nsenter,unshare: share common code; fix a bug of not closing all fds

function                                             old     new   delta
xvfork_parent_waits_and_exits                          -      64     +64
exec_prog_or_SHELL                                     -      39     +39
unshare_main                                         873     810     -63
nsenter_main                                         663     596     -67
------------------------------------------------------------------------------
(add/remove: 2/0 grow/shrink: 0/2 up/down: 106/-130)          Total: -27 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

include/libbb.h

@@ -992,9 +992,10 @@ int BB_EXECVP(const char *file, char *const argv[]) FAST_FUNC;
 #define BB_EXECVP(prog,cmd)     execvp(prog,cmd)
 #define BB_EXECLP(prog,cmd,...) execlp(prog,cmd,__VA_ARGS__)
 #endif
-int BB_EXECVP_or_die(char **argv) NORETURN FAST_FUNC;
+void BB_EXECVP_or_die(char **argv) NORETURN FAST_FUNC;
+void exec_prog_or_SHELL(char **argv) NORETURN FAST_FUNC;
 
-/* xvfork() can't be a _function_, return after vfork mangles stack
+/* xvfork() can't be a _function_, return after vfork in child mangles stack
  * in the parent. It must be a macro. */
 #define xvfork() \
 ({ \
@@ -1006,6 +1007,7 @@ int BB_EXECVP_or_die(char **argv) NORETURN FAST_FUNC;
 #if BB_MMU
 pid_t xfork(void) FAST_FUNC;
 #endif
+void xvfork_parent_waits_and_exits(void) FAST_FUNC;
 
 /* NOMMU friendy fork+exec: */
 pid_t spawn(char **argv) FAST_FUNC;

libbb/executable.c

@@ -83,10 +83,19 @@ int FAST_FUNC BB_EXECVP(const char *file, char *const argv[])
 }
 #endif
 
-int FAST_FUNC BB_EXECVP_or_die(char **argv)
+void FAST_FUNC BB_EXECVP_or_die(char **argv)
 {
 	BB_EXECVP(argv[0], argv);
 	/* SUSv3-mandated exit codes */
 	xfunc_error_retval = (errno == ENOENT) ? 127 : 126;
 	bb_perror_msg_and_die("can't execute '%s'", argv[0]);
 }
+
+/* Typical idiom for applets which exec *optional* PROG [ARGS] */
+void FAST_FUNC exec_prog_or_SHELL(char **argv)
+{
+	if (argv[0]) {
+		BB_EXECVP_or_die(argv);
+	}
+	run_shell(getenv("SHELL"), /*login:*/ 1, NULL, NULL);
+}

libbb/xfuncs_printf.c

@@ -659,3 +659,19 @@ pid_t FAST_FUNC xfork(void)
 	return pid;
 }
 #endif
+
+void FAST_FUNC xvfork_parent_waits_and_exits(void)
+{
+	pid_t pid;
+
+	fflush_all();
+	pid = xvfork();
+	if (pid > 0) {
+		/* Parent */
+		int exit_status = wait_for_exitstatus(pid);
+		if (WIFSIGNALED(exit_status))
+			kill_myself_with_sig(WTERMSIG(exit_status));
+		_exit(WEXITSTATUS(exit_status));
+	}
+	/* Child continues */
+}

util-linux/nsenter.c

@@ -230,7 +230,7 @@ int nsenter_main(int argc UNUSED_PARAM, char **argv)
 				ns->ns_nsfile8 + 3 /* skip over "ns/" */
 			);
 		}
-		/*close(ns_ctx->fd);*/
+		close(ns_ctx->fd); /* should close fds, to not confuse exec'ed PROG */
 		/*ns_ctx->fd = -1;*/
 	}
 
@@ -244,13 +244,13 @@ int nsenter_main(int argc UNUSED_PARAM, char **argv)
 		}
 		xfchdir(root_fd);
 		xchroot(".");
-		/*close(root_fd);*/
+		close(root_fd);
 		/*root_fd = -1;*/
 	}
 
 	if (wd_fd >= 0) {
 		xfchdir(wd_fd);
-		/*close(wd_fd);*/
+		close(wd_fd);
 		/*wd_fd = -1;*/
 	}
 
@@ -259,14 +259,7 @@ int nsenter_main(int argc UNUSED_PARAM, char **argv)
 	 * explicitly requested by the user not to.
 	 */
 	if (!(opts & OPT_nofork) && (opts & OPT_pid)) {
-		pid_t pid = xvfork();
+		xvfork_parent_waits_and_exits();
-		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 */
 	}
 
@@ -278,9 +271,5 @@ int nsenter_main(int argc UNUSED_PARAM, char **argv)
 	if (opts & OPT_setuid)
 		xsetuid(uid);
 
-	if (*argv) {
+	exec_prog_or_SHELL(argv);
-		BB_EXECVP_or_die(argv);
-	}
-
-	run_shell(getenv("SHELL"), /*login:*/ 1, NULL, NULL);
 }

util-linux/unshare.c

@@ -281,7 +281,7 @@ int unshare_main(int argc UNUSED_PARAM, char **argv)
 
 	if (fdp.wr >= 0) {
 		close(fdp.wr); /* Release child */
-		/*close(fdp.rd);*/
+		close(fdp.rd); /* should close fd, to not confuse exec'ed PROG */
 	}
 
 	if (need_mount) {
@@ -307,14 +307,7 @@ int unshare_main(int argc UNUSED_PARAM, char **argv)
 	 * that'll become PID 1 in this new namespace.
 	 */
 	if (opts & OPT_fork) {
-		pid_t pid = xfork();
+		xvfork_parent_waits_and_exits();
-		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 */
 	}
 
@@ -354,11 +347,5 @@ int unshare_main(int argc UNUSED_PARAM, char **argv)
 		mount_or_die("proc", proc_mnt_target, "proc", MS_NOSUID | MS_NOEXEC | MS_NODEV);
 	}
 
-	if (argv[0]) {
+	exec_prog_or_SHELL(argv);
-		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);
 }