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    SubjectRe: [PATCH] incorrect error handling inside generic_file_direct_write
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    Andrew Morton <akpm@osdl.org> writes:

    > On Mon, 11 Dec 2006 16:34:27 +0300
    > Dmitriy Monakhov <dmonakhov@openvz.org> wrote:
    >
    >> OpenVZ team has discovered error inside generic_file_direct_write()
    >> If generic_file_direct_IO() has fail (ENOSPC condition) it may have instantiated
    >> a few blocks outside i_size. And fsck will complain about wrong i_size
    >> (ext2, ext3 and reiserfs interpret i_size and biggest block difference as error),
    >> after fsck will fix error i_size will be increased to the biggest block,
    >> but this blocks contain gurbage from previous write attempt, this is not
    >> information leak, but its silence file data corruption.
    >> We need truncate any block beyond i_size after write have failed , do in simular
    >> generic_file_buffered_write() error path.
    >>
    >> Exampe:
    >> open("mnt2/FILE3", O_WRONLY|O_CREAT|O_DIRECT, 0666) = 3
    >> write(3, "aaaaaa"..., 4096) = -1 ENOSPC (No space left on device)
    >>
    >> stat mnt2/FILE3
    >> File: `mnt2/FILE3'
    >> Size: 0 Blocks: 4 IO Block: 4096 regular empty file
    >> >>>>>>>>>>>>>>>>>>>>>>^^^^^^^^^^ file size is less than biggest block idx
    >> Device: 700h/1792d Inode: 14 Links: 1
    >> Access: (0644/-rw-r--r--) Uid: ( 0/ root) Gid: ( 0/ root)
    >>
    >> fsck.ext2 -f -n mnt1/fs_img
    >> Pass 1: Checking inodes, blocks, and sizes
    >> Inode 14, i_size is 0, should be 2048. Fix? no
    >>
    >> Signed-off-by: Dmitriy Monakhov <dmonakhov@openvz.org>
    >> ----------
    >>
    >> diff --git a/mm/filemap.c b/mm/filemap.c
    >> index 7b84dc8..bf7cf6c 100644
    >> --- a/mm/filemap.c
    >> +++ b/mm/filemap.c
    >> @@ -2041,6 +2041,14 @@ generic_file_direct_write(struct kiocb *
    >> mark_inode_dirty(inode);
    >> }
    >> *ppos = end;
    >> + } else if (written < 0) {
    >> + loff_t isize = i_size_read(inode);
    >> + /*
    >> + * generic_file_direct_IO() may have instantiated a few blocks
    >> + * outside i_size. Trim these off again.
    >> + */
    >> + if (pos + count > isize)
    >> + vmtruncate(inode, isize);
    >> }
    >>
    >
    > XFS (at least) can call generic_file_direct_write() with i_mutex not held.
    > And vmtruncate() expects i_mutex to be held.
    >
    > I guess a suitable solution would be to push this problem back up to the
    > callers: let them decide whether to run vmtruncate() and if so, to ensure
    > that i_mutex is held.
    >
    > The existence of generic_file_aio_write_nolock() makes that rather messy
    > though.
    This means we may call generic_file_aio_write_nolock() without i_mutex, right?
    but call trace is :
    generic_file_aio_write_nolock()
    ->generic_file_buffered_write() /* i_mutex not held here */
    but according to filemaps locking rules: mm/filemap.c:77
    ..
    * ->i_mutex (generic_file_buffered_write)
    * ->mmap_sem (fault_in_pages_readable->do_page_fault)
    ..
    I'm confused a litle bit, where is the truth?

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