From 6668ed271eaefaa63e686bdfbedaeb7b8e492722 Mon Sep 17 00:00:00 2001
From: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Date: Fri, 4 Sep 2026 10:43:19 +0200
Subject: selftests/bpf: Reject graph kptr use after RCU unlock

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>
---
 Documentation/filesystems/ext4/mmp.rst | 77 ++++++++++++++++++++++++++++++++++
 1 file changed, 77 insertions(+)
 create mode 100644 Documentation/filesystems/ext4/mmp.rst

(limited to 'Documentation/filesystems/ext4/mmp.rst')

diff --git a/Documentation/filesystems/ext4/mmp.rst b/Documentation/filesystems/ext4/mmp.rst
new file mode 100644
index 000000000..174dd6538
--- /dev/null
+++ b/Documentation/filesystems/ext4/mmp.rst
@@ -0,0 +1,77 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+Multiple Mount Protection
+-------------------------
+
+Multiple mount protection (MMP) is a feature that protects the
+filesystem against multiple hosts trying to use the filesystem
+simultaneously. When a filesystem is opened (for mounting, or fsck,
+etc.), the MMP code running on the node (call it node A) checks a
+sequence number. If the sequence number is EXT4_MMP_SEQ_CLEAN, the
+open continues. If the sequence number is EXT4_MMP_SEQ_FSCK, then
+fsck is (hopefully) running, and open fails immediately. Otherwise, the
+open code will wait for twice the specified MMP check interval and check
+the sequence number again. If the sequence number has changed, then the
+filesystem is active on another machine and the open fails. If the MMP
+code passes all of those checks, a new MMP sequence number is generated
+and written to the MMP block, and the mount proceeds.
+
+While the filesystem is live, the kernel sets up a timer to re-check the
+MMP block at the specified MMP check interval. To perform the re-check,
+the MMP sequence number is re-read; if it does not match the in-memory
+MMP sequence number, then another node (node B) has mounted the
+filesystem, and node A remounts the filesystem read-only. If the
+sequence numbers match, the sequence number is incremented both in
+memory and on disk, and the re-check is complete.
+
+The hostname and device filename are written into the MMP block whenever
+an open operation succeeds. The MMP code does not use these values; they
+are provided purely for informational purposes.
+
+The checksum is calculated against the FS UUID and the MMP structure.
+The MMP structure (``struct mmp_struct``) is as follows:
+
+.. list-table::
+   :widths: 8 12 20 40
+   :header-rows: 1
+
+   * - Offset
+     - Type
+     - Name
+     - Description
+   * - 0x0
+     - __le32
+     - mmp_magic
+     - Magic number for MMP, 0x004D4D50 (“MMP”).
+   * - 0x4
+     - __le32
+     - mmp_seq
+     - Sequence number, updated periodically.
+   * - 0x8
+     - __le64
+     - mmp_time
+     - Time that the MMP block was last updated.
+   * - 0x10
+     - char[64]
+     - mmp_nodename
+     - Hostname of the node that opened the filesystem.
+   * - 0x50
+     - char[32]
+     - mmp_bdevname
+     - Block device name of the filesystem.
+   * - 0x70
+     - __le16
+     - mmp_check_interval
+     - The MMP re-check interval, in seconds.
+   * - 0x72
+     - __le16
+     - mmp_pad1
+     - Zero.
+   * - 0x74
+     - __le32[226]
+     - mmp_pad2
+     - Zero.
+   * - 0x3FC
+     - __le32
+     - mmp_checksum
+     - Checksum of the MMP block.
-- 
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