util, conf: Add and use ns_is_init() helper

We'll need this in isolate_initial(). While at it, don't rely on
BUFSIZ: the earlier issue we had with musl reminded me it's not a
magic "everything will fit" value. Size the read buffer to what we
actually need from uid_map, and check for the final newline too,
because uid_map is organised in lines.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

conf.c

@@ -1096,10 +1096,6 @@ static int conf_runas(char *opt, unsigned int *uid, unsigned int *gid)
  */
 static void conf_ugid(char *runas, uid_t *uid, gid_t *gid)
 {
-	const char root_uid_map[] = "         0          0 4294967295";
-	char buf[BUFSIZ];
-	int fd;
-
 	/* If user has specified --runas, that takes precedence... */
 	if (runas) {
 		if (conf_runas(runas, uid, gid))
@@ -1116,18 +1112,8 @@ static void conf_ugid(char *runas, uid_t *uid, gid_t *gid)
 		return;
 
 	/* ...or at least not root in the init namespace... */
-	if ((fd = open("/proc/self/uid_map", O_RDONLY | O_CLOEXEC)) < 0) {
+	if (!ns_is_init())
-		die("Can't determine if we're in init namespace: %s",
-		    strerror(errno));
-	}
-
-	if (read(fd, buf, BUFSIZ) != sizeof(root_uid_map) ||
-	    strncmp(buf, root_uid_map, sizeof(root_uid_map) - 1)) {
-		close(fd);
 		return;
-	}
-
-	close(fd);
 
 	/* ...otherwise use nobody:nobody */
 	warn("Don't run as root. Changing to nobody...");

util.c

@@ -390,6 +390,31 @@ int ns_enter(const struct ctx *c)
 	return 0;
 }
 
+/**
+ * ns_is_init() - Is the caller running in the "init" user namespace?
+ *
+ * Return: true if caller is in init, false otherwise, won't return on failure
+ */
+bool ns_is_init(void)
+{
+	const char root_uid_map[] = "         0          0 4294967295\n";
+	char buf[sizeof(root_uid_map)] = { 0 };
+	bool ret = true;
+	int fd;
+
+	if ((fd = open("/proc/self/uid_map", O_RDONLY | O_CLOEXEC)) < 0) {
+		die("Can't determine if we're in init namespace: %s",
+		    strerror(errno));
+	}
+
+	if (read(fd, buf, sizeof(root_uid_map)) != sizeof(root_uid_map) - 1 ||
+	    strncmp(buf, root_uid_map, sizeof(root_uid_map)))
+		ret = false;
+
+	close(fd);
+	return ret;
+}
+
 /**
  * pid_file() - Write PID to file, if requested to do so, and close it
  * @fd:		Open PID file descriptor, closed on exit, -1 to skip writing it

util.h

@@ -8,6 +8,7 @@
 
 #include <stdlib.h>
 #include <stdarg.h>
+#include <stdbool.h>
 
 #include "log.h"
 
@@ -216,6 +217,7 @@ char *line_read(char *buf, size_t len, int fd);
 void procfs_scan_listen(struct ctx *c, uint8_t proto, int ip_version, int ns,
 			uint8_t *map, uint8_t *exclude);
 int ns_enter(const struct ctx *c);
+bool ns_is_init(void);
 void write_pidfile(int fd, pid_t pid);
 int __daemon(int pidfile_fd, int devnull_fd);
 int fls(unsigned long x);