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

diff --git a/tools/bpf/bpftool/Documentation/bpftool-feature.rst b/tools/bpf/bpftool/Documentation/bpftool-feature.rst
index ca085944e4cf1fa462583c97ef7ea136fecee58a..1fa755f55e0c1925d4f651f4edf5389750a589db 100644 (file)
--- a/tools/bpf/bpftool/Documentation/bpftool-feature.rst
+++ b/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

diff --git a/tools/bpf/bpftool/Makefile b/tools/bpf/bpftool/Makefile
index 89d7962a4a44a869a9a5b18903e9dbc996fe50ed..2759f9cc32897bec48e3cd0f5670d394faac5e45 100644 (file)
--- a/tools/bpf/bpftool/Makefile
+++ b/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

diff --git a/tools/bpf/bpftool/feature.c b/tools/bpf/bpftool/feature.c
index 952f4b1987c0849458cfb90095fd7672f6b2cb9a..f54347f55ee0335dc2be099476244a81ecd6d4db 100644 (file)
--- a/tools/bpf/bpftool/feature.c
+++ b/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 @@ exit_free:
        }
 
        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);