bpftool: Allow explicitly skip llvm, libbfd and libcrypto dependencies

Introduce SKIP_LLVM, SKIP_LIBBFD, and SKIP_CRYPTO build flags that let
users build bpftool without these optional dependencies.

SKIP_LLVM=1 skips LLVM even when detected. SKIP_LIBBFD=1 prevents the
libbfd JIT disassembly fallback when LLVM is absent. Together, they
produce a bpftool with no disassembly support.

SKIP_CRYPTO=1 excludes sign.c and removes the -lcrypto link dependency.
Inline stubs in main.h return errors with a clear message if signing
functions are called at runtime.

Use BPFTOOL_WITHOUT_CRYPTO (not HAVE_LIBCRYPTO_SUPPORT) as the C
define, following the BPFTOOL_WITHOUT_SKELETONS naming convention for
bpftool-internal build config, leaving HAVE_LIBCRYPTO_SUPPORT free for
proper feature detection in the future.

All three flags are propagated through the selftests Makefile to bpftool
sub-builds.

Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20260312-b4-bpftool_build-v2-1-4c9d57133644@meta.com

tools/bpf/bpftool/Makefile

@@ -97,6 +97,15 @@ RM ?= rm -f
 
 FEATURE_USER = .bpftool
 
+# Skip optional dependencies: LLVM (JIT disasm), libbfd (fallback
+# disasm), libcrypto (program signing).
+SKIP_LLVM ?=
+SKIP_LIBBFD ?=
+SKIP_CRYPTO ?=
+ifneq ($(SKIP_CRYPTO),1)
+  CRYPTO_LIBS := -lcrypto
+endif
+
 FEATURE_TESTS := clang-bpf-co-re
 FEATURE_TESTS += llvm
 FEATURE_TESTS += libcap
@@ -130,8 +139,8 @@ include $(FEATURES_DUMP)
 endif
 endif
 
-LIBS = $(LIBBPF) -lelf -lcrypto -lz
-LIBS_BOOTSTRAP = $(LIBBPF_BOOTSTRAP) -lelf -lcrypto -lz
+LIBS = $(LIBBPF) -lelf $(CRYPTO_LIBS) -lz
+LIBS_BOOTSTRAP = $(LIBBPF_BOOTSTRAP) -lelf $(CRYPTO_LIBS) -lz
 
 ifeq ($(feature-libelf-zstd),1)
 LIBS += -lzstd
@@ -150,7 +159,12 @@ all: $(OUTPUT)bpftool
 SRCS := $(wildcard *.c)
 
 ifeq ($(feature-llvm),1)
-  # If LLVM is available, use it for JIT disassembly
+ifneq ($(SKIP_LLVM),1)
+HAS_LLVM := 1
+endif
+endif
+
+ifeq ($(HAS_LLVM),1)
   CFLAGS  += -DHAVE_LLVM_SUPPORT
   LLVM_CONFIG_LIB_COMPONENTS := mcdisassembler all-targets
   # llvm-config always adds -D_GNU_SOURCE, however, it may already be in CFLAGS
@@ -165,6 +179,7 @@ ifeq ($(feature-llvm),1)
   endif
   LDFLAGS += $(shell $(LLVM_CONFIG) --ldflags)
 else
+  ifneq ($(SKIP_LIBBFD),1)
   # Fall back on libbfd
   ifeq ($(feature-libbfd),1)
     LIBS += -lbfd -ldl -lopcodes
@@ -186,15 +201,22 @@ else
       CFLAGS += -DDISASM_INIT_STYLED
     endif
   endif
+  endif # SKIP_LIBBFD
 endif
 ifeq ($(filter -DHAVE_LLVM_SUPPORT -DHAVE_LIBBFD_SUPPORT,$(CFLAGS)),)
   # No support for JIT disassembly
   SRCS := $(filter-out jit_disasm.c,$(SRCS))
 endif
 
+ifeq ($(SKIP_CRYPTO),1)
+  CFLAGS += -DBPFTOOL_WITHOUT_CRYPTO
+  HOST_CFLAGS += -DBPFTOOL_WITHOUT_CRYPTO
+  SRCS := $(filter-out sign.c,$(SRCS))
+endif
+
 BPFTOOL_BOOTSTRAP := $(BOOTSTRAP_OUTPUT)bpftool
 
-BOOTSTRAP_OBJS = $(addprefix $(BOOTSTRAP_OUTPUT),main.o common.o json_writer.o gen.o btf.o sign.o)
+BOOTSTRAP_OBJS = $(addprefix $(BOOTSTRAP_OUTPUT),main.o common.o json_writer.o gen.o btf.o $(if $(CRYPTO_LIBS),sign.o))
 $(BOOTSTRAP_OBJS): $(LIBBPF_BOOTSTRAP)
 
 OBJS = $(patsubst %.c,$(OUTPUT)%.o,$(SRCS)) $(OUTPUT)disasm.o

tools/bpf/bpftool/main.c

@@ -131,6 +131,11 @@ static int do_version(int argc, char **argv)
 	const bool has_skeletons = false;
 #else
 	const bool has_skeletons = true;
+#endif
+#ifdef BPFTOOL_WITHOUT_CRYPTO
+	const bool has_crypto = false;
+#else
+	const bool has_crypto = true;
 #endif
 	bool bootstrap = false;
 	int i;
@@ -163,6 +168,7 @@ static int do_version(int argc, char **argv)
 		jsonw_start_object(json_wtr);	/* features */
 		jsonw_bool_field(json_wtr, "libbfd", has_libbfd);
 		jsonw_bool_field(json_wtr, "llvm", has_llvm);
+		jsonw_bool_field(json_wtr, "crypto", has_crypto);
 		jsonw_bool_field(json_wtr, "skeletons", has_skeletons);
 		jsonw_bool_field(json_wtr, "bootstrap", bootstrap);
 		jsonw_end_object(json_wtr);	/* features */
@@ -181,6 +187,7 @@ static int do_version(int argc, char **argv)
 		printf("features:");
 		print_feature("libbfd", has_libbfd, &nb_features);
 		print_feature("llvm", has_llvm, &nb_features);
+		print_feature("crypto", has_crypto, &nb_features);
 		print_feature("skeletons", has_skeletons, &nb_features);
 		print_feature("bootstrap", bootstrap, &nb_features);
 		printf("\n");

tools/bpf/bpftool/main.h

@@ -293,6 +293,20 @@ struct kernel_config_option {
 int read_kernel_config(const struct kernel_config_option *requested_options,
 		       size_t num_options, char **out_values,
 		       const char *define_prefix);
+#ifndef BPFTOOL_WITHOUT_CRYPTO
 int bpftool_prog_sign(struct bpf_load_and_run_opts *opts);
 __u32 register_session_key(const char *key_der_path);
+#else
+static inline int bpftool_prog_sign(struct bpf_load_and_run_opts *opts)
+{
+	p_err("bpftool was built without signing support");
+	return -ENOTSUP;
+}
+
+static inline __u32 register_session_key(const char *key_der_path)
+{
+	p_err("bpftool was built without signing support");
+	return -1;
+}
+#endif
 #endif

tools/testing/selftests/bpf/Makefile

@@ -41,6 +41,8 @@ LIBELF_LIBS	:= $(shell $(PKG_CONFIG) libelf --libs 2>/dev/null || echo -lelf)
 
 SKIP_DOCS	?=
 SKIP_LLVM	?=
+SKIP_LIBBFD	?=
+SKIP_CRYPTO	?=
 
 ifeq ($(srctree),)
 srctree := $(patsubst %/,%,$(dir $(CURDIR)))
@@ -333,6 +335,9 @@ $(DEFAULT_BPFTOOL): $(wildcard $(BPFTOOLDIR)/*.[ch] $(BPFTOOLDIR)/Makefile)    \
 		    OUTPUT=$(HOST_BUILD_DIR)/bpftool/			       \
 		    LIBBPF_OUTPUT=$(HOST_BUILD_DIR)/libbpf/		       \
 		    LIBBPF_DESTDIR=$(HOST_SCRATCH_DIR)/			       \
+		    SKIP_LLVM=$(SKIP_LLVM)				       \
+		    SKIP_LIBBFD=$(SKIP_LIBBFD)				       \
+		    SKIP_CRYPTO=$(SKIP_CRYPTO)				       \
 		    prefix= DESTDIR=$(HOST_SCRATCH_DIR)/ install-bin
 
 ifneq ($(CROSS_COMPILE),)
@@ -345,6 +350,9 @@ $(CROSS_BPFTOOL): $(wildcard $(BPFTOOLDIR)/*.[ch] $(BPFTOOLDIR)/Makefile)	\
 		    OUTPUT=$(BUILD_DIR)/bpftool/				\
 		    LIBBPF_OUTPUT=$(BUILD_DIR)/libbpf/				\
 		    LIBBPF_DESTDIR=$(SCRATCH_DIR)/				\
+		    SKIP_LLVM=$(SKIP_LLVM)					\
+		    SKIP_LIBBFD=$(SKIP_LIBBFD)					\
+		    SKIP_CRYPTO=$(SKIP_CRYPTO)					\
 		    prefix= DESTDIR=$(SCRATCH_DIR)/ install-bin
 endif