fat: work around race with userspace's read via blockdev while mounting
authorOGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Mon, 23 Sep 2019 22:32:53 +0000 (15:32 -0700)
committerLinus Torvalds <torvalds@linux-foundation.org>
Tue, 24 Sep 2019 22:54:06 +0000 (15:54 -0700)
commit07bfa4415ab607e459b69bd86aa7e7602ce10b4f
tree62a7bc60deca51a1a795c8a22293e658d88efc07
parent619e17cf75dd58905aa67ccd494a6ba5f19d6cc6
fat: work around race with userspace's read via blockdev while mounting

If userspace reads the buffer via blockdev while mounting,
sb_getblk()+modify can race with buffer read via blockdev.

For example,

            FS                               userspace
    bh = sb_getblk()
    modify bh->b_data
                                  read
    ll_rw_block(bh)
      fill bh->b_data by on-disk data
      /* lost modified data by FS */
      set_buffer_uptodate(bh)
    set_buffer_uptodate(bh)

Userspace should not use the blockdev while mounting though, the udev
seems to be already doing this.  Although I think the udev should try to
avoid this, workaround the race by small overhead.

Link: http://lkml.kernel.org/r/87pnk7l3sw.fsf_-_@mail.parknet.co.jp
Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Reported-by: Jan Stancek <jstancek@redhat.com>
Tested-by: Jan Stancek <jstancek@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
fs/fat/dir.c
fs/fat/fatent.c