From a954c9e3168cdf0c3cad07b43dfc8ca2945d773a Mon Sep 17 00:00:00 2001 From: Andrii Nakryiko Date: Tue, 30 Jun 2026 13:54:18 -0700 Subject: [PATCH] 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 Acked-by: Quentin Monnet Link: https://lore.kernel.org/bpf/20260630205418.3483969-1-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