cutils: return the remote uid on netlink recv failure

This change adds a layer of indirection to uevent's
uevent_kernel_multicast_recv, allowing dependent code to
get the remote endpoint if desired.

Change-Id: If2f00708a1dd6196c415b1ebc2a7eeb72399a90e

1 files changed, 25 insertions, 6 deletions

diff --git a/libcutils/uevent.c b/libcutils/uevent.c
index 4add29c3d..97a81e309 100644
--- a/libcutils/uevent.c
+++ b/libcutils/uevent.c
@@ -29,7 +29,24 @@
 /**
  * Like recv(), but checks that messages actually originate from the kernel.
  */
-ssize_t uevent_kernel_multicast_recv(int socket, void *buffer, size_t length) {
+ssize_t uevent_kernel_multicast_recv(int socket, void *buffer, size_t length)
+{
+    uid_t user = -1;
+    return uevent_kernel_multicast_uid_recv(socket, buffer, length, &user);
+}
+
+/**
+ * Like the above, but passes a uid_t in by reference. In the event that this
+ * fails due to a bad uid check, the uid_t will be set to the uid of the
+ * socket's peer.
+ *
+ * If this method rejects a netlink message from outside the kernel, it
+ * returns -1, sets errno to EIO, and sets "user" to the UID associated with the
+ * message. If the peer UID cannot be determined, "user" is set to -1."
+ */
+ssize_t uevent_kernel_multicast_uid_recv(int socket, void *buffer,
+                                         size_t length, uid_t *user)
+{
     struct iovec iov = { buffer, length };
     struct sockaddr_nl addr;
     char control[CMSG_SPACE(sizeof(struct ucred))];
@@ -43,16 +60,12 @@ ssize_t uevent_kernel_multicast_recv(int socket, void *buffer, size_t length) {
         0,
     };
 
+    *user = -1;
     ssize_t n = recvmsg(socket, &hdr, 0);
     if (n <= 0) {
         return n;
     }
 
-    if (addr.nl_groups == 0 || addr.nl_pid != 0) {
-        /* ignoring non-kernel or unicast netlink message */
-        goto out;
-    }
-
     struct cmsghdr *cmsg = CMSG_FIRSTHDR(&hdr);
     if (cmsg == NULL || cmsg->cmsg_type != SCM_CREDENTIALS) {
         /* ignoring netlink message with no sender credentials */
@@ -60,11 +73,17 @@ ssize_t uevent_kernel_multicast_recv(int socket, void *buffer, size_t length) {
     }
 
     struct ucred *cred = (struct ucred *)CMSG_DATA(cmsg);
+    *user = cred->uid;
     if (cred->uid != 0) {
         /* ignoring netlink message from non-root user */
         goto out;
     }
 
+    if (addr.nl_groups == 0 || addr.nl_pid != 0) {
+        /* ignoring non-kernel or unicast netlink message */
+        goto out;
+    }
+
     return n;
 
 out: