Re: [PATCH] iomap_dio_rw: Prevent reading file data beyond iomap_dio->i_size

[Chandan Rajendra <chandan@linux.vnet.ibm.com>]

On Friday, April 07, 2017 09:25:28 AM Darrick J. Wong wrote:
> On Fri, Apr 07, 2017 at 03:02:43PM +0530, Chandan Rajendra wrote:
> > 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>
> 
> Looks ok to me, but I'll wait for the test before making further decisions.
> 

The command 'fsstress -d /mnt -l 1000 -n 1000 -p 1000' takes a long time for
execution. It does not complete even after 30 mins of run time.

Hence I tried the following command line sequence for the new xfstests' test,

# mkfs.xfs -f -d size=256m /dev/loop0
# mount /dev/loop0 /mnt/
# xfs_io -f -c 'pwrite 0 64k' /mnt/test-file
# dd if=/dev/zero of=/mnt/filler bs=4k
# for i in $(seq 1 2 32); do offset=$(($i * 4096)); xfs_io -f -c "fpunch $offset 4k" -c sync /mnt/filler; done
# xfs_io -f -c 'pwrite 64k 4k' /mnt/test-file # Prealloc blocks are reserved beyond file offset (68k - 1).
# xfs_io -f -d -c 'pread 64k 64k' /mnt/test-file

The last command which performs a direct read operation beyond file's EOF
should have triggered the call trace. But I noticed (via printk statements)
that XFS was able to successfully allocate (4k + prealloc blocks) worth of
contiguous space for /mnt/test-file.

I am still trying to figure out how XFS was able to allocate the EOF prealloc
blocks when it shouldn't have had any contiguous space larger than 4k bytes.

-- 
chandan