PFS, inet: use static UID to drop privileges

Previously, services would obtain the user ID of "service" through
getpwnam(3).  While this approach is conceptually better, it also
imposes linking against libc which in turn causes problems with
printf(3), which already led to PFS no longer dropping privileges at
all.  For now, we hardcode SERVICE_UID and use that instead.

In the future, two changes should allow removal of SERVICE_UID again:
- "service edit" should cause RS to request that a service (such as
  PFS) drop privileges through SEF, using the user ID resolved by
  service(8), or something similar;
- a future devfs should make it possible for inet to start without
  root privileges altogether.

Change-Id: Ie02a1e888cde325806fc0ae76909943ac42c9b96

minix/fs/pfs/pfs.c

@@ -3,6 +3,7 @@
 #include <minix/drivers.h>
 #include <minix/fsdriver.h>
 #include <minix/vfsif.h>
+#include <minix/rs.h>
 #include <assert.h>
 
 /*
@@ -385,6 +386,20 @@ pfs_signal(int signo)
 	fsdriver_terminate();
 }
 
+/*
+ * Initialize PFS.
+ */
+static int
+pfs_init(int __unused type, sef_init_info_t * __unused info)
+{
+
+	/* Drop privileges. */
+	if (setuid(SERVICE_UID) != 0)
+		printf("PFS: warning, unable to drop privileges\n");
+
+	return OK;
+}
+
 /*
  * Perform SEF initialization.
  */
@@ -393,7 +408,7 @@ pfs_startup(void)
 {
 
 	/* Register initialization callbacks. */
-	sef_setcb_init_fresh(sef_cb_init_null);
+	sef_setcb_init_fresh(pfs_init);
 	sef_setcb_init_restart(sef_cb_init_fail);
 
 	/* No live update support for now. */

minix/include/minix/rs.h

@@ -12,6 +12,12 @@ Interface to the reincarnation server
 
 #define SERVICE_LOGIN	"service"	/* passwd file entry for services */
 
+/* The following definition should be kept in sync with the actual /etc/passwd
+ * value for SERVICE_LOGIN for now, and removed altogether once we are able to
+ * obtain its value dynamically everywhere.
+ */
+#define SERVICE_UID		12		/* user ID for services */
+
 /* RSS definitions. */
 #define RSS_NR_IRQ		16
 #define RSS_NR_IO		16

minix/net/inet/inet.c

@@ -161,7 +161,6 @@ static int sef_cb_init_fresh(int type, sef_init_info_t *info)
 	int timerand, fd;
 	u8_t randbits[32];
 	struct timeval tv;
-	struct passwd *pw;
 
 #if DEBUG
 	printf("Starting inet...\n");
@@ -236,13 +235,9 @@ static int sef_cb_init_fresh(int type, sef_init_info_t *info)
 		ip_panic(("inet: can't subscribe to driver events"));
 	}
 
-	/* Drop root privileges */
+	/* Drop privileges. */
-	if ((pw = getpwnam(SERVICE_LOGIN)) == NULL) {
+	if (setuid(SERVICE_UID) != 0)
-		printf("inet: unable to retrieve uid of SERVICE_LOGIN, "
+		printf("inet: warning, unable to drop privileges\n");
-			"still running as root");
-	} else if (setuid(pw->pw_uid) != 0) {
-		ip_panic(("inet: unable to drop privileges"));
-	}
 
 	/* Announce we are up. INET announces its presence to VFS just like
 	 * any other character driver.