From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: stable@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	patches@lists.linux.dev,
	Andrii Nakryiko <andrii@kernel.org>,
	Quentin Monnet <qmo@kernel.org>,
	Sasha Levin <sashal@kernel.org>
Subject: [PATCH 6.12 0466/1102] bpftool: Strip all -Wformat* flags from bootstrap libbpf build
Date: Thu, 17 Sep 2026 16:06:49 +0100
Message-ID: <20260917151551.389749085@linuxfoundation.org>
In-Reply-To: <20260917151539.408551884@linuxfoundation.org>
References: <20260917151539.408551884@linuxfoundation.org>
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6.12-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Andrii Nakryiko <andrii@kernel.org>

[ Upstream commit a954c9e3168cdf0c3cad07b43dfc8ca2945d773a ]

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>
Acked-by: Quentin Monnet <qmo@kernel.org>
Link: https://lore.kernel.org/bpf/20260630205418.3483969-1-andrii@kernel.org
Signed-off-by: Sasha Levin <sashal@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 f92cdb7dbd428..6c2d32c874a6a 100644
--- a/tools/bpf/bpftool/Makefile
+++ b/tools/bpf/bpftool/Makefile
@@ -95,7 +95,7 @@ HOST_CFLAGS := $(subst -I$(LIBBPF_INCLUDE),-I$(LIBBPF_BOOTSTRAP_INCLUDE),\
 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