In 2.6.23 kernel, commit a32ea1e1
("Fix read/truncate race") fixed a race in the generic code, and as a
side effect, now do_generic_file_read() can ask us to readpage() past
the i_size. This seems to be correctly handled by the block routines
(e.g. block_read_full_page() fills the page with zeroes in case if
somebody is trying to read past the last inode's block).

JFFS2 doesn't handle this; it assumes that it won't be asked to read
pages which don't exist -- and thus that there will be at least _one_
valid 'frag' on the page it's being asked to read. It will fill any
holes with the following memset:

  memset(buf, 0, min(end, frag->ofs + frag->size) - offset);

When the 'closest smaller match' returned by jffs2_lookup_node_frag() is
actually on a previous page and ends before 'offset', that results in:

  memset(buf, 0, <huge unsigned negative>);

Hopefully, in most cases the corruption is fatal, and quickly causing
random oopses, like this:

  root@10.0.0.4:~/ltp-fs-20090531# ./testcases/kernel/fs/ftest/ftest01
  Unable to handle kernel paging request for data at address 0x00000008
  Faulting instruction address: 0xc01cd980
  Oops: Kernel access of bad area, sig: 11 [#1

]
  [...]
  NIP [c01cd980] rb_insert_color+0x38/0x184
  LR [c0043978] enqueue_hrtimer+0x88/0xc4
  Call Trace:
  [c6c63b60] [c004f9a8] tick_sched_timer+0xa0/0xe4 (unreliable)
  [c6c63b80] [c0043978] enqueue_hrtimer+0x88/0xc4
  [c6c63b90] [c0043a48] __run_hrtimer+0x94/0xbc
  [c6c63bb0] [c0044628] hrtimer_interrupt+0x140/0x2b8
  [c6c63c10] [c000f8e8] timer_interrupt+0x13c/0x254
  [c6c63c30] [c001352c] ret_from_except+0x0/0x14
  --- Exception: 901 at memset+0x38/0x5c
      LR = jffs2_read_inode_range+0x144/0x17c
  [c6c63cf0] [00000000] (null) (unreliable)

This patch fixes the issue, plus fixes all LTP tests on NAND/UBI with
JFFS2 filesystem that were failing since 2.6.23 (seems like the bug
above also broke the truncation).

Reported-By: Anton Vorontsov <avorontsov@ru.mvista.com>
Tested-By: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>