kernel crash in mknod

[Paulo Alcantara]

Al Viro <viro at zeniv.linux.org.uk> writes:

> On Mon, Mar 25, 2024 at 11:26:59AM -0500, Steve French wrote:
>
>> A loosely related question.  Do I need to change cifs.ko to return the
>> pointer to inode on mknod now?  dentry->inode is NULL in the case of mknod
>> from cifs.ko (and presumably some other fs as Al noted), unlike mkdir and
>> create where it is filled in.   Is there a perf advantage in filling in the
>> dentry->inode in the mknod path in the fs or better to leave it as is?  Is
>> there a good example to borrow from on this?
>
> AFAICS, that case in in CIFS is the only instance of ->mknod() that does this
> "skip lookups, just unhash and return 0" at the moment.
>
> What's more, it really had been broken all along for one important case -
> AF_UNIX bind(2) with address (== socket pathname) being on the filesystem
> in question.

Yes, except that we currently return -EPERM for such cases.  I don't
even know if this SFU thing supports sockets.

> Note that cifs_sfu_make_node() is the only case in CIFS where that happens -
> other codepaths (both in cifs_make_node() and in smb2_make_node()) will
> instantiate.  How painful would it be for cifs_sfu_make_node()?
> AFAICS, you do open/sync_write/close there; would it be hard to do
> an eqiuvalent of fstat and set the inode up?

This should be pretty straightforward as it would only require an extra
query info call and then {smb311_posix,cifs}_get_inode_info() ->
d_instantiate().  We could even make it a single compound request of
open/write/getinfo/close for SMB2+ case.