7164 zdb should be able to open the root dataset

Reviewed by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>

usr/src/cmd/zdb/zdb.c

@@ -3533,6 +3533,7 @@ main(int argc, char **argv)
 	uint64_t max_txg = UINT64_MAX;
 	int rewind = ZPOOL_NEVER_REWIND;
 	char *spa_config_path_env;
+	boolean_t target_is_spa = B_TRUE;
 
 	(void) setrlimit(RLIMIT_NOFILE, &rl);
 	(void) enable_extended_FILE_stdio(-1, -1);
@@ -3711,8 +3712,23 @@ main(int argc, char **argv)
 		}
 	}
 
+	if (strpbrk(target, "/@") != NULL) {
+		size_t targetlen;
+
+		target_is_spa = B_FALSE;
+		/*
+		 * Remove any trailing slash.  Later code would get confused
+		 * by it, but we want to allow it so that "pool/" can
+		 * indicate that we want to dump the topmost filesystem,
+		 * rather than the whole pool.
+		 */
+		targetlen = strlen(target);
+		if (targetlen != 0 && target[targetlen - 1] == '/')
+			target[targetlen - 1] = '\0';
+	}
+
 	if (error == 0) {
-		if (strpbrk(target, "/@") == NULL || dump_opt['R']) {
+		if (target_is_spa || dump_opt['R']) {
 			error = spa_open_rewind(target, &spa, FTAG, policy,
 			    NULL);
 			if (error) {

usr/src/man/man1m/zdb.1m

@@ -55,6 +55,11 @@ The output of this command in general reflects the on-disk structure of a ZFS
 pool, and is inherently unstable. The precise output of most invocations is
 not documented, a knowledge of ZFS internals is assumed.
 
+.P
+If the \fIdataset\fR argument does not contain any \fB/\fR or \fB@\fR
+characters, it is interpreted as a pool name.  The root dataset can be
+specified as \fIpool\fB/\fR (pool name followed by a slash).
+
 .P
 When operating on an imported and active pool it is possible, though unlikely,
 that zdb may interpret inconsistent pool data and behave erratically.