Re: [PATCH] ext4: Fix i_disksize exceeding i_size problem in paritally written case

[Zhihao Cheng]

Hi, Jan
> On Fri 17-03-23 09:35:53, Zhihao Cheng wrote:
> > Following process makes i_disksize exceed i_size:
> >
> > generic_perform_write
> >   copied = iov_iter_copy_from_user_atomic(len) // copied < len
> >   ext4_da_write_end
> >   | ext4_update_i_disksize
> >   |  new_i_size = pos + copied;
> >   |  WRITE_ONCE(EXT4_I(inode)->i_disksize, newsize) // update i_disksize
> >   | generic_write_end
> >   |  copied = block_write_end(copied, len) // copied = 0
> >   |   if (unlikely(copied < len))
> >   |    if (!PageUptodate(page))
> >   |     copied = 0;
> >   |  if (pos + copied > inode->i_size) // return false
> >   if (unlikely(copied == 0))
> >    goto again;
> >   if (unlikely(iov_iter_fault_in_readable(i, bytes))) {
> >    status = -EFAULT;
> >    break;
> >   }
> >
> > We get i_disksize greater than i_size here, which could trigger WARNING
> > check 'i_size_read(inode) < EXT4_I(inode)->i_disksize' while doing dio:
> >
> > ext4_dio_write_iter
> >   iomap_dio_rw
> >    __iomap_dio_rw // return err, length is not aligned to 512
> >   ext4_handle_inode_extension
> >    WARN_ON_ONCE(i_size_read(inode) < EXT4_I(inode)->i_disksize) // Oops
> >
> >   WARNING: CPU: 2 PID: 2609 at fs/ext4/file.c:319
> >   CPU: 2 PID: 2609 Comm: aa Not tainted 6.3.0-rc2
> >   RIP: 0010:ext4_file_write_iter+0xbc7
> >   Call Trace:
> >    vfs_write+0x3b1
> >    ksys_write+0x77
> >    do_syscall_64+0x39
> >
> > Fix it by putting block_write_end() before i_disksize updating just
> > like ext4_write_end() does.
> >
> > Fetch a reproducer in [Link].
> >
> > Link: https://bugzilla.kernel.org/show_bug.cgi?id=217209
> > Fixes: 64769240bd07f ("ext4: Add delayed allocation support in data=writeback mode")
> > Signed-off-by: Zhihao Cheng <chengzhihao1@xxxxxxxxxx>
>
> Good catch (although practically this will hardly have any negative
> effect). But rather than opencoding generic_write_end() I'd do:
>
>          if (unlikely(copied < len) && !PageUptodate(page))
>                  copied = 0;
>
> at the beginning of ext4_da_write_end() and that should solve these
> problems as well?

Yes, your suggestion looks good, and I think we can put the checking 
just after ext4_write_inline_data_end(Line 3150)? On the one hand, we 
can pass original 'copied' value in trace_ext4_da_write_end(), one the 
other hand, ext4_write_inline_data_end() already has this checking.