Re: [RFC dwarves 5/6] btf_encoder: Do not error out if BTF is not found in some input files

[Eduard Zingerman <eddyz87@gmail.com>]

On Thu, 2025-08-07 at 10:12 -0700, Alexei Starovoitov wrote:
> On Thu, Aug 7, 2025 at 9:37 AM Jose E. Marchesi
> <jose.marchesi@oracle.com> wrote:
> >
> >
> > > On Thu, Aug 7, 2025 at 7:42 AM Alan Maguire <alan.maguire@oracle.com> wrote:
> > > >
> > > > This is no substitute for link-time BTF deduplication of course, but
> > > > it does provide a simple way to see the BTF that gcc generates for vmlinux.
> > > >
> > > > The idea is that we can explore differences in BTF generation across
> > > > the various combinations
> > > >
> > > > 1. debug info source: DWARF; dedup done via pahole (traditional)
> > > > 2. debug info source: compiler-generated BTF; dedup done via pahole (above)
> > > > 3. debug info source: compiler-generated BTF; dedup done via linker (TBD)
> > > >
> > > > Handling 3 - linker-based dedup - will require BTF archives so that is the
> > > > next step we need to explore.
> > >
> > > Overall, the patch set makes sense and we need to make this step in pahole,
> > > but before we start any discussion about 3 and BTF archives
> > > the 1 and 2 above need to reach parity.
> > > Not just being close enough, but an exact equivalence.
> > >
> > > But, frankly, gcc support for btf_decl_tags is much much higher priority
> > > than any of this.
> > >
> > > We're tired of adding hacks through the bpf subsystem, because
> > > gcc cannot do decl_tags.
> > > Here are the hacks that will be removed:
> > > 1. BTF_TYPE_SAFE*
> > > 2. raw_tp_null_args[]
> > > 3. KF_ARENA_ARG
> > > and probably other cases.
> >
> > We are getting there.  The C front-end maintainer just looked at the
> > latest version of the series [1] and, other than a small observation
> > concerning wide char strings, he seems to be ok with the attributes.
> >
> > [1] https://gcc.gnu.org/pipermail/gcc-patches/2025-August/692057.html
>
> Good to know.
>
> Yonghong, what does llvm do with wchar?

The literal is copied as-is with a warning.

  $ cat wide-string-test.c
  __attribute__((btf_decl_tag(u8"üüü")))
  int foo(void) { return 42; }

  $ clang --target=bpf -O2 -g wide-string-test.c -c -o wide-string-test.o
  wide-string-test.c:1:29: warning: encoding prefix 'u8' on an unevaluated string literal has no effect [-Winvalid-unevaluated-string]
      1 | __attribute__((btf_decl_tag(u8"üüü")))
        |                             ^~
  1 warning generated.

  $ bpftool btf dump file wide-string-test.o
  [1] FUNC_PROTO '(anon)' ret_type_id=2 vlen=0
  [2] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
  [3] FUNC 'foo' type_id=1 linkage=global
  [4] DECL_TAG 'üüü' type_id=3 component_idx=-1

"As-is" means using compiler internal encoding (UTF8),
e.g. above u8"üüü" is encoded as "c3 bc c3 bc c3 bc" in the final .BTF
section, same happens for UTF32 literal U"üüü".