bcachefs: fix NULL pointer dereference in try_alloc_bucket

On Mon, 29 May 2023, Mikulas Patocka wrote:

> The oops happens in set_btree_iter_dontneed and it is caused by the fact
> that iter->path is NULL. The code in try_alloc_bucket is buggy because it
> sets "struct btree_iter iter = { NULL };" and then jumps to the "err"
> label that tries to dereference values in "iter".

Here I'm sending a patch for it.

From: Mikulas Patocka <mpatocka@redhat.com>

The function try_alloc_bucket sets the variable "iter" to NULL and then
(on various error conditions) jumps to the label "err". On the "err"
label, it calls "set_btree_iter_dontneed" that tries to dereference
"iter->trans" and "iter->path".

So, we get an oops on error condition.

This patch fixes the crash by testing that iter.trans and iter.path is
non-zero before calling set_btree_iter_dontneed.
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/alloc_foreground.c

@@ -371,7 +371,8 @@ static struct open_bucket *try_alloc_bucket(struct btree_trans *trans, struct bc
 	if (!ob)
 		iter.path->preserve = false;
 err:
-	set_btree_iter_dontneed(&iter);
+	if (iter.trans && iter.path)
+		set_btree_iter_dontneed(&iter);
 	bch2_trans_iter_exit(trans, &iter);
 	printbuf_exit(&buf);
 	return ob;