selftests/bpf: Reject graph kptr use after RCU unlockgrafted

Add a sleepable verifier test that loads a graph-node local kptr in an
explicit RCU read-side critical section, then passes its node to
bpf_rbtree_remove() after the section ends.

Before the verifier fix, the stale NON_OWN_REF flag makes the node look like
a live borrowed reference and the program is accepted. After the fix, the
pointer is demoted without NON_OWN_REF and the graph kfunc argument is
rejected.

Also exercise a graph kptr loaded while a spin lock provides implicit RCU
protection. The pointer must be invalidated when the lock is released, which
guards the required ordering between non-owning-reference invalidation and
RCU demotion.

Update the existing fault-protected load test state description. The
post-unlock pointer no longer carries NON_OWN_REF, but remains readable
because the load is rewritten to use BPF_PROBE_MEM.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20260904084325.52250-7-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 21 insertions, 0 deletions

diff --git a/Documentation/filesystems/ext4/bitmaps.rst b/Documentation/filesystems/ext4/bitmaps.rst
new file mode 100644
index 000000000..9d7d7b083
--- /dev/null
+++ b/Documentation/filesystems/ext4/bitmaps.rst
@@ -0,0 +1,21 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+Block and inode Bitmaps
+-----------------------
+
+The data block bitmap tracks the usage of data blocks within the block
+group.
+
+The inode bitmap records which entries in the inode table are in use.
+
+As with most bitmaps, one bit represents the usage status of one data
+block or inode table entry. This implies a block group size of 8 *
+number_of_bytes_in_a_logical_block.
+
+NOTE: If ``BLOCK_UNINIT`` is set for a given block group, various parts
+of the kernel and e2fsprogs code pretends that the block bitmap contains
+zeros (i.e. all blocks in the group are free). However, it is not
+necessarily the case that no blocks are in use -- if ``meta_bg`` is set,
+the bitmaps and group descriptor live inside the group. Unfortunately,
+ext2fs_test_block_bitmap2() will return '0' for those locations,
+which produces confusing debugfs output.