bcache: fix variable length array abuse in btree_iter

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: bcache: fix variable length array abuse in btree_iter btree_iter is used in two ways: either allocated on the stack with a fixed size MAX_BSETS, or from a mempool with a dynamic size based on the specific cache set. Previously, the struct had a fixed-length array of size MAX_BSETS which was indexed out-of-bounds for the dynamically-sized iterators, which causes UBSAN to complain. This patch uses the same approach as in bcachefs's sort_iter and splits the iterator into a btree_iter with a flexible array member and a btree_iter_stack which embeds a btree_iter as well as a fixed-length data array.

References

• https://git.kernel.org/stable/c/2c3d7b03b658dc8bfa6112b194b67b92a87e081b
• https://git.kernel.org/stable/c/5a1922adc5798b7ec894cd3f197afb6f9591b023
• https://git.kernel.org/stable/c/934e1e4331859183a861f396d7dfaf33cb5afb02
• https://git.kernel.org/stable/c/6479b9f41583b013041943c4602e1ad61cec8148
• https://git.kernel.org/stable/c/0c31344e22dd8d6b1394c6e4c41d639015bdc671
• https://git.kernel.org/stable/c/3a861560ccb35f2a4f0a4b8207fa7c2a35fc7f31