Sync with upstream 41655e1c:

Reduce memory footprint of fsck_msdosfs.

This utility was initially written for FAT12/16, which were inherently
small. When FAT32 support was added, the old data structure and
algorithms remain used with minimal changes.

With growing size of FAT32 media, the current data structure that
requires 4 32-bit variables per each FAT32 table entry would consume up
to 4 GiB of RAM, which can be too big for systems with limited RAM
available.

Address this by taking a different approach of validating the FAT.

The FAT is essentially a set of linked lists of chains that was
referenced by directory entries, and the checker needs to make sure that
the linked chains of clusters do not have cross-linked chains, and every
chain were referenced by one and only one directory entry.  Instead of
keeping track of the chain's 'head' cluster number, the size of the
chain, the used status of the chain and the "next" pointer which is
content of the FAT table, we create accessors for the FAT table data
for the "next" pointer, and keep only one bit to indicate if the
current cluster is a 'head' node of a cluster chain, in a bitmap.

We further overhaul the FAT checker to find out the possible head nodes
by excluding ones that are not (in other words, nodes that have some
other nodes claiming them as the next node) instead of marking the head
nodes for each node on the chain.  This approach greatly reduced the
complexiety of computation from O(N^2) worst case, to an O(N) scan for
worst case.  The file (cluster chain) length is not useful for the FAT
checker, so don't bother to calculate them in the FAT checker and
instead leave the task to the directory structure check, at which point
we would have non-crossed cluster chains, and we are guaranteed that
each cluster will be visited for at most one time.

When checking the directory structures, we use the head node indicator
to as the visited (used) flag: every cluster chain can only be
referenced by one directory entry, so we clear them when calculating
the length of the chain, and we can immediately tell if there are
anomalies in the directory entry.

As a result, the required RAM size is now 1 bit per each entry of
the FAT table, plus memory needed to hold the FAT table in memory,
instead of 16 bytes (=128 bits) per each entry.  For FAT12 and FAT16,
we will load the whole FAT table into memory as they are smaller than
128KiB, and for FAT32, we first attempt to mmap() it into memory, and
when that fails, we would fall back to a simple LRU cache of 4 MiB of
RAM.

Bug: 37055905
Bug: 130028193
Bug: 143230734
Bug: 139179199
Change-Id: Ia10258e098f0dce4c870ca38f2294d74f033819a

1 files changed, 1 insertions, 0 deletions

diff --git a/Android.bp b/Android.bp
index 67858c3..3d57578 100644
--- a/Android.bp
+++ b/Android.bp
@@ -21,5 +21,6 @@ cc_binary {
         "-Wno-unused-const-variable",
         "-Wno-format",
         "-Wno-sign-compare",
+        "-include freebsd-compat.h",
     ],
 }