On Wed 14-12-22 13:52:32, Theodore Ts'o wrote:
> On Wed, Dec 14, 2022 at 06:01:25PM +0100, Jan Kara wrote:
> >
> > Besides some naming nits (see below) I think this should work. But I have
> > to say I'm a bit uneasy about this because we will now be changing block
> > mapping from unwritten to written only with shared i_rwsem. OTOH that
> > happens during writeback as well so we should be fine and the gain is very
> > nice.
>
> Hmm.... when I was looking potential impacts of the change what
> ext4_overwrite_io() would do, I looked at the current user of that
> function in ext4_dio_write_checks().
>
> /*
> * Determine whether the IO operation will overwrite allocated
> * and initialized blocks.
> * We need exclusive i_rwsem for changing security info
> * in file_modified().
> */
> if (*ilock_shared && (!IS_NOSEC(inode) || *extend ||
> !ext4_overwrite_io(inode, offset, count))) {
> if (iocb->ki_flags & IOCB_NOWAIT) {
> ret = -EAGAIN;
> goto out;
> }
> inode_unlock_shared(inode);
> *ilock_shared = false;
> inode_lock(inode);
> goto restart;
> }
>
> ret = file_modified(file);
> if (ret < 0)
> goto out;
>
> What is confusing me is the comment, "We need exclusive i_rwsem for
> changing security info in file_modified().". But then we end up
> calling file_modified() unconditionally, regardless of whether we've
> transitioned from a shared lock to an exclusive lock.
>
> So file_modified() can get called either with or without the inode
> locked r/w. I realize that this patch doesn't change this
> inconsistency, but it appears either the comment is wrong, or the code
> is wrong.
Maybe the comment needs rephrasing but it seems correct. file_modified()
does multiple things. It updates timestamps - these are fine with shared
i_rwsem - and is calls into __file_remove_privs() to remove SUID bits etc.
Now if __file_remove_privs() is going to modify the inode, we need i_rwsem
exclusively. And we determine whether __file_remove_privs() will do
anything by checking !IS_NOSEC(inode) in the condition above. So the
sentence you're confused about speaks about this part of the condition.
Honza
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
