[XFS] Fix xfs_free_extent related NULL pointer dereference.

We recently fixed an out-of-space deadlock in XFS, and part of that fix
involved the addition of the XFS_ALLOC_FLAG_FREEING flag to some of the
space allocator calls to indicate they're freeing space, not allocating
it. There was a missed xfs_alloc_fix_freelist condition test that did not
correctly test "flags". The same test would also test an uninitialised
structure field (args->userdata) and depending on its value either would
or would not return early with a critical buffer pointer set to NULL.

This fixes that up, adds asserts to several places to catch future botches
of this nature, and skips sections of xfs_alloc_fix_freelist that are
irrelevent for the space-freeing case.

SGI-PV: 955303
SGI-Modid: xfs-linux-melb:xfs-kern:26743a
Signed-off-by: Nathan Scott <nathans@sgi.com>

fs/xfs/xfs_alloc.c

@@ -1835,21 +1835,27 @@ xfs_alloc_fix_freelist(
 				&agbp)))
 			return error;
 		if (!pag->pagf_init) {
+			ASSERT(flags & XFS_ALLOC_FLAG_TRYLOCK);
+			ASSERT(!(flags & XFS_ALLOC_FLAG_FREEING));
 			args->agbp = NULL;
 			return 0;
 		}
 	} else
 		agbp = NULL;
 
-	/* If this is a metadata preferred pag and we are user data
+	/*
+	 * If this is a metadata preferred pag and we are user data
 	 * then try somewhere else if we are not being asked to
 	 * try harder at this point
 	 */
-	if (pag->pagf_metadata && args->userdata && flags) {
+	if (pag->pagf_metadata && args->userdata &&
+	    (flags & XFS_ALLOC_FLAG_TRYLOCK)) {
+		ASSERT(!(flags & XFS_ALLOC_FLAG_FREEING));
 		args->agbp = NULL;
 		return 0;
 	}
 
+	if (!(flags & XFS_ALLOC_FLAG_FREEING)) {
 		need = XFS_MIN_FREELIST_PAG(pag, mp);
 		delta = need > pag->pagf_flcount ? need - pag->pagf_flcount : 0;
 		/*
@@ -1859,16 +1865,17 @@ xfs_alloc_fix_freelist(
 		longest = (pag->pagf_longest > delta) ?
 			(pag->pagf_longest - delta) :
 			(pag->pagf_flcount > 0 || pag->pagf_longest > 0);
-	if (args->minlen + args->alignment + args->minalignslop - 1 > longest ||
-	    (!(flags & XFS_ALLOC_FLAG_FREEING) &&
-	     (int)(pag->pagf_freeblks + pag->pagf_flcount -
-		   need - args->total) <
-	     (int)args->minleft)) {
+		if ((args->minlen + args->alignment + args->minalignslop - 1) >
+				longest ||
+		    ((int)(pag->pagf_freeblks + pag->pagf_flcount -
+			   need - args->total) < (int)args->minleft)) {
 			if (agbp)
 				xfs_trans_brelse(tp, agbp);
 			args->agbp = NULL;
 			return 0;
 		}
+	}
+
 	/*
 	 * Get the a.g. freespace buffer.
 	 * Can fail if we're not blocking on locks, and it's held.
@@ -1878,6 +1885,8 @@ xfs_alloc_fix_freelist(
 				&agbp)))
 			return error;
 		if (agbp == NULL) {
+			ASSERT(flags & XFS_ALLOC_FLAG_TRYLOCK);
+			ASSERT(!(flags & XFS_ALLOC_FLAG_FREEING));
 			args->agbp = NULL;
 			return 0;
 		}
@@ -1887,23 +1896,25 @@ xfs_alloc_fix_freelist(
 	 */
 	agf = XFS_BUF_TO_AGF(agbp);
 	need = XFS_MIN_FREELIST(agf, mp);
-	delta = need > be32_to_cpu(agf->agf_flcount) ?
-		(need - be32_to_cpu(agf->agf_flcount)) : 0;
 	/*
 	 * If there isn't enough total or single-extent, reject it.
 	 */
+	if (!(flags & XFS_ALLOC_FLAG_FREEING)) {
+		delta = need > be32_to_cpu(agf->agf_flcount) ?
+			(need - be32_to_cpu(agf->agf_flcount)) : 0;
 		longest = be32_to_cpu(agf->agf_longest);
 		longest = (longest > delta) ? (longest - delta) :
 			(be32_to_cpu(agf->agf_flcount) > 0 || longest > 0);
-	if (args->minlen + args->alignment + args->minalignslop - 1 > longest ||
-	     (!(flags & XFS_ALLOC_FLAG_FREEING) &&
-		(int)(be32_to_cpu(agf->agf_freeblks) +
+		if ((args->minlen + args->alignment + args->minalignslop - 1) >
+				longest ||
+		    ((int)(be32_to_cpu(agf->agf_freeblks) +
 		     be32_to_cpu(agf->agf_flcount) - need - args->total) <
 				(int)args->minleft)) {
 			xfs_trans_brelse(tp, agbp);
 			args->agbp = NULL;
 			return 0;
 		}
+	}
 	/*
 	 * Make the freelist shorter if it's too long.
 	 */
@@ -1950,13 +1961,12 @@ xfs_alloc_fix_freelist(
 		 * on a completely full ag.
 		 */
 		if (targs.agbno == NULLAGBLOCK) {
-			if (!(flags & XFS_ALLOC_FLAG_FREEING)) {
+			if (flags & XFS_ALLOC_FLAG_FREEING)
+				break;
 			xfs_trans_brelse(tp, agflbp);
 			args->agbp = NULL;
 			return 0;
 		}
-			break;
-		}
 		/*
 		 * Put each allocated block on the list.
 		 */
@@ -2442,31 +2452,26 @@ xfs_free_extent(
 	xfs_fsblock_t	bno,	/* starting block number of extent */
 	xfs_extlen_t	len)	/* length of extent */
 {
-#ifdef DEBUG
-	xfs_agf_t	*agf;	/* a.g. freespace header */
-#endif
-	xfs_alloc_arg_t	args;	/* allocation argument structure */
+	xfs_alloc_arg_t	args;
 	int		error;
 
 	ASSERT(len != 0);
+	memset(&args, 0, sizeof(xfs_alloc_arg_t));
 	args.tp = tp;
 	args.mp = tp->t_mountp;
 	args.agno = XFS_FSB_TO_AGNO(args.mp, bno);
 	ASSERT(args.agno < args.mp->m_sb.sb_agcount);
 	args.agbno = XFS_FSB_TO_AGBNO(args.mp, bno);
-	args.alignment = 1;
-	args.minlen = args.minleft = args.minalignslop = 0;
 	down_read(&args.mp->m_peraglock);
 	args.pag = &args.mp->m_perag[args.agno];
 	if ((error = xfs_alloc_fix_freelist(&args, XFS_ALLOC_FLAG_FREEING)))
 		goto error0;
 #ifdef DEBUG
 	ASSERT(args.agbp != NULL);
-	agf = XFS_BUF_TO_AGF(args.agbp);
-	ASSERT(args.agbno + len <= be32_to_cpu(agf->agf_length));
+	ASSERT((args.agbno + len) <=
+		be32_to_cpu(XFS_BUF_TO_AGF(args.agbp)->agf_length));
 #endif
-	error = xfs_free_ag_extent(tp, args.agbp, args.agno, args.agbno,
-		len, 0);
+	error = xfs_free_ag_extent(tp, args.agbp, args.agno, args.agbno, len, 0);
 error0:
 	up_read(&args.mp->m_peraglock);
 	return error;