[PATCH 4/5] unshare: Add option to automatically create user and group maps

[Sean Anderson <seanga2@gmail.com>]

This option is designed to handle the "garden path" user/group ID
mapping:

- The user has one big map in /etc/sub[u,g]id
- The user wants to map as many user and group IDs as they can,
  especially the first 1000 users and groups.

The "auto" map is designed to handle this. We find the first map
matching the current user, and then map the whole thing to the ID range
starting at ID 0.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
---

 sys-utils/unshare.c | 74 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 74 insertions(+)

diff --git a/sys-utils/unshare.c b/sys-utils/unshare.c
index 247925aa6..33d1a829c 100644
--- a/sys-utils/unshare.c
+++ b/sys-utils/unshare.c
@@ -373,6 +373,72 @@ static struct map_range *get_map_range(const char *s)
 	return ret;
 }
 
+/**
+ * read_subid_range() - Look up a user's sub[gu]id range
+ * @filename: The file to look up the range from. This should be either
+ *            ``/etc/subuid`` or ``/etc/subgid``.
+ * @uid: The uid of the user whose range we should look up.
+ *
+ * This finds the first subid range matching @uid in @filename.
+ */
+static struct map_range *read_subid_range(char *filename, uid_t uid)
+{
+        char *line = NULL, *pwbuf;
+        FILE *idmap;
+        size_t n;
+	struct passwd *pw;
+	struct map_range *map;
+
+	map = xmalloc(sizeof(*map));
+	map->inner = 0;
+
+	pw = xgetpwuid(uid, &pwbuf);
+	if (!pw)
+		errx(EXIT_FAILURE, _("you (user %d) don't exist."), uid);
+
+        idmap = fopen(filename, "r");
+        if (!idmap)
+                err(EXIT_FAILURE, _("could not open '%s'"), filename);
+
+        /*
+         * Each line in sub[ug]idmap looks like
+         * username:subuid:count
+         * OR
+         * uid:subuid:count
+         */
+        while (getline(&line, &n, idmap) != -1) {
+                char *rest, *s;
+
+                rest = strchr(line, ':');
+                if (!rest)
+                        continue;
+                *rest = '\0';
+
+                if (strcmp(line, pw->pw_name) &&
+                    strtoul(line, NULL, 10) != pw->pw_uid)
+                        continue;
+
+                s = rest + 1;
+                rest = strchr(s, ':');
+                if (!rest)
+                        continue;
+                *rest = '\0';
+		map->outer = strtoul_or_err(s, _("failed to parse subid map"));
+
+                s = rest + 1;
+                rest = strchr(s, '\n');
+                if (rest)
+                        *rest = '\0';
+		map->count = strtoul_or_err(s, _("failed to parse subid map"));
+
+                fclose(idmap);
+                return map;
+        }
+
+        err(EXIT_FAILURE, _("no line matching user \"%s\" in %s"),
+	    pw->pw_name, filename);
+}
+
 /**
  * map_ids() - Create a new uid/gid map
  * @idmapper: Either newuidmap or newgidmap
@@ -571,6 +637,7 @@ static void __attribute__((__noreturn__)) usage(void)
 	fputs(_(" --map-group=<gid>|<name>  map current group to gid (implies --user)\n"), out);
 	fputs(_(" -r, --map-root-user       map current user to root (implies --user)\n"), out);
 	fputs(_(" -c, --map-current-user    map current user to itself (implies --user)\n"), out);
+	fputs(_(" --map-auto                map users and groups automatically (implies --user)\n"), out);
 	fputs(_(" --map-users=<outeruid>,<inneruid>,<count>\n"
 		"                           map count users from outeruid to inneruid (implies --user)\n"), out);
 	fputs(_(" --map-groups=<outergid>,<innergid>,<count>\n"
@@ -612,6 +679,7 @@ int main(int argc, char *argv[])
 		OPT_MAPUSERS,
 		OPT_MAPGROUP,
 		OPT_MAPGROUPS,
+		OPT_MAPAUTO,
 	};
 	static const struct option longopts[] = {
 		{ "help",          no_argument,       NULL, 'h'             },
@@ -635,6 +703,7 @@ int main(int argc, char *argv[])
 		{ "map-groups",    required_argument, NULL, OPT_MAPGROUPS   },
 		{ "map-root-user", no_argument,       NULL, 'r'             },
 		{ "map-current-user", no_argument,    NULL, 'c'             },
+		{ "map-auto",      no_argument,       NULL, OPT_MAPAUTO     },
 		{ "propagation",   required_argument, NULL, OPT_PROPAGATION },
 		{ "setgroups",     required_argument, NULL, OPT_SETGROUPS   },
 		{ "keep-caps",     no_argument,       NULL, OPT_KEEPCAPS    },
@@ -753,6 +822,11 @@ int main(int argc, char *argv[])
 			unshare_flags |= CLONE_NEWUSER;
 			groupmap = get_map_range(optarg);
 			break;
+		case OPT_MAPAUTO:
+			unshare_flags |= CLONE_NEWUSER;
+			usermap = read_subid_range("/etc/subuid", real_euid);
+			groupmap = read_subid_range("/etc/subgid", real_egid);
+			break;
 		case OPT_SETGROUPS:
 			setgrpcmd = setgroups_str2id(optarg);
 			break;
-- 
2.33.0