wishlist: add item about SCM_RIGHTS/SELinux nastiness

Author: poettering

README.md

@@ -772,6 +772,43 @@ to thread-group leader pidfd.
 a PID namespace corresponds to in the caller's PID namespace. For example, to
 figure out what the PID of PID 1 inside of a given PID namespace is.
 
+### Useful handling of LSM denials on SCM_RIGHTS
+
+Right now if some LSM such as SELinux denies an `AF_UNIX` socket peer
+to receive an `SCM_RIGHTS` fd the `SCM_RIGHTS` fd array will be cut
+short at that point, and `MSG_CTRUNC` is set on return of
+`recvmsg()`. This is highly problematic behaviour, because it leaves
+the receiver wondering what happened. As per man page `MSG_CTRUNC` is
+supposed to indicate that the control buffer was sized too short, but
+suddenly a permission error might result in the exact same flag being
+set. Moreover, the receiver has no chance to determine how many fds
+got originally sent and how many were suppressed.
+
+Ideas how to improve things:
+
+1. Maybe introduce a new flag `MSG_RIGHTS_DENIAL` or so which is set
+   on `recvmsg()` return, which tells us that fds where dropped from
+   the `SCM_RIGHTS` array because of an LSM error. This new flag could
+   be set in addition to `CMSG_CTRUNC`, for compatibility.
+
+2. Maybe, define a new flag `MSG_RIGHTS_FILTER` or so which when
+   passed to `recvmsg()` will ensure that the `SCM_RIGHTS` fd array is
+   always passed through in its full, original size. Entries for which
+   an LSM says no are suppressed, and replaced by a special value, for
+   example `-EPERM`.
+
+3. It would be good if the relevant man page would at least document
+   this pitfall, even if it right now cannot reasonably be handled.
+
+Ideally both ideas would be implemented, but of course, strictly
+speaking the 2nd idea makes the 1st idea half-way redundant.
+
+**Use-Case:** Any code that uses `SCM_RIGHTS` generically (D-Bus and
+so on) needs this, so that it can reasonably handle SELinux AVC errors
+on received messages.
+
+---
+
 ## Finished Items
 
 ### Unmounting of obstructed mounts