[PATCH] assoc_array: Fix BUG_ON during garbage collect

[Stephen Brennan <stephen.s.brennan@oracle.com>]

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

Fixes: 3cb989501c26 ("Add a generic associative array implementation.")
Cc: <stable@vger.kernel.org>
Signed-off-by: Stephen Brennan <stephen.s.brennan@oracle.com>
---

Hi Andrew,

As far as I can tell, lib/assoc_array.c has no maintainer, so I'm
sending this bugfix to you. Hopefully David can take a look at this and
verify sure it's all sane. I tested it on my userspace reproducer, and
also by booting it and exercising the keyring_gc functions a bit.

Thanks,
Stephen

 lib/assoc_array.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/lib/assoc_array.c b/lib/assoc_array.c
index 079c72e26493..7ed20233a770 100644
--- a/lib/assoc_array.c
+++ b/lib/assoc_array.c
@@ -1462,6 +1462,7 @@ int assoc_array_gc(struct assoc_array *array,
 	struct assoc_array_ptr *new_root, *new_parent, **new_ptr_pp;
 	unsigned long nr_leaves_on_tree;
 	int keylen, slot, nr_free, next_slot, i;
+	bool retained;
 
 	pr_devel("-->%s()\n", __func__);
 
@@ -1536,6 +1537,7 @@ int assoc_array_gc(struct assoc_array *array,
 		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
@@ -1554,6 +1556,7 @@ int assoc_array_gc(struct assoc_array *array,
 
 	/* See what we can fold in */
 	next_slot = 0;
+	retained = 0;
 	for (slot = 0; slot < ASSOC_ARRAY_FAN_OUT; slot++) {
 		struct assoc_array_shortcut *s;
 		struct assoc_array_node *child;
@@ -1602,9 +1605,14 @@ int assoc_array_gc(struct assoc_array *array,
 			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 neough 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;
-- 
2.30.2