From: Volker Lendecke <vl@samba.org>
Date: Mon, 7 Nov 2016 20:40:15 +0000 (+0100)
Subject: tdb: Do lock upgrades properly
X-Git-Tag: tdb-1.3.13~143
X-Git-Url: http://git.samba.org/?a=commitdiff_plain;h=a6f1532d7fedc6b6f36b511aebe8998e9452b7ff;p=samba.git

tdb: Do lock upgrades properly

When a process holds a readlock and wants to upgrade, this needs to be
reflected in the underlying lock. Without this, it is possible to cheat:
One process holds a readlock, and another process wants to write this
record. All the writer has to do is take a readonly lock on the key and
then do the store.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
---

diff --git a/lib/tdb/common/lock.c b/lib/tdb/common/lock.c
index 594c28751f1..4ad70cff3fe 100644
--- a/lib/tdb/common/lock.c
+++ b/lib/tdb/common/lock.c
@@ -321,6 +321,22 @@ int tdb_nest_lock(struct tdb_context *tdb, uint32_t offset, int ltype,
 
 	new_lck = find_nestlock(tdb, offset);
 	if (new_lck) {
+		if ((new_lck->ltype == F_RDLCK) && (ltype == F_WRLCK)) {
+			if (!tdb_have_mutexes(tdb)) {
+				int ret;
+				/*
+				 * Upgrade the underlying fcntl
+				 * lock. Mutexes don't do readlocks,
+				 * so this only applies to fcntl
+				 * locking.
+				 */
+				ret = tdb_brlock(tdb, ltype, offset, 1, flags);
+				if (ret != 0) {
+					return ret;
+				}
+			}
+			new_lck->ltype = F_WRLCK;
+		}
 		/*
 		 * Just increment the in-memory struct, posix locks
 		 * don't stack.