unshare: new applet

function                                             old     new   delta
unshare_main                                           -     873    +873
.rodata                                           154444  155131    +687
packed_usage                                       30329   30520    +191
unshare_longopts                                       -     106    +106
mount_namespaces                                       -      99     +99
mount_or_die                                           -      51     +51
ns_list                                                -      48     +48
wait_for_exitstatus                                    -      41     +41
opt_str                                                -      17     +17
applet_names                                        2510    2518      +8
applet_main                                         2912    2920      +8
applet_suid                                           91      92      +1
applet_install_loc                                   182     183      +1
------------------------------------------------------------------------------
(add/remove: 8/0 grow/shrink: 6/0 up/down: 2131/0)           Total: 2131 bytes
   text	   data	    bss	    dec	    hex	filename
 826110	   4070	   9080	 839260	  cce5c	busybox_old
 827961	   4078	   9080	 841119	  cd59f	busybox_unstripped

Signed-off-by: Bartosz Golaszewski <bartekgola@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 380 insertions, 0 deletions

diff --git a/util-linux/unshare.c b/util-linux/unshare.c
new file mode 100644
index 000000000..f1a9cdf19
--- /dev/null
+++ b/util-linux/unshare.c
@@ -0,0 +1,380 @@
+/* vi: set sw=4 ts=4: */
+/*
+ * Mini unshare implementation for busybox.
+ *
+ * Copyright (C) 2016 by Bartosz Golaszewski <bartekgola@gmail.com>
+ *
+ * Licensed under GPLv2 or later, see file LICENSE in this source tree.
+ */
+
+//config:config UNSHARE
+//config:	bool "unshare"
+//config:	default y
+//config:	depends on LONG_OPTS && !NOMMU
+//config:	select PLATFORM_LINUX
+//config:	help
+//config:	  Run program with some namespaces unshared from parent.
+
+// depends on LONG_OPTS: it is awkward to exclude code which handles --propagation
+// and --setgroups based on LONG_OPTS, so instead applet requires LONG_OPTS.
+// depends on !NOMMU: we need fork()
+
+//applet:IF_UNSHARE(APPLET(unshare, BB_DIR_USR_BIN, BB_SUID_DROP))
+
+//kbuild:lib-$(CONFIG_UNSHARE) += unshare.o
+
+//usage:#define unshare_trivial_usage
+//usage:       "[OPTIONS] [PROG [ARGS]]"
+//usage:#define unshare_full_usage "\n"
+//usage:     "\n	-m, --mount[=FILE]	Unshare mount namespace"
+//usage:     "\n	-u, --uts[=FILE]	Unshare UTS namespace (hostname etc.)"
+//usage:     "\n	-i, --ipc[=FILE]	Unshare System V IPC namespace"
+//usage:     "\n	-n, --net[=FILE]	Unshare network namespace"
+//usage:     "\n	-p, --pid[=FILE]	Unshare PID namespace"
+//usage:     "\n	-U, --user[=FILE}	Unshare user namespace"
+//usage:     "\n	-f, --fork		Fork before execing PROG"
+//usage:     "\n	-r, --map-root-user	Map current user to root (implies -u)"
+//usage:     "\n	--mount-proc[=DIR]	Mount /proc filesystem first (implies -m)"
+//usage:     "\n	--propagation slave|shared|private|unchanged"
+//usage:     "\n				Modify mount propagation in mount namespace"
+//usage:     "\n	--setgroups allow|deny	Control the setgroups syscall in user namespaces"
+
+#include <sched.h>
+#include <sys/mount.h>
+#include "libbb.h"
+
+static void mount_or_die(const char *source, const char *target,
+                 const char *fstype, unsigned long mountflags)
+{
+	if (mount(source, target, fstype, mountflags, NULL)) {
+		bb_perror_msg_and_die("can't mount %s on %s (flags:0x%lx)",
+			source, target, mountflags);
+		/* fstype is always either NULL or "proc".
+		 * "proc" is only used to mount /proc.
+		 * No need to clutter up error message with fstype,
+		 * it is easily deductible.
+		 */
+	}
+}
+
+// TODO: move to libbb
+static int wait_for_exitstatus(pid_t pid)
+{
+	int exit_status, n;
+
+	n = safe_waitpid(pid, &exit_status, 0);
+	if (n < 0)
+		bb_perror_msg_and_die("waitpid");
+	return exit_status;
+}
+
+/*
+ * Longest possible path to a procfs file used in unshare. Must be able to
+ * contain the '/proc/' string, the '/ns/user' string which is the longest
+ * namespace name and a 32-bit integer representing the process ID.
+ */
+#define PATH_PROC_SETGROUPS	"/proc/self/setgroups"
+#define PATH_PROC_UIDMAP	"/proc/self/uid_map"
+#define PATH_PROC_GIDMAP	"/proc/self/gid_map"
+
+struct namespace_descr {
+	int flag;
+	const char nsfile4[4];
+};
+
+struct namespace_ctx {
+	char *path;
+};
+
+enum {
+	OPT_mount	= 1 << 0,
+	OPT_uts		= 1 << 1,
+	OPT_ipc		= 1 << 2,
+	OPT_network	= 1 << 3,
+	OPT_pid		= 1 << 4,
+	OPT_user	= 1 << 5, /* OPT_user, NS_USR_POS, and ns_list[] index must match! */
+	OPT_fork	= 1 << 6,
+	OPT_map_root	= 1 << 7,
+	OPT_mount_proc	= 1 << 8,
+	OPT_propagation	= 1 << 9,
+	OPT_setgroups	= 1 << 10,
+};
+enum {
+	NS_MNT_POS = 0,
+	NS_UTS_POS,
+	NS_IPC_POS,
+	NS_NET_POS,
+	NS_PID_POS,
+	NS_USR_POS, /* OPT_user, NS_USR_POS, and ns_list[] index must match! */
+	NS_COUNT,
+};
+static const struct namespace_descr ns_list[] = {
+	{ CLONE_NEWNS,   "mnt"  },
+	{ CLONE_NEWUTS,  "uts"  },
+	{ CLONE_NEWIPC,  "ipc"  },
+	{ CLONE_NEWNET,  "net"  },
+	{ CLONE_NEWPID,  "pid"  },
+	{ CLONE_NEWUSER, "user" }, /* OPT_user, NS_USR_POS, and ns_list[] index must match! */
+};
+
+/*
+ * Upstream unshare doesn't support short options for --mount-proc,
+ * --propagation, --setgroups.
+ * Optional arguments (namespace mountpoints) exist only for long opts,
+ * we are forced to use "fake" letters for them.
+ * '+': stop at first non-option.
+ */
+static const char opt_str[] = "+muinpU""fr""\xfd::""\xfe:""\xff:";
+static const char unshare_longopts[] ALIGN1 =
+	"mount\0"		Optional_argument	"\xf0"
+	"uts\0"			Optional_argument	"\xf1"
+	"ipc\0"			Optional_argument	"\xf2"
+	"network\0"		Optional_argument	"\xf3"
+	"pid\0"			Optional_argument	"\xf4"
+	"user\0"		Optional_argument	"\xf5"
+	"fork\0"		No_argument		"f"
+	"map-root-user\0"	No_argument		"r"
+	"mount-proc\0"		Optional_argument	"\xfd"
+	"propagation\0"		Required_argument	"\xfe"
+	"setgroups\0"		Required_argument	"\xff"
+;
+
+/* Ugly-looking string reuse trick */
+#define PRIVATE_STR   "private\0""unchanged\0""shared\0""slave\0"
+#define PRIVATE_UNCHANGED_SHARED_SLAVE   PRIVATE_STR
+
+static unsigned long parse_propagation(const char *prop_str)
+{
+	int i = index_in_strings(PRIVATE_UNCHANGED_SHARED_SLAVE, prop_str);
+	if (i < 0)
+		bb_error_msg_and_die("unrecognized: --%s=%s", "propagation", prop_str);
+	if (i == 0)
+		return MS_REC | MS_PRIVATE;
+	if (i == 1)
+		return 0;
+	if (i == 2)
+		return MS_REC | MS_SHARED;
+	return MS_REC | MS_SLAVE;
+}
+
+static void mount_namespaces(pid_t pid, struct namespace_ctx *ns_ctx_list)
+{
+	const struct namespace_descr *ns;
+	struct namespace_ctx *ns_ctx;
+	int i;
+
+	for (i = 0; i < NS_COUNT; i++) {
+		char nsf[sizeof("/proc/%u/ns/AAAA") + sizeof(int)*3];
+
+		ns = &ns_list[i];
+		ns_ctx = &ns_ctx_list[i];
+		if (!ns_ctx->path)
+			continue;
+		sprintf(nsf, "/proc/%u/ns/%.4s", (unsigned)pid, ns->nsfile4);
+		mount_or_die(nsf, ns_ctx->path, NULL, MS_BIND);
+	}
+}
+
+int unshare_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
+int unshare_main(int argc UNUSED_PARAM, char **argv)
+{
+	int i;
+	unsigned int opts;
+	int unsflags;
+	uintptr_t need_mount;
+	const char *proc_mnt_target;
+	const char *prop_str;
+	const char *setgrp_str;
+	unsigned long prop_flags;
+	uid_t reuid = geteuid();
+	gid_t regid = getegid();
+	struct fd_pair fdp;
+	pid_t child = child; /* for compiler */
+	struct namespace_ctx ns_ctx_list[NS_COUNT];
+
+	memset(ns_ctx_list, 0, sizeof(ns_ctx_list));
+	proc_mnt_target = "/proc";
+	prop_str = PRIVATE_STR;
+	setgrp_str = NULL;
+
+	opt_complementary =
+		"\xf0""m" /* long opts (via their "fake chars") imply short opts */
+		":\xf1""u"
+		":\xf2""i"
+		":\xf3""n"
+		":\xf4""p"
+		":\xf5""U"
+		":ru"	   /* --map-root-user or -r implies -u */
+		":\xfd""m" /* --mount-proc implies -m */
+	;
+	applet_long_options = unshare_longopts;
+	opts = getopt32(argv, opt_str,
+			&proc_mnt_target, &prop_str, &setgrp_str,
+			&ns_ctx_list[NS_MNT_POS].path,
+			&ns_ctx_list[NS_UTS_POS].path,
+			&ns_ctx_list[NS_IPC_POS].path,
+			&ns_ctx_list[NS_NET_POS].path,
+			&ns_ctx_list[NS_PID_POS].path,
+			&ns_ctx_list[NS_USR_POS].path
+	);
+	argv += optind;
+	//bb_error_msg("opts:0x%x", opts);
+	//bb_error_msg("mount:%s", ns_ctx_list[NS_MNT_POS].path);
+	//bb_error_msg("proc_mnt_target:%s", proc_mnt_target);
+	//bb_error_msg("prop_str:%s", prop_str);
+	//bb_error_msg("setgrp_str:%s", setgrp_str);
+	//exit(1);
+
+	if (setgrp_str) {
+		if (strcmp(setgrp_str, "allow") == 0) {
+			if (opts & OPT_map_root) {
+				bb_error_msg_and_die(
+					"--setgroups=allow and --map-root-user "
+					"are mutually exclusive"
+				);
+			}
+		} else {
+			/* It's not "allow", must be "deny" */
+			if (strcmp(setgrp_str, "deny") != 0)
+				bb_error_msg_and_die("unrecognized: --%s=%s",
+					"setgroups", setgrp_str);
+		}
+	}
+
+	unsflags = 0;
+	need_mount = 0;
+	for (i = 0; i < NS_COUNT; i++) {
+		const struct namespace_descr *ns = &ns_list[i];
+		struct namespace_ctx *ns_ctx = &ns_ctx_list[i];
+
+		if (opts & (1 << i))
+			unsflags |= ns->flag;
+
+		need_mount |= (uintptr_t)(ns_ctx->path);
+	}
+	/* need_mount != 0 if at least one FILE was given */
+
+	prop_flags = MS_REC | MS_PRIVATE;
+	/* Silently ignore --propagation if --mount is not requested. */
+	if (opts & OPT_mount)
+		prop_flags = parse_propagation(prop_str);
+
+	/*
+	 * Special case: if we were requested to unshare the mount namespace
+	 * AND to make any namespace persistent (by bind mounting it) we need
+	 * to spawn a child process which will wait for the parent to call
+	 * unshare(), then mount parent's namespaces while still in the
+	 * previous namespace.
+	 */
+	fdp.wr = -1;
+	if (need_mount && (opts & OPT_mount)) {
+		/*
+		 * Can't use getppid() in child, as we can be unsharing the
+		 * pid namespace.
+		 */
+		pid_t ppid = getpid();
+
+		xpiped_pair(fdp);
+
+		child = xfork();
+		if (child == 0) {
+			/* Child */
+			close(fdp.wr);
+
+			/* Wait until parent calls unshare() */
+			read(fdp.rd, ns_ctx_list, 1); /* ...using bogus buffer */
+			/*close(fdp.rd);*/
+
+			/* Mount parent's unshared namespaces. */
+			mount_namespaces(ppid, ns_ctx_list);
+			return EXIT_SUCCESS;
+		}
+		/* Parent continues */
+	}
+
+	if (unshare(unsflags) != 0)
+		bb_perror_msg_and_die("unshare(0x%x)", unsflags);
+
+	if (fdp.wr >= 0) {
+		close(fdp.wr); /* Release child */
+		/*close(fdp.rd);*/
+	}
+
+	if (need_mount) {
+		/* Wait for the child to finish mounting the namespaces. */
+		if (opts & OPT_mount) {
+			int exit_status = wait_for_exitstatus(child);
+			if (WIFEXITED(exit_status) &&
+			    WEXITSTATUS(exit_status) != EXIT_SUCCESS)
+				return WEXITSTATUS(exit_status);
+		} else {
+			/*
+			 * Regular way - we were requested to mount some other
+			 * namespaces: mount them after the call to unshare().
+			 */
+			mount_namespaces(getpid(), ns_ctx_list);
+		}
+	}
+
+	/*
+	 * When we're unsharing the pid namespace, it's not the process that
+	 * calls unshare() that is put into the new namespace, but its first
+	 * child. The user may want to use this option to spawn a new process
+	 * that'll become PID 1 in this new namespace.
+	 */
+	if (opts & OPT_fork) {
+		pid_t pid = xfork();
+		if (pid > 0) {
+			/* Parent */
+			int exit_status = wait_for_exitstatus(pid);
+			if (WIFSIGNALED(exit_status))
+				kill_myself_with_sig(WTERMSIG(exit_status));
+			return WEXITSTATUS(exit_status);
+		}
+		/* Child continues */
+	}
+
+	if (opts & OPT_map_root) {
+		char uidmap_buf[sizeof("%u 0 1") + sizeof(int)*3];
+
+		/*
+		 * Since Linux 3.19 unprivileged writing of /proc/self/gid_map
+		 * has been disabled unless /proc/self/setgroups is written
+		 * first to permanently disable the ability to call setgroups
+		 * in that user namespace.
+		 */
+		xopen_xwrite_close(PATH_PROC_SETGROUPS, "deny");
+		sprintf(uidmap_buf, "%u 0 1", (unsigned)reuid);
+		xopen_xwrite_close(PATH_PROC_UIDMAP, uidmap_buf);
+		sprintf(uidmap_buf, "%u 0 1", (unsigned)regid);
+		xopen_xwrite_close(PATH_PROC_GIDMAP, uidmap_buf);
+	} else
+	if (setgrp_str) {
+		/* Write "allow" or "deny" */
+		xopen_xwrite_close(PATH_PROC_SETGROUPS, setgrp_str);
+	}
+
+	if (opts & OPT_mount) {
+		mount_or_die("none", "/", NULL, prop_flags);
+	}
+
+	if (opts & OPT_mount_proc) {
+		/*
+		 * When creating a new pid namespace, we might want the pid
+		 * subdirectories in /proc to remain consistent with the new
+		 * process IDs. Without --mount-proc the pids in /proc would
+		 * still reflect the old pid namespace. This is why we make
+		 * /proc private here and then do a fresh mount.
+		 */
+		mount_or_die("none", proc_mnt_target, NULL, MS_PRIVATE | MS_REC);
+		mount_or_die("proc", proc_mnt_target, "proc", MS_NOSUID | MS_NOEXEC | MS_NODEV);
+	}
+
+	if (argv[0]) {
+		BB_EXECVP_or_die(argv);
+	}
+	/* unshare from util-linux 2.27.1, despite not documenting it,
+	 * runs a login shell (argv0="-sh") if no PROG is given
+	 */
+	run_shell(getenv("SHELL"), /*login:*/ 1, NULL, NULL);
+}