[PATCH v6 3/4] ntfs: update index root allocated size before shrink

[DaeMyung Kang <charsyam@gmail.com>]

ntfs_ir_truncate() currently shrinks the resident $INDEX_ROOT value first
and only updates index.allocated_size after re-looking up the attribute.
During that relookup, the resident value_length can already be smaller
while index.allocated_size still contains the old larger size.

That leaves a transiently inconsistent $INDEX_ROOT layout and prevents
lookup-time $INDEX_ROOT validation from being enabled: validation can
correctly reject allocated_size extending past the newly shrunk resident
value.

When shrinking, lower index.allocated_size before shrinking value_length.
If the truncate fails, restore the old allocated_size. Keep the existing
grow ordering because the old allocated_size remains within the enlarged
resident value until it is updated after the relookup. The shrink path is
safe because the new value_length still covers struct index_root, so the
index.allocated_size field remains present while it is updated first.

Signed-off-by: DaeMyung Kang <charsyam@gmail.com>
---
 fs/ntfs/index.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/fs/ntfs/index.c b/fs/ntfs/index.c
index 052d80fddbbc..c5f2cf75b750 100644
--- a/fs/ntfs/index.c
+++ b/fs/ntfs/index.c
@@ -1365,9 +1365,16 @@ static int ntfs_ir_reparent(struct ntfs_index_context *icx)
 static int ntfs_ir_truncate(struct ntfs_index_context *icx, int data_size)
 {
 	int ret;
+	u32 old_allocated_size;
+	bool shrink;
 
 	ntfs_debug("Entering\n");
 
+	old_allocated_size = le32_to_cpu(icx->ir->index.allocated_size);
+	shrink = data_size < old_allocated_size;
+	if (shrink)
+		icx->ir->index.allocated_size = cpu_to_le32(data_size);
+
 	/*
 	 *  INDEX_ROOT must be resident and its entries can be moved to
 	 *  struct index_block, so ENOSPC isn't a real error.
@@ -1379,9 +1386,14 @@ static int ntfs_ir_truncate(struct ntfs_index_context *icx, int data_size)
 		if (!icx->ir)
 			return -ENOENT;
 
-		icx->ir->index.allocated_size = cpu_to_le32(data_size);
-	} else if (ret != -ENOSPC)
-		ntfs_error(icx->idx_ni->vol->sb, "Failed to truncate INDEX_ROOT");
+		if (!shrink)
+			icx->ir->index.allocated_size = cpu_to_le32(data_size);
+	} else {
+		if (shrink)
+			icx->ir->index.allocated_size = cpu_to_le32(old_allocated_size);
+		if (ret != -ENOSPC)
+			ntfs_error(icx->idx_ni->vol->sb, "Failed to truncate INDEX_ROOT");
+	}
 
 	return ret;
 }
-- 
2.43.0