Re: [PATCH v3 13/18] perf build: Remove libbfd support

[Ian Rogers]

On Tue, Jan 28, 2025 at 4:31 PM Daniel Xu <dxu@xxxxxxxxx> wrote:
>
> Hi Ian,
>
> On Wed, Jan 22, 2025 at 09:43:03AM -0800, Ian Rogers wrote:
> > libbfd is license incompatible with perf and building requires the
> > BUILD_NONDISTRO=1 build flag. Remove the code to simplify the code
> > base.
> >
> > Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
> > ---
> >  tools/perf/Documentation/perf-check.txt |   1 -
> >  tools/perf/Makefile.config              |  38 +---
> >  tools/perf/builtin-check.c              |   1 -
> >  tools/perf/tests/Build                  |   1 -
> >  tools/perf/tests/builtin-test.c         |   1 -
> >  tools/perf/tests/pe-file-parsing.c      | 101 ----------
> >  tools/perf/tests/tests.h                |   1 -
> >  tools/perf/util/demangle-cxx.cpp        |  13 +-
> >  tools/perf/util/disasm_bpf.c            | 166 ----------------
> >  tools/perf/util/srcline.c               | 243 +-----------------------
> >  tools/perf/util/symbol-elf.c            |  86 +--------
> >  tools/perf/util/symbol.c                | 135 -------------
> >  tools/perf/util/symbol.h                |   4 -
> >  13 files changed, 7 insertions(+), 784 deletions(-)
> >  delete mode 100644 tools/perf/tests/pe-file-parsing.c
>
> [..]
>
> I was briefly investigating why the centos build of perf was not
> demangling rust v0 symbols [0]. From looking at the rust issue [1], it
> appears the rust team somehow delivered support for v0 demangling
> through libbfd. The code itself looked a bit odd (relying on cxx
> demangle to run first?), but that's a separate thing.

There is still C++ demangling support by way of cxxabi:
https://git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/util/demangle-cxx.cpp?h=perf-tools-next#n44
that was in libstdc++ (GNU) and libcxx (LLVM) when I looked.

> The centos build does not build with libbfd for the license issues you
> mentioned. So your change probably won't regress any distro use cases.
> But it does remove support for motivated users who don't have
> re-distribution requirements.
>
> But since this patchset came up first in my search, I thought it'd be
> good to mention that someone probably needs to add v0 support to
> tools/perf/util/demangle-rust.c.

So I don't see any libbfd dependencies in demangle-rust.c:
https://git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/util/demangle-rust.c?h=perf-tools-next#n8
Unusually we don't have any tests on the Rust demangling, we do for
Java and OCaml:
https://git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/tests/demangle-java-test.c?h=perf-tools-next
https://git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/tests/demangle-ocaml-test.c?h=perf-tools-next

Reading a bit more it seems that previous libiberty was coming to the
rescue by way of C++ demangling. I'll see if I can write a demangler
by way of lex and yacc. If we have a v0 standard one is there any
value in the existing demangler or legacy demangling? It seems this
has been broken for the best part of 5 years.

Thanks,
Ian

> Thanks,
> Daniel
>
>
> [0]: https://doc.rust-lang.org/rustc/symbol-mangling/v0.html
> [1]: https://github.com/rust-lang/rust/issues/60705