--- /dev/null
+From d1dc87763f406d4e67caf16dbe438a5647692395 Mon Sep 17 00:00:00 2001
+From: Stephen Brennan <stephen.s.brennan@oracle.com>
+Date: Thu, 19 May 2022 09:50:30 +0100
+Subject: assoc_array: Fix BUG_ON during garbage collect
+
+From: Stephen Brennan <stephen.s.brennan@oracle.com>
+
+commit d1dc87763f406d4e67caf16dbe438a5647692395 upstream.
+
+A rare BUG_ON triggered in assoc_array_gc:
+
+ [3430308.818153] kernel BUG at lib/assoc_array.c:1609!
+
+Which corresponded to the statement currently at line 1593 upstream:
+
+ BUG_ON(assoc_array_ptr_is_meta(p));
+
+Using the data from the core dump, I was able to generate a userspace
+reproducer[1] and determine the cause of the bug.
+
+[1]: https://github.com/brenns10/kernel_stuff/tree/master/assoc_array_gc
+
+After running the iterator on the entire branch, an internal tree node
+looked like the following:
+
+ NODE (nr_leaves_on_branch: 3)
+ SLOT [0] NODE (2 leaves)
+ SLOT [1] NODE (1 leaf)
+ SLOT [2..f] NODE (empty)
+
+In the userspace reproducer, the pr_devel output when compressing this
+node was:
+
+ -- compress node 0x5607cc089380 --
+ free=0, leaves=0
+ [0] retain node 2/1 [nx 0]
+ [1] fold node 1/1 [nx 0]
+ [2] fold node 0/1 [nx 2]
+ [3] fold node 0/2 [nx 2]
+ [4] fold node 0/3 [nx 2]
+ [5] fold node 0/4 [nx 2]
+ [6] fold node 0/5 [nx 2]
+ [7] fold node 0/6 [nx 2]
+ [8] fold node 0/7 [nx 2]
+ [9] fold node 0/8 [nx 2]
+ [10] fold node 0/9 [nx 2]
+ [11] fold node 0/10 [nx 2]
+ [12] fold node 0/11 [nx 2]
+ [13] fold node 0/12 [nx 2]
+ [14] fold node 0/13 [nx 2]
+ [15] fold node 0/14 [nx 2]
+ after: 3
+
+At slot 0, an internal node with 2 leaves could not be folded into the
+node, because there was only one available slot (slot 0). Thus, the
+internal node was retained. At slot 1, the node had one leaf, and was
+able to be folded in successfully. The remaining nodes had no leaves,
+and so were removed. By the end of the compression stage, there were 14
+free slots, and only 3 leaf nodes. The tree was ascended and then its
+parent node was compressed. When this node was seen, it could not be
+folded, due to the internal node it contained.
+
+The invariant for compression in this function is: whenever
+nr_leaves_on_branch < ASSOC_ARRAY_FAN_OUT, the node should contain all
+leaf nodes. The compression step currently cannot guarantee this, given
+the corner case shown above.
+
+To fix this issue, retry compression whenever we have retained a node,
+and yet nr_leaves_on_branch < ASSOC_ARRAY_FAN_OUT. This second
+compression will then allow the node in slot 1 to be folded in,
+satisfying the invariant. Below is the output of the reproducer once the
+fix is applied:
+
+ -- compress node 0x560e9c562380 --
+ free=0, leaves=0
+ [0] retain node 2/1 [nx 0]
+ [1] fold node 1/1 [nx 0]
+ [2] fold node 0/1 [nx 2]
+ [3] fold node 0/2 [nx 2]
+ [4] fold node 0/3 [nx 2]
+ [5] fold node 0/4 [nx 2]
+ [6] fold node 0/5 [nx 2]
+ [7] fold node 0/6 [nx 2]
+ [8] fold node 0/7 [nx 2]
+ [9] fold node 0/8 [nx 2]
+ [10] fold node 0/9 [nx 2]
+ [11] fold node 0/10 [nx 2]
+ [12] fold node 0/11 [nx 2]
+ [13] fold node 0/12 [nx 2]
+ [14] fold node 0/13 [nx 2]
+ [15] fold node 0/14 [nx 2]
+ internal nodes remain despite enough space, retrying
+ -- compress node 0x560e9c562380 --
+ free=14, leaves=1
+ [0] fold node 2/15 [nx 0]
+ after: 3
+
+Changes
+=======
+DH:
+ - Use false instead of 0.
+ - Reorder the inserted lines in a couple of places to put retained before
+ next_slot.
+
+ver #2)
+ - Fix typo in pr_devel, correct comparison to "<="
+
+Fixes: 3cb989501c26 ("Add a generic associative array implementation.")
+Cc: <stable@vger.kernel.org>
+Signed-off-by: Stephen Brennan <stephen.s.brennan@oracle.com>
+Signed-off-by: David Howells <dhowells@redhat.com>
+cc: Andrew Morton <akpm@linux-foundation.org>
+cc: keyrings@vger.kernel.org
+Link: https://lore.kernel.org/r/20220511225517.407935-1-stephen.s.brennan@oracle.com/ # v1
+Link: https://lore.kernel.org/r/20220512215045.489140-1-stephen.s.brennan@oracle.com/ # v2
+Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
+Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
+Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
+---
+ lib/assoc_array.c | 8 ++++++++
+ 1 file changed, 8 insertions(+)
+
+--- a/lib/assoc_array.c
++++ b/lib/assoc_array.c
+@@ -1478,6 +1478,7 @@ int assoc_array_gc(struct assoc_array *a
+ struct assoc_array_ptr *cursor, *ptr;
+ struct assoc_array_ptr *new_root, *new_parent, **new_ptr_pp;
+ unsigned long nr_leaves_on_tree;
++ bool retained;
+ int keylen, slot, nr_free, next_slot, i;
+
+ pr_devel("-->%s()\n", __func__);
+@@ -1554,6 +1555,7 @@ continue_node:
+ goto descend;
+ }
+
++retry_compress:
+ pr_devel("-- compress node %p --\n", new_n);
+
+ /* Count up the number of empty slots in this node and work out the
+@@ -1571,6 +1573,7 @@ continue_node:
+ pr_devel("free=%d, leaves=%lu\n", nr_free, new_n->nr_leaves_on_branch);
+
+ /* See what we can fold in */
++ retained = false;
+ next_slot = 0;
+ for (slot = 0; slot < ASSOC_ARRAY_FAN_OUT; slot++) {
+ struct assoc_array_shortcut *s;
+@@ -1620,9 +1623,14 @@ continue_node:
+ pr_devel("[%d] retain node %lu/%d [nx %d]\n",
+ slot, child->nr_leaves_on_branch, nr_free + 1,
+ next_slot);
++ retained = true;
+ }
+ }
+
++ if (retained && new_n->nr_leaves_on_branch <= ASSOC_ARRAY_FAN_OUT) {
++ pr_devel("internal nodes remain despite enough space, retrying\n");
++ goto retry_compress;
++ }
+ pr_devel("after: %lu\n", new_n->nr_leaves_on_branch);
+
+ nr_leaves_on_tree = new_n->nr_leaves_on_branch;
projects / thirdparty / kernel / stable-queue.git / commitdiff
