f2fs: optimize iteration over sparse directories

f2fs: optimize iteration over sparse directories

[ Upstream commit 59237a21776f70ffb0420611c23e7158e1317037 ]

Wei Chen reports a kernel bug as blew:

INFO: task syz-executor.0:29056 blocked for more than 143 seconds.
      Not tainted 5.15.0-rc5 #1
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:syz-executor.0  state:D stack:14632 pid:29056 ppid:  6574 flags:0x00000004
Call Trace:
 __schedule+0x4a1/0x1720
 schedule+0x36/0xe0
 rwsem_down_write_slowpath+0x322/0x7a0
 fscrypt_ioctl_set_policy+0x11f/0x2a0
 __f2fs_ioctl+0x1a9f/0x5780
 f2fs_ioctl+0x89/0x3a0
 __x64_sys_ioctl+0xe8/0x140
 do_syscall_64+0x34/0xb0
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Eric did some investigation on this issue, quoted from reply of Eric:

"Well, the quality of this bug report has a lot to be desired (not on
upstream kernel, reproducer is full of totally irrelevant stuff, not
sent to the mailing list of the filesystem whose disk image is being
fuzzed, etc.).  But what is going on is that f2fs_empty_dir() doesn't
consider the case of a directory with an extremely large i_size on a
malicious disk image.

Specifically, the reproducer mounts an f2fs image with a directory
that has an i_size of 14814520042850357248, then calls
FS_IOC_SET_ENCRYPTION_POLICY on it.

That results in a call to f2fs_empty_dir() to check whether the
directory is empty.  f2fs_empty_dir() then iterates through all
3616826182336513 blocks the directory allegedly contains to check
whether any contain anything.  i_rwsem is held during this, so
anything else that tries to take it will hang."

In order to solve this issue, let's use f2fs_get_next_page_offset()
to speed up iteration by skipping holes for all below functions:
- f2fs_empty_dir
- f2fs_readdir
- find_in_level

The way why we can speed up iteration was described in
'commit 3cf4574705b4 ("f2fs: introduce get_next_page_offset to speed
up SEEK_DATA")'.

Meanwhile, in f2fs_empty_dir(), let's use f2fs_find_data_page()
instead f2fs_get_lock_data_page(), due to i_rwsem was held in
caller of f2fs_empty_dir(), there shouldn't be any races, so it's
fine to not lock dentry page during lookuping dirents in the page.

Link: https://lore.kernel.org/lkml/536944df-a0ae-1dd8-148f-510b476e1347@kernel.org/T/
Reported-by: Wei Chen <harperchen1110@gmail.com>
Cc: Eric Biggers <ebiggers@google.com>
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Stable-dep-of: d2d9bb3b6d2f ("f2fs: get out of a repeat loop when getting a locked data page")
Signed-off-by: Sasha Levin <sashal@kernel.org>