capsicum: introduce cap_rights_is_empty Function

Before this commit, we only had the capability to check if a specific
capability was set (using cap_rights_is_set function). However, there
was no efficient method to determine if a cap_rights_t structure doesn't
contain any capability. The cap_rights_is_empty function addresses
this gap.

PR:		275330
Reported by:	vini.ipsmaker@gmail.com
Reviewed by:	emaste, markj
Differential Revision:	https://reviews.freebsd.org/D42780

contrib/capsicum-test/capability-fd.cc

@@ -1342,3 +1342,18 @@ TEST(Capability, NoBypassDACIfRoot) {
   close(fd);
   unlink(TmpFile("cap_root_owned"));
 }
+
+TEST(Capability, CheckIsEmpty) {
+  cap_rights_t rights;
+
+  cap_rights_init(&rights);
+  EXPECT_TRUE(cap_rights_is_empty(&rights));
+
+  size_t num_known = (sizeof(known_rights)/sizeof(known_rights[0]));
+  for (size_t ii = 0; ii < num_known; ii++) {
+    cap_rights_init(&rights, known_rights[ii].right);
+    EXPECT_FALSE(cap_rights_is_empty(&rights));
+    cap_rights_clear(&rights, known_rights[ii].right);
+    EXPECT_TRUE(cap_rights_is_empty(&rights));
+  }
+}

lib/libc/capability/Symbol.map

@@ -8,3 +8,7 @@ FBSD_1.3 {
 	cap_rights_remove;
 	__cap_rights_set;
 };
+
+FBSD_1.8 {
+	cap_rights_is_empty;
+};

lib/libc/capability/cap_rights_init.3

@@ -25,7 +25,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd May 5, 2020
+.Dd November 25, 2023
 .Dt CAP_RIGHTS_INIT 3
 .Os
 .Sh NAME
@@ -33,6 +33,7 @@
 .Nm cap_rights_set ,
 .Nm cap_rights_clear ,
 .Nm cap_rights_is_set ,
+.Nm cap_rights_is_empty ,
 .Nm cap_rights_is_valid ,
 .Nm cap_rights_merge ,
 .Nm cap_rights_remove ,
@@ -51,6 +52,8 @@
 .Ft bool
 .Fn cap_rights_is_set "const cap_rights_t *rights" "..."
 .Ft bool
+.Fn cap_rights_is_empty "const cap_rights_t *rights"
+.Ft bool
 .Fn cap_rights_is_valid "const cap_rights_t *rights"
 .Ft cap_rights_t *
 .Fn cap_rights_merge "cap_rights_t *dst" "const cap_rights_t *src"
@@ -118,6 +121,12 @@ function checks if all the given capability rights are set for the given
 structure.
 .Pp
 The
+.Fn cap_rights_is_empty
+function checks if the
+.Fa rights
+structure is empty.
+.Pp
+The
 .Fn cap_rights_is_valid
 function verifies if the given
 .Vt cap_rights_t
@@ -182,6 +191,14 @@ if all the given capability rights are set in the
 argument.
 .Pp
 The
+.Fn cap_rights_is_empty
+function returns
+.Va true
+if none of the capability rights are set in the
+.Fa rights
+structure.
+.Pp
+The
 .Fn cap_rights_is_valid
 function performs various checks to see if the given
 .Vt cap_rights_t

sys/kern/subr_capability.c

@@ -306,6 +306,25 @@ __cap_rights_is_set(const cap_rights_t *rights, ...)
 	return (ret);
 }
 
+bool
+cap_rights_is_empty(const cap_rights_t *rights)
+{
+#ifndef _KERNEL
+	cap_rights_t cap_no_rights;
+	cap_rights_init(&cap_no_rights);
+#endif
+
+	assert(CAPVER(rights) == CAP_RIGHTS_VERSION_00);
+	assert(CAPVER(&cap_no_rights) == CAP_RIGHTS_VERSION_00);
+
+	for (int i = 0; i < CAPARSIZE(rights); i++) {
+		if (rights->cr_rights[i] != cap_no_rights.cr_rights[i])
+			return (false);
+	}
+
+	return (true);
+}
+
 bool
 cap_rights_is_valid(const cap_rights_t *rights)
 {

sys/sys/capsicum.h

@@ -336,6 +336,8 @@ cap_rights_t *__cap_rights_clear(cap_rights_t *rights, ...);
 	__cap_rights_is_set(__VA_ARGS__, 0ULL)
 bool __cap_rights_is_set(const cap_rights_t *rights, ...);
 
+bool cap_rights_is_empty(const cap_rights_t *rights);
+
 bool cap_rights_is_valid(const cap_rights_t *rights);
 cap_rights_t *cap_rights_merge(cap_rights_t *dst, const cap_rights_t *src);
 cap_rights_t *cap_rights_remove(cap_rights_t *dst, const cap_rights_t *src);