Re: [syzbot] [exfat?] [ocfs2?] kernel BUG in link_path_walk

[Al Viro <viro@zeniv.linux.org.uk>]

On Thu, Dec 04, 2025 at 08:45:08AM +0100, Mateusz Guzik wrote:

> Or to put it differently, lookup got entered with a bogus state of a
> dentry claiming it is a directory, with an inode which is not. Per the
> i_mode reported in the opening mail it is a regular file instead.
> 
> While I don't see how this can happen,

->i_op set to something with ->lookup != NULL, ->i_mode - to regular.
Which is to say, bogus ->i_mode change somewhere.

Theoretically it should bail out, having detected the type change
(on inode_wrong_type()).  I'd suggest slapping
	BUG_ON(inode_wrong_type(inode, new_i_mode_value));
in front of all reassignments (ocfs2_populate_inode() is the initialization
and thus exempt; all other stores to ->i_mode of struct inode in there
are, in principle, suspect.  Something like inode->i_mode &= ~S_ISUID
doesn't need checking - we obviously can't change the type there.
Unpleasant part is that struct ocfs2_dinode also has a member called
i_mode (__le16, that one), so stores to that clutter the grep results...