Attempt to further comment and clarify System V IPC logic: document

why certain exceptions are made, note an inconsistency between
FreeBSD and some other implementations regarding IPC_M, and let
suser() generate our EPERM rather than forcing it ourselves.
Remove a carriage return that crept in in the last commit.

Reviewed by:	gordon
Obtained from:	TrustedBSD Project
Sponsored by:	DARPA, Network Associates Laboratories

1 files changed, 24 insertions, 9 deletions

diff --git a/sys/kern/sysv_ipc.c b/sys/kern/sysv_ipc.c
index 30939e4..bf91258 100644
--- a/sys/kern/sysv_ipc.c
+++ b/sys/kern/sysv_ipc.c
@@ -76,23 +76,38 @@ ipcperm(td, perm, mode)
 	int mode;
 {
 	struct ucred *cred = td->td_ucred;
+	int error;
 
-	/* Check for user match. */
 	if (cred->cr_uid != perm->cuid && cred->cr_uid != perm->uid) {
-		if (mode & IPC_M)
-			return (suser(td) == 0 ? 0 : EPERM);
-		/* Check for group match. */
+		/*
+		 * For a non-create/owner, we require privilege to
+		 * modify the object protections.  Note: some other
+		 * implementations permit IPC_M to be delegated to
+		 * unprivileged non-creator/owner uids/gids.
+		 */
+		if (mode & IPC_M) {
+			error = suser(td);
+			if (error)
+				return (error);
+		}
+		/*
+		 * Try to match against creator/owner group; if not, fall
+		 * back on other.
+		 */
 		mode >>= 3;
 		if (!groupmember(perm->gid, cred) &&
 		    !groupmember(perm->cgid, cred))
-			/* Check for `other' match. */
 			mode >>= 3;
+	} else {
+		/*
+		 * Always permit the creator/owner to update the object
+		 * protections regardless of whether the object mode
+		 * permits it.
+		 */
+		if (mode & IPC_M)
+			return (0);
 	}
 
-	if (mode & IPC_M)
-		return (0);
-
-
 	if ((mode & perm->mode) != mode) {
 		if (suser(td) != 0)
 			return (EACCES);