Re: [RFC][PATCH] fix short copy handling in copy_mc_pipe_to_iter()

[Al Viro]

On Tue, Jun 14, 2022 at 12:25:03AM +0100, Al Viro wrote:

> The more I'm looking at that thing, the more it smells like a bug;
> it had the same 3 callers since the time it had been introduced.
> 
> 1) pipe_get_pages().  We are about to try and allocate up to that
> many pipe buffers.  Allocation (done in push_pipe()) is done only
> if we have !pipe_full(pipe->head, pipe->tail, pipe->max_usage).
> 
> It simply won't give you more than max_usage - occupancy.
> Your function returns min(ring_size - occupancy, max_usage), which
> is always greater than or equal to that (ring_size >= max_usage).
> 
> 2) pipe_get_pages_alloc().  Same story, same push_pipe() being
> called, same "we'll never get that much - it'll hit the limit
> first".
> 
> 3) iov_iter_npages() in case of ITER_PIPE.  Again, the value
> is bogus - it should not be greater than the amount of pages
> we would be able to write there.
> 
> AFAICS, 6718b6f855a0 "pipe: Allow pipes to have kernel-reserved slots"
> broke it for cases when ring_size != max_usage...

Unless I'm missing something, the following would do the right thing.
Dave?

diff --git a/include/linux/pipe_fs_i.h b/include/linux/pipe_fs_i.h
index 4ea496924106..c22173d6e500 100644
--- a/include/linux/pipe_fs_i.h
+++ b/include/linux/pipe_fs_i.h
@@ -165,15 +165,10 @@ static inline bool pipe_full(unsigned int head, unsigned int tail,
 static inline unsigned int pipe_space_for_user(unsigned int head, unsigned int tail,
 					       struct pipe_inode_info *pipe)
 {
-	unsigned int p_occupancy, p_space;
-
-	p_occupancy = pipe_occupancy(head, tail);
+	unsigned int p_occupancy = pipe_occupancy(head, tail);
 	if (p_occupancy >= pipe->max_usage)
 		return 0;
-	p_space = pipe->ring_size - p_occupancy;
-	if (p_space > pipe->max_usage)
-		p_space = pipe->max_usage;
-	return p_space;
+	return pipe->max_usage - p_occupancy;
 }
 
 /**