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

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

[Andrii Nakryiko]

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>
---
 tools/bpf/bpftool/Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/bpf/bpftool/Makefile b/tools/bpf/bpftool/Makefile
index 271a7dc77273..b0f7168e7943 100644
--- a/tools/bpf/bpftool/Makefile
+++ b/tools/bpf/bpftool/Makefile
@@ -99,7 +99,7 @@ endif
 HOST_LDFLAGS := $(LDFLAGS)
 
 # Remove warnings for libbpf bootstrap build
-LIBBPF_BOOTSTRAP_CFLAGS := $(filter-out -W -Wall -Wextra -Wformat -Wformat-signedness,$(HOST_CFLAGS))
+LIBBPF_BOOTSTRAP_CFLAGS := $(filter-out -W -Wall -Wextra -Wformat%,$(HOST_CFLAGS))
 
 INSTALL ?= install
 RM ?= rm -f
-- 
2.53.0-Meta

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

[Quentin Monnet]

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>

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

[Andrii Nakryiko]

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

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

[Quentin Monnet]

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

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

[Andrii Nakryiko]

On Thu, Jul 2, 2026 at 7:57 AM Quentin Monnet <qmo@kernel.org> wrote:
>
> 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!
>

Nice, finally warning-free wprof build :) Thanks!

> Quentin

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

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to bpf/bpf-next.git (master)
by Andrii Nakryiko <andrii@kernel.org>:

On Tue, 30 Jun 2026 13:54:18 -0700 you wrote:
> 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:
> 
> [...]

Here is the summary with links:
  - [bpf-next] bpftool: Strip all -Wformat* flags from bootstrap libbpf build
    https://git.kernel.org/bpf/bpf-next/c/a954c9e3168c

You are awesome, thank you!
-- 
Deet-doot-dot, I am a bot.
https://korg.docs.kernel.org/patchwork/pwbot.html