From 1ae4b36f2a717cd52601d10de6337f97bf4bcf50 Mon Sep 17 00:00:00 2001
From: Christopher Haster <chaster@utexas.edu>
Date: Fri, 20 Nov 2020 00:18:13 -0600
Subject: [PATCH] Removed unnecessary randomization of offsets in
 lfs_alloc_reset

On first read, randomizing the allocators offset may seem appropriate
for lfs_alloc_reset. However, it ends up using the filesystem-fed
pseudorandom seed in situations it wasn't designed for.

As noted by gtaska, the combination of using xors for feeding the seed
and multiple traverses of the same CRCs can cause the seed to flip to
zeros with concerning frequency.

Removed the randomization from lfs_alloc_reset, leaving it in only
lfs_mount.

Found by gtaska
---
 lfs.c | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/lfs.c b/lfs.c
index 7f99ccb..cc8ca38 100644
--- a/lfs.c
+++ b/lfs.c
@@ -452,14 +452,16 @@ static int lfs_alloc_lookahead(void *p, lfs_block_t block) {
     return 0;
 }
 
+// indicate allocated blocks have been committed into the filesystem, this
+// is to prevent blocks from being garbage collected in the middle of a
+// commit operation
 static void lfs_alloc_ack(lfs_t *lfs) {
     lfs->free.ack = lfs->cfg->block_count;
 }
 
-// Invalidate the lookahead buffer. This is done during mounting and
-// failed traversals
-static void lfs_alloc_reset(lfs_t *lfs) {
-    lfs->free.off = lfs->seed % lfs->cfg->block_count;
+// drop the lookahead buffer, this is done during mounting and failed
+// traversals in order to avoid invalid lookahead state
+static void lfs_alloc_drop(lfs_t *lfs) {
     lfs->free.size = 0;
     lfs->free.i = 0;
     lfs_alloc_ack(lfs);
@@ -505,7 +507,7 @@ static int lfs_alloc(lfs_t *lfs, lfs_block_t *block) {
         memset(lfs->free.buffer, 0, lfs->cfg->lookahead_size);
         int err = lfs_fs_traverseraw(lfs, lfs_alloc_lookahead, lfs, true);
         if (err) {
-            lfs_alloc_reset(lfs);
+            lfs_alloc_drop(lfs);
             return err;
         }
     }
@@ -3797,8 +3799,10 @@ int lfs_mount(lfs_t *lfs, const struct lfs_config *cfg) {
     lfs->gstate.tag += !lfs_tag_isvalid(lfs->gstate.tag);
     lfs->gdisk = lfs->gstate;
 
-    // setup free lookahead
-    lfs_alloc_reset(lfs);
+    // setup free lookahead, to distribute allocations uniformly across
+    // boots, we start the allocator at a random location
+    lfs->free.off = lfs->seed % lfs->cfg->block_count;
+    lfs_alloc_drop(lfs);
 
     LFS_TRACE("lfs_mount -> %d", 0);
     return 0;