[PATCH v1] erofs-utils: lib: Fix 8MB bug on uncompressed extent size

[Kelvin Zhang]

Previously, uncompressed extent can be at most 8MB before mkfs.erofs
crashes on some error condition. This is due to a minor bug in how
compressed indices are encoded. This patch fixes the issue.

Signed-off-by: Kelvin Zhang <zhangkelvin at google.com>
---
 include/erofs_fs.h |  2 +-
 lib/compress.c     | 21 ++++++++++++++++++++-
 2 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/include/erofs_fs.h b/include/erofs_fs.h
index 9a91877..13eaf24 100644
--- a/include/erofs_fs.h
+++ b/include/erofs_fs.h
@@ -353,7 +353,7 @@ enum {
  * compressed block count of a compressed extent (in logical clusters, aka.
  * block count of a pcluster).
  */
-#define Z_EROFS_VLE_DI_D0_CBLKCNT		(1 << 11)
+#define Z_EROFS_VLE_DI_D0_CBLKCNT		(1U << 11)
 
 struct z_erofs_vle_decompressed_index {
 	__le16 di_advise;
diff --git a/lib/compress.c b/lib/compress.c
index 98be7a2..23e571c 100644
--- a/lib/compress.c
+++ b/lib/compress.c
@@ -97,7 +97,26 @@ static void vle_write_indexes(struct z_erofs_vle_compress_ctx *ctx,
 		} else if (d0) {
 			type = Z_EROFS_VLE_CLUSTER_TYPE_NONHEAD;
 
-			di.di_u.delta[0] = cpu_to_le16(d0);
+			/* If the |Z_EROFS_VLE_DI_D0_CBLKCNT| bit is set, parser
+			 * will interpret |delta[0]| as size of pcluster, rather
+			 * than distance to last head cluster. Normally this
+			 * isn't a problem, because uncompressed extent size are
+			 * below Z_EROFS_VLE_DI_D0_CBLKCNT * BLOCK_SIZE = 8MB.
+			 * But with large pcluster it's possible to go over this
+			 * number, resulting in corrupted compressed indices.
+			 * To solve this, we use Z_EROFS_VLE_DI_D0_CBLKCNT-1 if
+			 * the uncompressed extent size goes above 8MB. This is
+			 * OK because if kernel sees another non-head cluster
+			 * after going back by |delta[0]| blocks, kernel will
+			 * just keep looking back.
+			 */
+			if (d0 & Z_EROFS_VLE_DI_D0_CBLKCNT) {
+				di.di_u.delta[0] = max(
+					d0 & (~Z_EROFS_VLE_DI_D0_CBLKCNT),
+					Z_EROFS_VLE_DI_D0_CBLKCNT-1);
+			} else {
+				di.di_u.delta[0] = cpu_to_le16(d0);
+			}
 			di.di_u.delta[1] = cpu_to_le16(d1);
 		} else {
 			type = raw ? Z_EROFS_VLE_CLUSTER_TYPE_PLAIN :
-- 
2.34.1.448.ga2b2bfdf31-goog