• Kirill Smelkov's avatar
    fs: stream_open - opener for stream-like files so that read and write can run... · b673f99c
    Kirill Smelkov authored
    fs: stream_open - opener for stream-like files so that read and write can run simultaneously without deadlock
    commit 10dce8af34226d90fa56746a934f8da5dcdba3df upstream.
    Commit 9c225f26 ("vfs: atomic f_pos accesses as per POSIX") added
    locking for file.f_pos access and in particular made concurrent read and
    write not possible - now both those functions take f_pos lock for the
    whole run, and so if e.g. a read is blocked waiting for data, write will
    deadlock waiting for that read to complete.
    This caused regression for stream-like files where previously read and
    write could run simultaneously, but after that patch could not do so
    anymore. See e.g. commit 581d21a2 ("xenbus: fix deadlock on writes
    to /proc/xen/xenbus") which fixes such regression for particular case of
    The patch that added f_pos lock in 2014 did so to guarantee POSIX thread
    safety for read/write/lseek and added the locking to file descriptors of
    all regular files. In 2014 that thread-safety problem was not new as it
    was already discussed earlier in 2006.
    However even though 2006'th version of Linus's patch was adding f_pos
    locking "only for files that are marked seekable with FMODE_LSEEK (thus
    avoiding the stream-like objects like pipes and sockets)", the 2014
    version - the one that actually made it into the tree as 9c225f26 -
    is doing so irregardless of whether a file is seekable or not.
    for historic context.
    The reason that it did so is, probably, that there are many files that
    are marked non-seekable, but e.g. their read implementation actually
    depends on knowing current position to correctly handle the read. Some
    	kernel/power/user.c		snapshot_read
    	fs/debugfs/file.c		u32_array_read
    	fs/fuse/control.c		fuse_conn_waiting_read + ...
    	drivers/hwmon/asus_atk0110.c	atk_debugfs_ggrp_read
    	arch/s390/hypfs/inode.c		hypfs_read_iter
    Despite that, many nonseekable_open users implement read and write with
    pure stream semantics - they don't depend on passed ppos at all. And for
    those cases where read could wait for something inside, it creates a
    situation similar to xenbus - the write could be never made to go until
    read is done, and read is waiting for some, potentially external, event,
    for potentially unbounded time -> deadlock.
    Besides xenbus, there are 14 such places in the kernel that I've found
    with semantic patch (see below):
    	drivers/xen/evtchn.c:667:8-24: ERROR: evtchn_fops: .read() can deadlock .write()
    	drivers/isdn/capi/capi.c:963:8-24: ERROR: capi_fops: .read() can deadlock .write()
    	drivers/input/evdev.c:527:1-17: ERROR: evdev_fops: .read() can deadlock .write()
    	drivers/char/pcmcia/cm4000_cs.c:1685:7-23: ERROR: cm4000_fops: .read() can deadlock .write()
    	net/rfkill/core.c:1146:8-24: ERROR: rfkill_fops: .read() can deadlock .write()
    	drivers/s390/char/fs3270.c:488:1-17: ERROR: fs3270_fops: .read() can deadlock .write()
    	drivers/usb/misc/ldusb.c:310:1-17: ERROR: ld_usb_fops: .read() can deadlock .write()
    	drivers/hid/uhid.c:635:1-17: ERROR: uhid_fops: .read() can deadlock .write()
    	net/batman-adv/icmp_socket.c:80:1-17: ERROR: batadv_fops: .read() can deadlock .write()
    	drivers/media/rc/lirc_dev.c:198:1-17: ERROR: lirc_fops: .read() can deadlock .write()
    	drivers/leds/uleds.c:77:1-17: ERROR: uleds_fops: .read() can deadlock .write()
    	drivers/input/misc/uinput.c:400:1-17: ERROR: uinput_fops: .read() can deadlock .write()
    	drivers/infiniband/core/user_mad.c:985:7-23: ERROR: umad_fops: .read() can deadlock .write()
    	drivers/gnss/core.c:45:1-17: ERROR: gnss_fops: .read() can deadlock .write()
    In addition to the cases above another regression caused by f_pos
    locking is that now FUSE filesystems that implement open with
    FOPEN_NONSEEKABLE flag, can no longer implement bidirectional
    stream-like files - for the same reason as above e.g. read can deadlock
    write locking on file.f_pos in the kernel.
    FUSE's FOPEN_NONSEEKABLE was added in 2008 in a7c1b990 ("fuse:
    implement nonseekable open") to support OSSPD. OSSPD implements /dev/dsp
    in userspace with FOPEN_NONSEEKABLE flag, with corresponding read and
    write routines not depending on current position at all, and with both
    read and write being potentially blocking operations:
    Corresponding libfuse example/test also describes FOPEN_NONSEEKABLE as
    "somewhat pipe-like files ..." with read handler not using offset.
    However that test implements only read without write and cannot exercise
    the deadlock scenario:
    I've actually hit the read vs write deadlock for real while implementing
    my FUSE filesystem where there is /head/watch file, for which open
    creates separate bidirectional socket-like stream in between filesystem
    and its user with both read and write being later performed
    simultaneously. And there it is semantically not easy to split the
    stream into two separate read-only and write-only channels:
    Let's fix this regression. The plan is:
    1. We can't change nonseekable_open to include &~FMODE_ATOMIC_POS -
       doing so would break many in-kernel nonseekable_open users which
       actually use ppos in read/write handlers.
    2. Add stream_open() to kernel to open stream-like non-seekable file
       descriptors. Read and write on such file descriptors would never use
       nor change ppos. And with that property on stream-like files read and
       write will be running without taking f_pos lock - i.e. read and write
       could be running simultaneously.
    3. With semantic patch search and convert to stream_open all in-kernel
       nonseekable_open users for which read and write actually do not
       depend on ppos and where there is no other methods in file_operations
       which assume @offset access.
    4. Add FOPEN_STREAM to fs/fuse/ and open in-kernel file-descriptors via
       steam_open if that bit is present in filesystem open reply.
       It was tempting to change fs/fuse/ open handler to use stream_open
       instead of nonseekable_open on just FOPEN_NONSEEKABLE flags, but
       grepping through Debian codesearch shows users of FOPEN_NONSEEKABLE,
       and in particular GVFS which actually uses offset in its read and
       write handlers
       so if we would do such a change it will break a real user.
    5. Add stream_open and FOPEN_STREAM handling to stable kernels starting
       from v3.14+ (the kernel where 9c225f26 first appeared).
       This will allow to patch OSSPD and other FUSE filesystems that
       provide stream-like files to return FOPEN_STREAM | FOPEN_NONSEEKABLE
       in their open handler and this way avoid the deadlock on all kernel
       versions. This should work because fs/fuse/ ignores unknown open
       flags returned from a filesystem and so passing FOPEN_STREAM to a
       kernel that is not aware of this flag cannot hurt. In turn the kernel
       that is not aware of FOPEN_STREAM will be < v3.14 where just
       FOPEN_NONSEEKABLE is sufficient to implement streams without read vs
       write deadlock.
    This patch adds stream_open, converts /proc/xen/xenbus to it and adds
    semantic patch to automatically locate in-kernel places that are either
    required to be converted due to read vs write deadlock, or that are just
    safe to be converted because read and write do not use ppos and there
    are no other funky methods in file_operations.
    Regarding semantic patch I've verified each generated change manually -
    that it is correct to convert - and each other nonseekable_open instance
    left - that it is either not correct to convert there, or that it is not
    converted due to current stream_open.cocci limitations.
    The script also does not convert files that should be valid to convert,
    but that currently have .llseek = noop_llseek or generic_file_llseek for
    unknown reason despite file being opened with nonseekable_open (e.g.
    Cc: Michael Kerrisk <mtk.manpages@gmail.com>
    Cc: Yongzhi Pan <panyongzhi@gmail.com>
    Cc: Jonathan Corbet <corbet@lwn.net>
    Cc: David Vrabel <david.vrabel@citrix.com>
    Cc: Juergen Gross <jgross@suse.com>
    Cc: Miklos Szeredi <miklos@szeredi.hu>
    Cc: Tejun Heo <tj@kernel.org>
    Cc: Kirill Tkhai <ktkhai@virtuozzo.com>
    Cc: Arnd Bergmann <arnd@arndb.de>
    Cc: Christoph Hellwig <hch@lst.de>
    Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    Cc: Julia Lawall <Julia.Lawall@lip6.fr>
    Cc: Nikolaus Rath <Nikolaus@rath.org>
    Cc: Han-Wen Nienhuys <hanwen@google.com>
    Signed-off-by: default avatarKirill Smelkov <kirr@nexedi.com>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>