Re: [PATCH 3/8] bpftool: Conditionally add -Wformat-signedness flag

[Leo Yan]

On Thu, Sep 25, 2025 at 11:42:39AM +0100, Quentin Monnet wrote:
> 2025-09-25 11:26 UTC+0100 ~ Leo Yan <leo.yan@xxxxxxx>
> > clang-18.1.3 on Ubuntu 24.04.2 reports warning:
> > 
> >   warning: unknown warning option '-Wformat-signedness' [-Wunknown-warning-option]
> > 
> > Conditionally add the option only when it is supported by compiler.
> > 
> > Signed-off-by: Leo Yan <leo.yan@xxxxxxx>
> 
> 
> Hi, how annoying is this warning?

Not too bad, it prints out 5~6 times warnings (see the build log below).
Though the warnings are a bit noisy, they does not break the build.

> I'm asking because as far as I
> understand, the option has been introduced in LLVM 19.1.0 [0] - the
> latest being 21.1.0 already - so we won't need this check once distros
> have transitioned, and I'm a bit reluctant to add it.

I agree the warning likely won't affect distros, as package maintainers
will hide it. But it can annoy _paranoid_ developers in day-to-day
builds ;)

As the warning doesn't block anything, so I am fine with not merging
this patch.

Thanks,
Leo

> [0]
> https://github.com/llvm/llvm-project/commit/ea92b1f9d0fc31f1fd97ad04eb0412003a37cb0d


---8<---

Auto-detecting system features:
...                         clang-bpf-co-re: [ on  ]
...                                    llvm: [ OFF ]
...                                  libcap: [ on  ]
...                                  libbfd: [ OFF ]
...                          libbfd-liberty: [ OFF ]
...                        libbfd-liberty-z: [ OFF ]
...                  disassembler-four-args: [ OFF ]
...                disassembler-init-styled: [ OFF ]
...                             libelf-zstd: [ o
  MKDIR   /build/util/bpf_skel/.tmp/bootstrap/libbpf/include/bpf
  MKDIR   /build/util/bpf_skel/.tmp/bootstrap/libbpf/
  MKDIR   /build/util/bpf_skel/.tmp/bootstrap/
  INSTALL /build/util/bpf_skel/.tmp/bootstrap/libbpf/include/bpf/hashmap.h
  INSTALL /build/util/bpf_skel/.tmp/bootstrap/libbpf/include/bpf/relo_core.h
  INSTALL /build/util/bpf_skel/.tmp/bootstrap/libbpf/include/bpf/libbpf_internal.h
warning: unknown warning option '-Wformat-signedness' [-Wunknown-warning-option]
1 warning generated.
  GEN     /build/util/bpf_skel/.tmp/bootstrap/libbpf/bpf_helper_defs.h
  INSTALL /build/util/bpf_skel/.tmp/bootstrap/libbpf/include/bpf/bpf.h
  INSTALL /build/util/bpf_skel/.tmp/bootstrap/libbpf/include/bpf/libbpf.h
  INSTALL /build/util/bpf_skel/.tmp/bootstrap/libbpf/include/bpf/libbpf_legacy.h
  INSTALL /build/util/bpf_skel/.tmp/bootstrap/libbpf/include/bpf/btf.h
  INSTALL /build/util/bpf_skel/.tmp/bootstrap/libbpf/include/bpf/bpf_helpers.h
  INSTALL /build/util/bpf_skel/.tmp/bootstrap/libbpf/include/bpf/libbpf_common.h
  INSTALL /build/util/bpf_skel/.tmp/bootstrap/libbpf/include/bpf/bpf_tracing.h
  INSTALL /build/util/bpf_skel/.tmp/bootstrap/libbpf/include/bpf/bpf_core_read.h
  INSTALL /build/util/bpf_skel/.tmp/bootstrap/libbpf/include/bpf/bpf_endian.h
  INSTALL /build/util/bpf_skel/.tmp/bootstrap/libbpf/include/bpf/libbpf_version.h
  INSTALL /build/util/bpf_skel/.tmp/bootstrap/libbpf/include/bpf/skel_internal.h
  INSTALL /build/util/bpf_skel/.tmp/bootstrap/libbpf/include/bpf/usdt.bpf.h
  INSTALL /build/util/bpf_skel/.tmp/bootstrap/libbpf/include/bpf/bpf_helper_defs.h
  INSTALL libbpf_headers
  CC      /build/util/bpf_skel/.tmp/bootstrap/libbpf/staticobjs/libbpf.o
  CC      /build/util/bpf_skel/.tmp/bootstrap/libbpf/staticobjs/bpf.o
  CC      /build/util/bpf_skel/.tmp/bootstrap/libbpf/staticobjs/nlattr.o
  CC      /build/util/bpf_skel/.tmp/bootstrap/libbpf/staticobjs/btf.o
  CC      /build/util/bpf_skel/.tmp/bootstrap/libbpf/staticobjs/libbpf_errno.o
  CC      /build/util/bpf_skel/.tmp/bootstrap/libbpf/staticobjs/str_error.o
  CC      /build/util/bpf_skel/.tmp/bootstrap/libbpf/staticobjs/netlink.o
  CC      /build/util/bpf_skel/.tmp/bootstrap/libbpf/staticobjs/bpf_prog_linfo.o
  CC      /build/util/bpf_skel/.tmp/bootstrap/libbpf/staticobjs/libbpf_probes.o
  CC      /build/util/bpf_skel/.tmp/bootstrap/libbpf/staticobjs/hashmap.o
  CC      /build/util/bpf_skel/.tmp/bootstrap/libbpf/staticobjs/btf_dump.o
  CC      /build/util/bpf_skel/.tmp/bootstrap/libbpf/staticobjs/ringbuf.o
  CC      /build/util/bpf_skel/.tmp/bootstrap/libbpf/staticobjs/strset.o
  CC      /build/util/bpf_skel/.tmp/bootstrap/libbpf/staticobjs/gen_loader.o
  CC      /build/util/bpf_skel/.tmp/bootstrap/libbpf/staticobjs/relo_core.o
  CC      /build/util/bpf_skel/.tmp/bootstrap/libbpf/staticobjs/linker.o
  CC      /build/util/bpf_skel/.tmp/bootstrap/libbpf/staticobjs/usdt.o
  CC      /build/util/bpf_skel/.tmp/bootstrap/libbpf/staticobjs/zip.o
  CC      /build/util/bpf_skel/.tmp/bootstrap/libbpf/staticobjs/elf.o
  CC      /build/util/bpf_skel/.tmp/bootstrap/libbpf/staticobjs/features.o
  CC      /build/util/bpf_skel/.tmp/bootstrap/libbpf/staticobjs/btf_iter.o
  CC      /build/util/bpf_skel/.tmp/bootstrap/libbpf/staticobjs/btf_relocate.o
  LD      /build/util/bpf_skel/.tmp/bootstrap/libbpf/staticobjs/libbpf-in.o
  LINK    /build/util/bpf_skel/.tmp/bootstrap/libbpf/libbpf.a
  CC      /build/util/bpf_skel/.tmp/bootstrap/main.o
  CC      /build/util/bpf_skel/.tmp/bootstrap/common.o
  CC      /build/util/bpf_skel/.tmp/bootstrap/json_writer.o
  CC      /build/util/bpf_skel/.tmp/bootstrap/gen.o
  CC      /build/util/bpf_skel/.tmp/bootstrap/btf.o
warning: unknown warning option '-Wformat-signedness' [-Wunknown-warning-option]
warning: unknown warning option '-Wformat-signedness' [-Wunknown-warning-option]
warning: unknown warning option '-Wformat-signedness' [-Wunknown-warning-option]warning:
unknown warning option '-Wformat-signedness' [-Wunknown-warning-option]
warning: unknown warning option '-Wformat-signedness' [-Wunknown-warning-option]
1 warning generated.
1 warning generated.
1 warning generated.
1 warning generated.
1 warning generated.
  LINK    /build/util/bpf_skel/.tmp/bootstrap/bpftool