Re: [PATCH bpf-next] bpftool: Strip all -Wformat* flags from bootstrap libbpf build

[Quentin Monnet <qmo@kernel.org>]

2026-07-01 11:15 UTC-0700 ~ Andrii Nakryiko <andrii.nakryiko@gmail.com>
> On Wed, Jul 1, 2026 at 5:54 AM Quentin Monnet <qmo@kernel.org> wrote:
>>
>> 2026-06-30 13:54 UTC-0700 ~ Andrii Nakryiko <andrii@kernel.org>
>>> Commit 9080b97689db ("bpftool: Pass host flags to bootstrap libbpf")
>>> started building the bootstrap libbpf with HOST_CFLAGS, stripping the
>>> warning options that are unsuitable for that build by filtering out
>>> -W -Wall -Wextra -Wformat -Wformat-signedness.
>>>
>>> HOST_CFLAGS inherits EXTRA_WARNINGS, which includes -Wformat-security
>>> and -Wformat-y2k. The filter drops -Wall and -Wformat (the latter being
>>> what actually enables -Wformat), but leaves those two -Wformat-* children
>>> in LIBBPF_BOOTSTRAP_CFLAGS. Building the bootstrap libbpf with it then
>>> warns:
>>>
>>>   cc1: warning: '-Wformat-y2k' ignored without '-Wformat'
>>>   cc1: warning: '-Wformat-security' ignored without '-Wformat'
>>>
>>> The warning is easy to miss in an in-tree build: tools/lib/bpf/Makefile
>>> re-adds -Wall via "override CFLAGS += -Wall", which re-enables -Wformat
>>> for the libbpf objects, so only libbpf's feature-detection probe (which
>>> uses the passed CFLAGS verbatim) leaks the two warnings. The standalone
>>> libbpf Makefile (github.com/libbpf/libbpf, used by the bpftool mirror)
>>> instead uses "CFLAGS ?= ... -Wall", which the passed-in CFLAGS overrides,
>>> so -Wall is never re-added and every bootstrap object warns.
>>>
>>> Use a -Wformat% wildcard in the filter-out so the orphaned children are
>>> removed together with the parent.
>>>
>>> Fixes: 9080b97689db ("bpftool: Pass host flags to bootstrap libbpf")
>>> Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
>>
>>
>> Looks good, thank you.
>>
>> Acked-by: Quentin Monnet <qmo@kernel.org>
> 
> thanks, applied to bpf-next, please do another bpftool sync when you
> get a chance


Done now, it should be all sorted in the GitHub mirror. Thanks!

Quentin