Re: iov_iter_pipe warning.

[Al Viro]

On Mon, Apr 10, 2017 at 11:05:32PM -0400, Dave Jones wrote:
> On Tue, Apr 11, 2017 at 01:22:15AM +0100, Al Viro wrote:
> 
>  > 	* in do_splice_to(): WARN_ON(pipe->nrbufs == pipe->buffers)
> 
> Hit this one.

But not WARN_ON(pipe->nrbufs) in its caller *or* WARN_ON(!pipe->buffers)
in do_splice_to() itself?

How the devil can that be possible?

Again, to make sure we are on the same page: in
	if (WARN_ON(pipe->nrbufs)) {
		printk(KERN_ERR "->splice_write = %p",
			sd->u.file->f_op->splice_write);
	}
        while (len) {
                size_t read_len;
                loff_t pos = sd->pos, prev_pos = pos;

                ret = do_splice_to(in, &pos, pipe, len, flags);
		...
		... (not a single continue in sight)
		...
		if (WARN_ON(pipe->nrbufs)) {
			printk(KERN_ERR "->splice_write = %p",
				sd->u.file->f_op->splice_write);
		}
	}
neither of those WARN_ON() triggers.  In do_splice_to()
	WARN_ON(pipe->nrbufs == pipe->buffers);
does trigger, but
	WARN_ON(!pipe->buffers);
does not.  And pipe is equal to current->splice_pipe, so nobody else could
see it, let alone be messing with it.

How can that be possible?  Non-triggering WARN_ON() in caller of do_splice_to()
mean that pipe->nrbufs is zero.  Triggering WARN_ON() in do_splice_to() means
that it's equal to pipe->buffers, but WARN_ON(!pipe->buffers) manages to avoid
being triggered?  Can you confirm all that?  Because if that's the case,
the next possibility is random memory corruption and/or pipe_info dangling
pointers/use-after-free/etc.