autotools: check for cap_get_file

configure.ac

@@ -368,7 +368,9 @@ AM_CONDITIONAL([ENABLE_CAP], [test "x$enable_capabilities" = "xyes"])
 AM_COND_IF([ENABLE_CAP],
 	[AC_CHECK_HEADER([sys/capability.h],[],[AC_MSG_ERROR([You must install the libcap development package in order to compile lxc])])
 	AC_CHECK_LIB(cap,cap_set_proc,[],[AC_MSG_ERROR([You must install the libcap development package in order to compile lxc])])
-	AC_SUBST([CAP_LIBS], [-lcap])])
+        # Test whether we support getting file capabilities via cap_get_file().
+        AC_CHECK_LIB(cap,cap_get_file, AC_DEFINE(LIBCAP_SUPPORTS_FILE_CAPABILITIES,1,[Have cap_get_file]),[],[])
+        AC_SUBST([CAP_LIBS], [-lcap])])
 
 # HAVE_SCMP_FILTER_CTX=1 will tell us we have libseccomp api >= 1.0.0
 OLD_CFLAGS="$CFLAGS"

src/lxc/caps.c

@@ -225,10 +225,7 @@ static bool lxc_cap_is_set(cap_t caps, cap_value_t cap, cap_flag_t flag)
 
 bool lxc_file_cap_is_set(const char *path, cap_value_t cap, cap_flag_t flag)
 {
-	/* Android's bionic currently seems to lack support for cap_get_file(). */
-	#if IS_BIONIC
-	return true;
-	#else
+	#if LIBCAP_SUPPORTS_FILE_CAPABILITIES
 	bool cap_is_set;
 	cap_t caps;
 
@@ -247,6 +244,9 @@ bool lxc_file_cap_is_set(const char *path, cap_value_t cap, cap_flag_t flag)
 	cap_is_set = lxc_cap_is_set(caps, cap, flag);
 	cap_free(caps);
 	return cap_is_set;
+	#else
+	errno = ENODATA;
+	return false;
 	#endif
 }
 

src/lxc/conf.c

@@ -3254,7 +3254,7 @@ static int idmaptool_on_path_and_privileged(const char *binary, cap_value_t cap)
 		goto cleanup;
 	}
 
-	#if HAVE_LIBCAP && !IS_BIONIC
+	#if HAVE_LIBCAP && LIBCAP_SUPPORTS_FILE_CAPABILITIES
 	/* Check if it has the CAP_SETUID capability. */
 	if ((cap & CAP_SETUID) &&
 	    lxc_file_cap_is_set(path, CAP_SETUID, CAP_EFFECTIVE) &&
@@ -3275,6 +3275,10 @@ static int idmaptool_on_path_and_privileged(const char *binary, cap_value_t cap)
 		goto cleanup;
 	}
 	#else
+	/* If we cannot check for file capabilities we need to give the benefit
+	 * of the doubt. Otherwise we might fail even though all the necessary
+	 * file capabilities are set.
+	 */
 	DEBUG("Cannot check for file capabilites as full capability support is "
 	      "missing. Manual intervention needed.");
 	fret = 1;