Re: iov_iter_pipe warning.

[Al Viro]

On Tue, Apr 11, 2017 at 08:06:07PM -0400, Dave Jones wrote:
> On Wed, Apr 12, 2017 at 12:56:41AM +0100, Al Viro wrote:
>  > On Wed, Apr 12, 2017 at 12:51:58AM +0100, Al Viro wrote:
>  > > On Tue, Apr 11, 2017 at 07:45:58PM -0400, Dave Jones wrote:
>  > > >  >     if (file->f_op->splice_write == generic_splice_sendpage) {
>  > > >  >             struct socket *sock = file->private_data;
>  > > >  >             printk(KERN_ERR "socket [%d, %p]\n", sock->type, 
> sock->ops);
>  > > >  >     }
>  > > >  >     printk(KERN_ERR "in->f_op = %p\n", in->f_op);
>  > > > 
>  > > > Ugh, this explodes with a million errors when I try to compile it. 
>  > > > It misses socket definition, and include <linux/net.h> causes another
>  > > > cascade of errors about linkage.h and nonsense.
>  > > 
>  > > Ignore the socket part - you've already triggered it with NFS file as
>  > > destination, so this is not particularly interesting.  I would still like
>  > > to see in->f_op and even more - the checks in default_file_splice_read().
>  > 
>  > ... and the latter had a braino - WARN_ON(size != ret), not len != ret.
>  > Diff follows:
>  
> super fast repro..

Alas, that's just another braino - it checks for non-zero ->buffers (always
true) rather than non-zero ->nrbufs (non-empty pipe).  Sorry.  Fixed diff
follows:

diff --git a/fs/splice.c b/fs/splice.c
index 006ba50f4ece..43dd9b3140ee 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -448,6 +448,18 @@ static ssize_t default_file_splice_read(struct file *in, 
loff_t *ppos,
                put_page(pages[i]);
        kvfree(pages);
        iov_iter_advance(&to, copied);  /* truncates and discards */
+       if (res > 0 && pipe == current->splice_pipe) {
+               int idx = pipe->curbuf;
+               int n = pipe->nrbufs;
+               size_t size = 0;
+               while (n--) {
+                       size += pipe->bufs[idx++].len;
+                       if (idx == pipe->buffers)
+                               idx = 0;
+               }
+               WARN_ON(size != res);
+       }
+
        return res;
 }
 
@@ -970,6 +982,11 @@ ssize_t splice_direct_to_actor(struct file *in, struct 
splice_desc *sd,
        while (len) {
                size_t read_len;
                loff_t pos = sd->pos, prev_pos = pos;
+               if (WARN_ON(pipe->nrbufs)) {
+                       printk(KERN_ERR "in->f_op = %p, ->splice_write = %p\n",
+                               in->f_op,
+                               sd->u.file->f_op->splice_write);
+               }
 
                ret = do_splice_to(in, &pos, pipe, len, flags);
                if (unlikely(ret <= 0))