change name to voidnsrun, add command-line arguments, rewrite readme

1 files changed, 174 insertions, 0 deletions

diff --git a/voidnsrun.c b/voidnsrun.c
new file mode 100644
index 0000000..e121c0f
--- /dev/null
+++ b/voidnsrun.c
@@ -0,0 +1,174 @@
+#define _GNU_SOURCE
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <errno.h>
+#include <sched.h>
+#include <stdbool.h>
+#include <sys/mount.h>
+#include <sys/stat.h>
+#include <linux/limits.h>
+
+const char *var_name = "VOIDNSRUN_DIR";
+const char *prog_version = "1.0";
+
+#define USERMOUNTS_MAX 8
+
+#define ARRAY_SIZE(x)  (sizeof(x) / sizeof((x)[0]))
+#define ERROR(f_, ...) fprintf(stderr, (f_), ##__VA_ARGS__)
+
+bool isdir(const char *s)
+{
+	struct stat st;
+	int result = stat(s, &st);
+	if (result == -1)
+		ERROR("stat: %s\n", strerror(errno));
+	return result == 0 && S_ISDIR(st.st_mode);
+}
+
+void usage(const char *progname)
+{
+	printf("Usage: %s [OPTIONS] [PROGRAM [ARGS]]\n", progname);
+	printf("\n"
+			"Options:\n"
+			"	-h:        print this help\n"
+			"	-m <path>: add bind mount\n"
+			"	-r <path>: altroot path. If this option is not present,\n"
+			"	           %s environment variable is used.\n"
+			"	-v:        print version\n",
+			var_name);
+}
+
+bool mount_list(
+	const char *dirptr,
+	size_t dirlen,
+	const char **mountpoints,
+	size_t len,
+	bool ignore_missing)
+{
+	char buf[PATH_MAX];
+	for (size_t i = 0; i < len; i++) {
+		/* Check if it's safe to proceed. */
+		if (dirlen + strlen(mountpoints[i]) >= PATH_MAX) {
+			ERROR("error: path %s%s is too large.\n", dirptr, mountpoints[i]);
+			return false;
+		}
+		strcpy(buf, dirptr);
+		strcat(buf, mountpoints[i]);
+		if (!isdir(buf)) {
+			if (!ignore_missing) {
+				ERROR("error: source mount dir %s does not exists.\n", buf);
+				return false;
+			} else
+				continue;
+		}
+		if (!isdir(mountpoints[i])) {
+			ERROR("error: mountpoint %s does not exists.\n", mountpoints[i]);
+			return false;
+		}
+		if (mount(buf, mountpoints[i], NULL, MS_BIND|MS_REC, NULL) == -1) {
+			ERROR("mount: failed to mount %s: %s\n",
+				mountpoints[i], strerror(errno));
+			return false;
+		}
+	}
+	return true;
+}
+
+int main(int argc, char **argv)
+{
+	if (argc < 2) {
+		usage(argv[0]);
+		return 0;
+	}
+	
+	int result;
+	char *dir = NULL;
+	size_t dirlen;
+	const char *usermounts[USERMOUNTS_MAX] = {0};
+	int usermounts_count = 0;
+	int c;
+	while ((c = getopt(argc, argv, "vhm:r:")) != -1) {
+		switch (c) {
+			case 'v':
+				printf("%s\n", prog_version);
+				return 0;
+			case 'h':
+				usage(argv[0]);
+				return 0;
+			case 'r':
+				dir = optarg;
+				break;
+			case 'm':
+				if (usermounts_count < USERMOUNTS_MAX) {
+					usermounts[usermounts_count++] = optarg;
+					break;
+				} else {
+					ERROR("error: only up to %d user mounts allowed.\n",
+						USERMOUNTS_MAX);
+					return 1;
+				}
+			case '?':
+				return 1;
+		}
+	}
+
+	/* Get alternative root dir. */
+	if (!dir)
+		dir = getenv(var_name);
+	if (!dir) {
+		ERROR("error: environment variable %s not found.\n", var_name);
+		return 1;
+	}
+
+	/* Validate it. */
+	if (!isdir(dir)) {
+		ERROR("error: %s is not a directory.\n", dir);
+		return 1;
+	}
+	
+	dirlen = strlen(dir);
+
+	/* Do the unshare magic. */
+	result = unshare(CLONE_NEWNS);
+	if (result == -1) {
+		ERROR("unshare: %s\n", strerror(errno));
+		return 1;
+	}
+
+	/* Mount stuff from altroot to our private namespace. */
+	const char *mountpoints[] = {"/usr", "/var/db/xbps", "/etc/xbps.d"};
+	
+	if (!mount_list(dir, dirlen, mountpoints, ARRAY_SIZE(mountpoints), true))
+		return 1;
+	
+	if (usermounts_count > 0 && 
+		!mount_list(dir, dirlen, usermounts, usermounts_count, false))
+		return 1;
+
+	/* Drop root. */
+	uid_t uid = getuid();
+	gid_t gid = getgid();
+
+	result = setreuid(uid, uid);
+	if (result == -1) {
+		ERROR("setreuid: %s\n", strerror(errno));
+		return 1;
+	}
+
+	result = setregid(gid, gid);
+	if (result == -1) {
+		ERROR("setregid: %s\n", strerror(errno));
+		return 1;
+	}
+	
+	/* Launch program. */
+	result = execvp(argv[optind], (char *const *)argv+optind);
+	if (result == -1) {
+		ERROR("execvp(%s): %s\n", argv[optind], strerror(errno));
+		return 1;
+	}
+
+	return 0;
+}