snapuserd: Fix PrepareReadAhead
When checking the instead of current, and when checking the position, we could underflow,
which would get caught by UBSAN
Bug: 177104308
Test: Map cow file with xor op or copy op at position 0
Change-Id: I4fba1982419e08b58759ec893e238175d245dbfc
diff --git a/fs_mgr/libsnapshot/snapuserd/snapuserd_readahead.cpp b/fs_mgr/libsnapshot/snapuserd/snapuserd_readahead.cpp
index ac3c222..3bb7a0a 100644
--- a/fs_mgr/libsnapshot/snapuserd/snapuserd_readahead.cpp
+++ b/fs_mgr/libsnapshot/snapuserd/snapuserd_readahead.cpp
@@ -223,7 +223,7 @@
if (op->type == kCowCopyOp) {
next_offset *= BLOCK_SZ;
}
- if (next_offset != (*source_offset - nr_consecutive * BLOCK_SZ)) {
+ if (next_offset + nr_consecutive * BLOCK_SZ != *source_offset) {
break;
}
nr_consecutive += 1;