Re: WARNING in iov_iter_pipe

[Eric Biggers <ebiggers@kernel.org>]

On Mon, Nov 11, 2019 at 07:16:29PM +1100, Matthew Bobrowski wrote:
> On Fri, Nov 08, 2019 at 11:31:48AM +0100, Jan Kara wrote:
> > On Thu 07-11-19 10:54:10, syzbot wrote:
> > > syzbot found the following crash on:
> > > 
> > > HEAD commit:    c68c5373 Add linux-next specific files for 20191107
> > > git tree:       linux-next
> > > console output: https://syzkaller.appspot.com/x/log.txt?x=13d6bcfce00000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=742545dcdea21726
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=991400e8eba7e00a26e1
> > > compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
> > > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1529829ae00000
> > > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=16a55c0ce00000
> > > 
> > > The bug was bisected to:
> > > 
> > > commit b1b4705d54abedfd69dcdf42779c521aa1e0fbd3
> > > Author: Matthew Bobrowski <mbobrowski@mbobrowski.org>
> > > Date:   Tue Nov 5 12:01:37 2019 +0000
> > > 
> > >     ext4: introduce direct I/O read using iomap infrastructure
> > 
> > Hum, interesting and from the first looks the problem looks real.
> > Deciphered reproducer is:
> > 
> > int fd0 = open("./file0", O_RDWR | O_CREAT | O_EXCL | O_DIRECT, 0);
> > int fd1 = open("./file0, O_RDONLY);
> > write(fd0, "some_data...", 512);
> > sendfile(fd0, fd1, NULL, 0x7fffffa7);
> >   -> this is interesting as it will result in reading data from 'file0' at
> >      offset X with buffered read and writing them with direct write to
> >      offset X+512. So this way we'll grow the file up to those ~2GB in
> >      512-byte chunks.
> > - not sure if we ever get there but the remainder of the reproducer is:
> > fd2 = open("./file0", O_RDWR | O_CREAT | O_NOATIME | O_SYNC, 0);
> > sendfile(fd2, fd0, NULL, 0xffffffff)
> >   -> doesn't seem too interesting as fd0 is at EOF so this shouldn't do
> >      anything.
> > 
> > Matthew, can you have a look?
> 
> Sorry Jan, I've been crazy busy lately and I'm out at training this
> week. Let me take a look at this and see whether I can determine
> what's happening here.
> 

FYI, syzbot is still seeing this on linux-next.

Also, a new thread was started to discuss this:
https://lkml.kernel.org/linux-ext4/20191113180032.GB12013@quack2.suse.cz/T/#u
(Mentioning this in case anyone is following this thread only.)

- Eric