Re: kernel BUG in ext4_writepages

[Jan Kara]

On Thu 19-05-22 16:14:17, Tadeusz Struk wrote:
> On 5/19/22 05:23, Jan Kara wrote:
> > Hi!
> > 
> > On Tue 10-05-22 15:28:38, Tadeusz Struk wrote:
> > > Syzbot found another BUG in ext4_writepages [1].
> > > This time it complains about inode with inline data.
> > > C reproducer can be found here [2]
> > > I was able to trigger it on 5.18.0-rc6
> > > 
> > > [1] https://syzkaller.appspot.com/bug?id=a1e89d09bbbcbd5c4cb45db230ee28c822953984
> > > [2] https://syzkaller.appspot.com/text?tag=ReproC&x=129da6caf00000
> > 
> > Thanks for report. This should be fixed by:
> > 
> > https://lore.kernel.org/all/20220516012752.17241-1-yebin10@xxxxxxxxxx/
> 
> 
> In case of the syzbot bug there is something messed up with PAGE DIRTY flags
> and the way syzbot sets up the write. This is what triggers the crash:

Can you tell me where exactly we hit the bug? I've now noticed that this is
on 5.10 kernel and on vanilla 5.10 there's no BUG_ON on line 2753.

> $ ftrace -f ./repro
> ...
> [pid  2395] open("./bus", O_RDWR|O_CREAT|O_SYNC|O_NOATIME, 000 <unfinished ...>
> [pid  2395] <... open resumed> )        = 6
> ...
> [pid  2395] write(6, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0", 22 <unfinished ...>
> ...
> [pid  2395] <... write resumed> )       = 22
> 
> One way I could fix it was to clear the PAGECACHE_TAG_DIRTY on the mapping in
> ext4_try_to_write_inline_data() after the page has been updated:
> 
> diff --git a/fs/ext4/inline.c b/fs/ext4/inline.c
> index 9c076262770d..e4bbb53fa26f 100644
> --- a/fs/ext4/inline.c
> +++ b/fs/ext4/inline.c
> @@ -715,6 +715,7 @@ int ext4_try_to_write_inline_data(struct address_space *mapping,
>  			put_page(page);
>  			goto out_up_read;
>  		}
> +		__xa_clear_mark(&mapping->i_pages, 0, PAGECACHE_TAG_DIRTY);
>  	}
>  	ret = 1;
> 
> Please let me know it if makes sense any I will send a proper patch.

No, this looks really wrong... We need to better understand what's going
on.

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR