Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn

Alan J. Wylie
2017-01-13T10:33:13Z

at 10:20 on Fri 13-Jan-2017 Al Viro (viro@ZenIV.linux.org.uk) wrote:

> OK, so it is iov_iter_advance() failing to free the shit allocated, either
> due to some breakage in pipe_advance() or buggered 'copied'... Let's
> see which one; could you apply the following and run your reproducer? The
> only difference from the previous is that it collects and prints a bit more,
> so it should be just as reproducible...

git reset --hard HEAD
vi ~/mail/INBOX patch2
git apply patch2

# uname -a
Linux frodo 4.9.3-dirty #2 SMP PREEMPT Fri Jan 13 10:22:47 GMT 2017 x86_64 AMD FX(tm)-8350 Eight-Core Processor AuthenticAMD GNU/Linux

[hanging as expected]

# dmesg | tail
...
[ 141.553455] nr: 0->16, cur: 5->5, buffers: 16->16
[ 141.553461] copied: 0, count:2147479552, idx:5, offs:0

>
> diff --git a/fs/splice.c b/fs/splice.c
> index 873d83104e79..11477609e7f7 100644
> --- a/fs/splice.c
> +++ b/fs/splice.c
> @@ -393,6 +393,10 @@ static ssize_t default_file_splice_read(struct file *in, loff_t *ppos,
> size_t offset, dummy, copied = 0;
> ssize_t res;
> int i;
> + unsigned nrbufs = pipe->nrbufs,
> + curbuf = pipe->curbuf,
> + buffers = pipe->buffers;
> + int idx, count, offs;
>
> if (pipe->nrbufs == pipe->buffers)
> return -EAGAIN;
> @@ -444,7 +448,22 @@ static ssize_t default_file_splice_read(struct file *in, loff_t *ppos,
> for (i = 0; i < nr_pages; i++)
> put_page(pages[i]);
> kvfree(pages);
> + count = to.count;
> + idx = to.idx;
> + offs = to.iov_offset;
> iov_iter_advance(&to, copied); /* truncates and discards */
> + if (res == -EAGAIN && (
> + pipe->nrbufs != nrbufs ||
> + pipe->curbuf != curbuf ||
> + pipe->buffers != buffers)
> + ) {
> + printk(KERN_ERR "nr: %d->%d, cur: %d->%d, buffers: %d->%d\n",
> + nrbufs, pipe->nrbufs,
> + curbuf, pipe->curbuf,
> + buffers, pipe->buffers);
> + printk(KERN_ERR "copied: %zd, count:%d, idx:%d, offs:%d\n",
> + copied, count, idx, offs);
> + }
> return res;
> }
>

--
Alan J. Wylie http://www.wylie.me.uk/

Dance like no-one's watching. / Encrypt like everyone is.
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