-ld=lld performs the linking step.