cmds: add ns/clone

usage: ns/clone pid cmd [args...]

It starts cmd with provided arguments in a clone of the namespace
available to pid.

sys/src/cmd/ns/build.json

@@ -5,7 +5,8 @@
 		],
 		"Install": "/arch/$ARCH/cmd/ns/",
 		"SourceFilesCmd": [
-			"cat.c"
+			"cat.c",
+			"clone.c"
 		]
 	}
 }

sys/src/cmd/ns/clone.c

@@ -0,0 +1,63 @@
+/*
+ * This file is part of Jehanne.
+ *
+ * Copyright (C) 2017 Giacomo Tesio <giacomo@tesio.it>
+ *
+ * This is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, version 3 of the License.
+ *
+ * Jehanne is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with Jehanne.  If not, see <http://www.gnu.org/licenses/>.
+ */
+#include <u.h>
+#include <lib9.h>
+#include <envvars.h>
+
+void
+main(int argc, char *argv[])
+{
+	int fd, npaths;
+	char buf[256], *cmd, **args, *paths[17];
+
+	rfork(RFNAMEG);
+	if(argc < 3){
+		fprint(2, "usage: ns/clone pid cmd [args...]\n");
+		exits("usage");
+	}
+
+	snprint(buf, sizeof(buf), "/proc/%s/ns", argv[1]);
+	fd = open(buf, OWRITE);
+	if(fd < 0){
+		print("cannot open %s\n", buf);
+		exits("invalid target pid");
+	}
+	write(fd, "clone", 6);
+	close(fd);
+
+	cmd = strdup(argv[2]);
+	args = &argv[2];
+	if(cmd[0] == '/' 
+	||(cmd[0] == '.' && (cmd[1] == '/' || (cmd[1] == '.' && cmd[2] == '/')))){
+		exec(cmd, args);
+		sysfatal("exec %s failed: %r", cmd);
+	}
+
+	npaths = getfields(getenv(ENV_PATH), paths, nelem(paths), 1, ":");
+
+	if(npaths == nelem(paths))
+		--npaths;	// ignore last (possibly spurious) path
+
+	fd = 0;
+	while(fd < npaths){
+		snprint(buf, sizeof(buf), "%s/%s", paths[fd], cmd);
+		exec(buf, args);
+		++fd;
+	}
+	sysfatal("exec %s failed: %r", cmd);
+}