[BUG] ocfs2: kernel BUG in ocfs2_populate_inode still reproducible on v7.3-rc1

[BUG] ocfs2: kernel BUG in ocfs2_populate_inode still reproducible on v7.3-rc1

[CJ]

Hi,


I am reporting a kernel BUG in ocfs2's inode population path, triggered by a
syzkaller reproducer that mounts a crafted ocfs2 image.  The issue is
reproducible with HEAD commit cee9395acd8043be0644b25c34bfa86623f2b935
(v7.3-rc1, Linux 7.3.0-rc1).


The reproducer mounts a small ocfs2 filesystem image on a loop device.  The
image carries a dinode with the OCFS2_SUPER_BLOCK_FL flag set that still passes
the existing inode-block validator, so the mount path accepts it as an ordinary
dinode.  During superblock setup the mount initializes the global system
inodes, reads that dinode through ocfs2_read_locked_inode and hands it to
ocfs2_populate_inode.


ocfs2_populate_inode then reaches the branch that handles a dinode flagged as a
superblock and calls BUG() unconditionally.  The target console shows
"kernel BUG at fs/ocfs2/inode.c:448!", an invalid-opcode Oops, and RIP in
ocfs2_populate_inode.  The path is the mount-time system-inode initialization:
ocfs2_read_locked_inode -> ocfs2_iget -> ocfs2_get_system_file_inode ->
ocfs2_initialize_super -> ocfs2_fill_super -> mount.


One possible cause is that nothing on the read path rejects a dinode carrying
OCFS2_SUPER_BLOCK_FL before it reaches ocfs2_populate_inode.  The comment at the
BUG() site notes that read_inode cannot handle superblocks, but
ocfs2_validate_inode_block only inspects i_flags for OCFS2_VALID_FL; it checks
the ECC, the dinode signature, i_blkno, i_links_count, i_mode and
i_fs_generation, and never looks at OCFS2_SUPER_BLOCK_FL.  A crafted image that
sets VALID_FL together with SUPER_BLOCK_FL therefore passes validation.  This
looks like a case where the invariant the BUG() asserts is not enforced on the
on-disk-input path.  I note that recent work in this file
(bb88131c9831, b2d31acbd3b1, 8914a3330b72) tightened adjacent dinode validation
but does not cover this flag.


This appears to be a recurrence of the syzbot issue whose external id is
09184149c7c0d9e6f83a (title "kernel BUG in ocfs2_populate_inode").  It remains
reproducible on v7.3-rc1.  I am aware that two patches were proposed earlier
(one replacing the BUG() with ocfs2_error() plus make_bad_inode(), one rejecting
the flag in ocfs2_validate_inode_block) and that a review argued the existing
BUG() is acceptable for metadata corruption.  I am reporting the reproduction
only; if the current behaviour is considered correct, please disregard this.


Reproducer:


syz reproducer: https://pastebin.com/raw/C3BdtqfE
console output: https://pastebin.com/raw/Ld7dqEc4
kernel config: https://pastebin.com/raw/SGA1HK95


Kernel:


HEAD commit: cee9395acd8043be0644b25c34bfa86623f2b935
git tree: upstream (linux.git), tested through the v7.3-rc1 annotated tag object
           e5e04726cdd043e309677071ab1b65a4b18f422b
kernel version: 7.3.0-rc1 #1 PREEMPT(full)
tested tag: v7.3-rc1 (Linux 7.3-rc1, 2026-08-30)


Let me know if you need more details or testing.


Best regards,
Changjian