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bpftool: Strip all -Wformat* flags from bootstrap libbpf build

PATCH Applied Andrii Nakryiko 4 participants 6 messages Jun 30 – Jul 2

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Andrii Nakryiko

author maintainer
#0 lore raw

PATCH 1/1 bpftool: Strip all -Wformat* flags from bootstrap libbpf build

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 <[email protected]>
---
 tools/bpf/bpftool/Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
tools/bpf/bpftool/Makefile +1 -1
index 271a7dc77273..b0f7168e7943 100644
@@ -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
Signature · 1 line
2.53.0-Meta
maintainer Acked-by on v1 1/1
#1 lore raw

2026-06-30 13:54 UTC-0700 ~ Andrii Nakryiko <[email protected]>

Quoted 22 lines

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.

Quoted 5 lines

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 <[email protected]>

Looks good, thank you.

Acked-by Quentin Monnet <[email protected]>
Applied
#2 lore raw
Quoted 27 lines · Quentin Monnet

2026-06-30 13:54 UTC-0700 ~ Andrii Nakryiko <[email protected]>

Quoted 25 lines

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.

Quoted 7 lines
Quoted 2 lines
Fixes 9080b97689db ("bpftool: Pass host flags to bootstrap libbpf")
Signed-off-by Andrii Nakryiko <[email protected]>

Looks good, thank you.

Acked-by Quentin Monnet <[email protected]>

thanks, applied to bpf-next, please do another bpftool sync when you get a chance

maintainer
#3 lore raw

2026-07-01 11:15 UTC-0700 ~ Andrii Nakryiko <[email protected]>

Quoted 32 lines · Quentin Monnet
Quoted 31 lines

2026-06-30 13:54 UTC-0700 ~ Andrii Nakryiko <[email protected]>

Quoted 27 lines

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 <[email protected]>
Quoted 6 lines
Quoted 3 lines

Looks good, thank you.

Acked-by Quentin Monnet <[email protected]>

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

#4 lore raw
Quoted 36 lines · Quentin Monnet

2026-07-01 11:15 UTC-0700 ~ Andrii Nakryiko <[email protected]>

Quoted 34 lines · Quentin Monnet
Quoted 33 lines

2026-06-30 13:54 UTC-0700 ~ Andrii Nakryiko <[email protected]>

Quoted 22 lines

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.

Quoted 5 lines

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 <[email protected]>

Looks good, thank you.

Quoted 8 lines
Quoted 4 lines
Quoted 1 line
Acked-by Quentin Monnet <[email protected]>

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!

Quoted 1 line

Quentin

CI Applied
#5 lore raw
In reply to Andrii Nakryiko
This patch was applied to bpf/bpf-next.git (master) by Andrii Nakryiko <[email protected]>:

Hello:

This patch was applied to bpf/bpf-next.git (master) by Andrii Nakryiko <[email protected]>:

Quoted 5 lines · 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.

Quoted 7 lines

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!

Signature · 2 lines
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