iomap_dio_rw: Prevent reading file data beyond iomap_dio->i_size

iomap_dio_rw: Prevent reading file data beyond iomap_dio->i_size

On a ppc64 machine executing overlayfs/019 with xfs as the lower and
upper filesystem causes the following call trace,

WARNING: CPU: 2 PID: 8034 at /root/repos/linux/fs/iomap.c:765 .iomap_dio_actor+0xcc/0x420
Modules linked in:
CPU: 2 PID: 8034 Comm: fsstress Tainted: G             L  4.11.0-rc5-next-20170405 #100
task: c000000631314880 task.stack: c0000003915d4000
NIP: c00000000035a72c LR: c00000000035a6f4 CTR: c00000000035a660
REGS: c0000003915d7570 TRAP: 0700   Tainted: G             L   (4.11.0-rc5-next-20170405)
MSR: 800000000282b032 <SF,VEC,VSX,EE,FP,ME,IR,DR,RI>
  CR: 24004284  XER: 00000000
CFAR: c0000000006f7190 SOFTE: 1
GPR00: c00000000035a6f4 c0000003915d77f0 c0000000015a3f00 000000007c22f600
GPR04: 000000000022d000 0000000000002600 c0000003b2d56360 c0000003915d7960
GPR08: c0000003915d7cd0 0000000000000002 0000000000002600 c000000000521cc0
GPR12: 0000000024004284 c00000000fd80a00 000000004b04ae64 ffffffffffffffff
GPR16: 000000001000ca70 0000000000000000 c0000003b2d56380 c00000000153d2b8
GPR20: 0000000000000010 c0000003bc87bac8 0000000000223000 000000000022f5ff
GPR24: c0000003b2d56360 000000000000000c 0000000000002600 000000000022d000
GPR28: 0000000000000000 c0000003915d7960 c0000003b2d56360 00000000000001ff
NIP [c00000000035a72c] .iomap_dio_actor+0xcc/0x420
LR [c00000000035a6f4] .iomap_dio_actor+0x94/0x420
Call Trace:
[c0000003915d77f0] [c00000000035a6f4] .iomap_dio_actor+0x94/0x420 (unreliable)
[c0000003915d78f0] [c00000000035b9f4] .iomap_apply+0xf4/0x1f0
[c0000003915d79d0] [c00000000035c320] .iomap_dio_rw+0x230/0x420
[c0000003915d7ae0] [c000000000512a14] .xfs_file_dio_aio_read+0x84/0x160
[c0000003915d7b80] [c000000000512d24] .xfs_file_read_iter+0x104/0x130
[c0000003915d7c10] [c0000000002d6234] .__vfs_read+0x114/0x1a0
[c0000003915d7cf0] [c0000000002d7a8c] .vfs_read+0xac/0x1a0
[c0000003915d7d90] [c0000000002d96b8] .SyS_read+0x58/0x100
[c0000003915d7e30] [c00000000000b8e0] system_call+0x38/0xfc
Instruction dump:
78630020 7f831b78 7ffc07b4 7c7ce039 40820360 a13d0018 2f890003 419e0288
2f890004 419e00a0 2f890001 419e02a8 <0fe00000> 3b80fffb 38210100 7f83e378

The above problem can also be recreated on a regular xfs filesystem
using the command,

$ fsstress -d /mnt -l 1000 -n 1000 -p 1000

The reason for the call trace is,
1. When 'reserving' blocks for delayed allocation , XFS reserves more
   blocks (i.e. past file's current EOF) than required. This is done
   because XFS assumes that userspace might write more data and hence
   'reserving' more blocks might lead to the file's new data being
   stored contiguously on disk.
2. The in-memory 'struct xfs_bmbt_irec' mapping the file's last extent would
   then cover the prealloc-ed EOF blocks in addition to the regular blocks.
3. When flushing the dirty blocks to disk, we only flush data till the
   file's EOF. But before writing out the dirty data, we allocate blocks
   on the disk for holding the file's new data. This allocation includes
   the blocks that are part of the 'prealloc EOF blocks'.
4. Later, when the last reference to the inode is being closed, XFS frees the
   unused 'prealloc EOF blocks' in xfs_inactive().

In step 3 above, When allocating space on disk for the delayed allocation
range, the space allocator might sometimes allocate less blocks than
required. If such an allocation ends right at the current EOF of the
file, We will not be able to clear the "delayed allocation" flag for the
'prealloc EOF blocks', since we won't have dirty buffer heads associated
with that range of the file.

In such a situation if a Direct I/O read operation is performed on file
range [X, Y] (where X < EOF and Y > EOF), we flush dirty data in the
range [X, Y] and invalidate page cache for that range (Refer to
iomap_dio_rw()). Later for performing the Direct I/O read, XFS obtains
the extent items (which are still cached in memory) for the file
range. When doing so we are not supposed to get an extent item with
IOMAP_DELALLOC flag set, since the previous "flush" operation should
have converted any delayed allocation data in the range [X, Y]. Hence we
end up hitting a WARN_ON_ONCE(1) statement in iomap_dio_actor().

This commit fixes the bug by preventing the read operation from going
beyond iomap_dio->i_size.

Reported-by: Santhosh G <santhog4@linux.vnet.ibm.com>
Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>