tools: bpftool: Make libcap dependency optional

The new libcap dependency is not used for an essential feature of
bpftool, and we could imagine building the tool without checks on
CAP_SYS_ADMIN by disabling probing features as an unprivileged users.

Make it so, in order to avoid a hard dependency on libcap, and to ease
packaging/embedding of bpftool.

Signed-off-by: Quentin Monnet <quentin@isovalent.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20200429144506.8999-4-quentin@isovalent.com

Author: qmonnet

tools/bpf/bpftool/Documentation/bpftool-feature.rst

@@ -55,7 +55,9 @@ DESCRIPTION
 		  that case usually represent a small subset of the parameters
 		  supported by the system. Unprivileged users MUST use the
 		  **unprivileged** keyword: This is to avoid misdetection if
-		  bpftool is inadvertently run as non-root, for example.
+		  bpftool is inadvertently run as non-root, for example. This
+		  keyword is unavailable if bpftool was compiled without
+		  libcap.
 
 	**bpftool feature probe dev** *NAME* [**full**] [**macros** [**prefix** *PREFIX*]]
 		  Probe network device for supported eBPF features and dump

tools/bpf/bpftool/Makefile

@@ -55,16 +55,15 @@ ifneq ($(EXTRA_LDFLAGS),)
 LDFLAGS += $(EXTRA_LDFLAGS)
 endif
 
-LIBS = $(LIBBPF) -lelf -lz -lcap
-
 INSTALL ?= install
 RM ?= rm -f
 CLANG ?= clang
 
 FEATURE_USER = .bpftool
-FEATURE_TESTS = libbfd disassembler-four-args reallocarray zlib \
+FEATURE_TESTS = libbfd disassembler-four-args reallocarray zlib libcap \
+	clang-bpf-global-var
+FEATURE_DISPLAY = libbfd disassembler-four-args zlib libcap \
 	clang-bpf-global-var
-FEATURE_DISPLAY = libbfd disassembler-four-args zlib clang-bpf-global-var
 
 check_feat := 1
 NON_CHECK_FEAT_TARGETS := clean uninstall doc doc-clean doc-install doc-uninstall
@@ -90,6 +89,12 @@ ifeq ($(feature-reallocarray), 0)
 CFLAGS += -DCOMPAT_NEED_REALLOCARRAY
 endif
 
+LIBS = $(LIBBPF) -lelf -lz
+ifeq ($(feature-libcap), 1)
+CFLAGS += -DUSE_LIBCAP
+LIBS += -lcap
+endif
+
 include $(wildcard $(OUTPUT)*.d)
 
 all: $(OUTPUT)bpftool

tools/bpf/bpftool/feature.c

@@ -6,7 +6,9 @@
 #include <string.h>
 #include <unistd.h>
 #include <net/if.h>
+#ifdef USE_LIBCAP
 #include <sys/capability.h>
+#endif
 #include <sys/utsname.h>
 #include <sys/vfs.h>
 
@@ -37,7 +39,9 @@ static const char * const helper_name[] = {
 #undef BPF_HELPER_MAKE_ENTRY
 
 static bool full_mode;
+#ifdef USE_LIBCAP
 static bool run_as_unprivileged;
+#endif
 
 /* Miscellaneous utility functions */
 
@@ -475,11 +479,13 @@ probe_prog_type(enum bpf_prog_type prog_type, bool *supported_types,
 		}
 
 	res = bpf_probe_prog_type(prog_type, ifindex);
+#ifdef USE_LIBCAP
 	/* Probe may succeed even if program load fails, for unprivileged users
 	 * check that we did not fail because of insufficient permissions
 	 */
 	if (run_as_unprivileged && errno == EPERM)
 		res = false;
+#endif
 
 	supported_types[prog_type] |= res;
 
@@ -535,12 +541,14 @@ probe_helper_for_progtype(enum bpf_prog_type prog_type, bool supported_type,
 
 	if (supported_type) {
 		res = bpf_probe_helper(id, prog_type, ifindex);
+#ifdef USE_LIBCAP
 		/* Probe may succeed even if program load fails, for
 		 * unprivileged users check that we did not fail because of
 		 * insufficient permissions
 		 */
 		if (run_as_unprivileged && errno == EPERM)
 			res = false;
+#endif
 	}
 
 	if (json_output) {
@@ -738,6 +746,7 @@ static void section_misc(const char *define_prefix, __u32 ifindex)
 
 static int handle_perms(void)
 {
+#ifdef USE_LIBCAP
 	cap_value_t cap_list[1] = { CAP_SYS_ADMIN };
 	bool has_sys_admin_cap = false;
 	cap_flag_value_t val;
@@ -793,6 +802,18 @@ static int handle_perms(void)
 	}
 
 	return res;
+#else
+	/* Detection assumes user has sufficient privileges (CAP_SYS_ADMIN).
+	 * We do not use libpcap so let's approximate, and restrict usage to
+	 * root user only.
+	 */
+	if (geteuid()) {
+		p_err("full feature probing requires root privileges");
+		return -1;
+	}
+
+	return 0;
+#endif /* USE_LIBCAP */
 }
 
 static int do_probe(int argc, char **argv)
@@ -852,8 +873,13 @@ static int do_probe(int argc, char **argv)
 				return -1;
 			define_prefix = GET_ARG();
 		} else if (is_prefix(*argv, "unprivileged")) {
+#ifdef USE_LIBCAP
 			run_as_unprivileged = true;
 			NEXT_ARG();
+#else
+			p_err("unprivileged run not supported, recompile bpftool with libcap");
+			return -1;
+#endif
 		} else {
 			p_err("expected no more arguments, 'kernel', 'dev', 'macros' or 'prefix', got: '%s'?",
 			      *argv);