run minadbd as shell user

Make minadbd drop its root privileges after initializing.  We need to
make the /tmp directory writable by the shell group so that it can
drop the sideloaded file there.

Change-Id: I67b292cf769383f0f67fb934e5a80d408a4c131d

etc/init.rc

@@ -15,6 +15,9 @@ on init
     mkdir /cache
     mount /tmp /tmp tmpfs
 
+    chown root shell /tmp
+    chmod 0775 /tmp
+
     write /sys/class/android_usb/android0/enable 0
     write /sys/class/android_usb/android0/idVendor 18D1
     write /sys/class/android_usb/android0/idProduct D001

minadbd/README.txt

@@ -4,7 +4,7 @@ the following changes:
 adb.c
   - much support for host mode and non-linux OS's stripped out; this
     version only runs as adbd on the device.
-  - does not setuid/setgid itself (always stays root)
+  - always setuid/setgid's itself to the shell user
   - only uses USB transport
   - references to JDWP removed
   - main() removed
@@ -25,3 +25,7 @@ services.c
 Android.mk
   - only builds in adbd mode; builds as static library instead of a
     standalone executable.
+
+sysdeps.h
+  - changes adb_creat() to use O_NOFOLLOW
+

minadbd/adb.c

@@ -858,6 +858,16 @@ int adb_main()
         usb_init();
     }
 
+    if (setgid(AID_SHELL) != 0) {
+        fprintf(stderr, "failed to setgid to shell\n");
+        exit(1);
+    }
+    if (setuid(AID_SHELL) != 0) {
+        fprintf(stderr, "failed to setuid to shell\n");
+        exit(1);
+    }
+    fprintf(stderr, "userid is %d\n", getuid());
+
     D("Event loop starting\n");
 
     fdevent_loop();

minadbd/services.c

@@ -53,6 +53,7 @@ static void sideload_service(int s, void *cookie)
 
     fd = adb_creat(ADB_SIDELOAD_FILENAME, 0644);
     if(fd < 0) {
+        fprintf(stderr, "failed to create %s\n", ADB_SIDELOAD_FILENAME);
         adb_close(s);
         return;
     }

minadbd/sysdeps.h

@@ -324,6 +324,18 @@ static __inline__ int  adb_open_mode( const char*  pathname, int  options, int
     return open( pathname, options, mode );
 }
 
+static __inline__  int  adb_creat(const char*  path, int  mode)
+{
+    int  fd = open(path, O_CREAT|O_WRONLY|O_TRUNC|O_NOFOLLOW, mode);
+
+    if ( fd < 0 )
+        return -1;
+
+    close_on_exec(fd);
+    return fd;
+}
+#undef   creat
+#define  creat  ___xxx_creat
 
 static __inline__ int  adb_open( const char*  pathname, int  options )
 {
@@ -380,19 +392,6 @@ static __inline__  int    adb_unlink(const char*  path)
 #undef  unlink
 #define unlink  ___xxx_unlink
 
-static __inline__  int  adb_creat(const char*  path, int  mode)
-{
-    int  fd = creat(path, mode);
-
-    if ( fd < 0 )
-        return -1;
-
-    close_on_exec(fd);
-    return fd;
-}
-#undef   creat
-#define  creat  ___xxx_creat
-
 static __inline__ int  adb_socket_accept(int  serverfd, struct sockaddr*  addr, socklen_t  *addrlen)
 {
     int fd;