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

[Quentin Monnet <qmo@kernel.org>]

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>