nilfs2: fix improper return values of nilfs_get_cpinfo ioctl

A few tool developers gave me requests for fixing inconvenient return
value of nilfs_get_cpinfo() ioctl; if the requested mode is NILFS_SNAPSHOT
and the specified start entry is not a snapshot, the ioctl unnaturally
returns one as the number of acquired snapshot item.

In addition, the ioctl function returns an ENOENT error for checkpoints
within blocks deleted by garbage collection.

These behaviors require corrections for programs which enumerate
snapshots.  This resolves the inconvenience by changing the return values
to zero for the above cases.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/nilfs2/cpfile.c

@@ -431,7 +431,7 @@ static ssize_t nilfs_cpfile_do_get_ssinfo(struct inode *cpfile, __u64 *cnop,
 	__u64 curr = *cnop, next;
 	unsigned long curr_blkoff, next_blkoff;
 	void *kaddr;
-	int n, ret;
+	int n = 0, ret;
 
 	down_read(&NILFS_MDT(cpfile)->mi_sem);
 
@@ -455,27 +455,33 @@ static ssize_t nilfs_cpfile_do_get_ssinfo(struct inode *cpfile, __u64 *cnop,
 
 	curr_blkoff = nilfs_cpfile_get_blkoff(cpfile, curr);
 	ret = nilfs_cpfile_get_checkpoint_block(cpfile, curr, 0, &bh);
-	if (ret < 0)
+	if (unlikely(ret < 0)) {
+		if (ret == -ENOENT)
+			ret = 0; /* No snapshots (started from a hole block) */
 		goto out;
-	kaddr = kmap_atomic(bh->b_page, KM_USER0);
-	for (n = 0; n < nci; n++) {
-		cp = nilfs_cpfile_block_get_checkpoint(
-			cpfile, curr, bh, kaddr);
-		nilfs_cpfile_checkpoint_to_cpinfo(cpfile, cp, &ci[n]);
-		next = le64_to_cpu(cp->cp_snapshot_list.ssl_next);
-		if (next == 0) {
-			curr = ~(__u64)0; /* Terminator */
-			n++;
-			break;
 	}
+	kaddr = kmap_atomic(bh->b_page, KM_USER0);
+	while (n < nci) {
+		cp = nilfs_cpfile_block_get_checkpoint(cpfile, curr, bh, kaddr);
+		curr = ~(__u64)0; /* Terminator */
+		if (unlikely(nilfs_checkpoint_invalid(cp) ||
+			     !nilfs_checkpoint_snapshot(cp)))
+			break;
+		nilfs_cpfile_checkpoint_to_cpinfo(cpfile, cp, &ci[n++]);
+		next = le64_to_cpu(cp->cp_snapshot_list.ssl_next);
+		if (next == 0)
+			break; /* reach end of the snapshot list */
+
 		next_blkoff = nilfs_cpfile_get_blkoff(cpfile, next);
 		if (curr_blkoff != next_blkoff) {
 			kunmap_atomic(kaddr, KM_USER0);
 			brelse(bh);
 			ret = nilfs_cpfile_get_checkpoint_block(cpfile, next,
 								0, &bh);
-			if (ret < 0)
+			if (unlikely(ret < 0)) {
+				WARN_ON(ret == -ENOENT);
 				goto out;
+			}
 			kaddr = kmap_atomic(bh->b_page, KM_USER0);
 		}
 		curr = next;