fs: stream_open - opener for stream-like files so that read and write can run simultaneously without deadlock

2019-06-11T10:24:13Z

commit 10dce8af34226d90fa56746a934f8da5dcdba3df upstream. Commit 9c225f2655e3 ("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 581d21a2d02a ("xenbus: fix deadlock on writes to /proc/xen/xenbus") which fixes such regression for particular case of /proc/xen/xenbus. 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 9c225f2655e3 - is doing so irregardless of whether a file is seekable or not. See https://lore.kernel.org/lkml/53022DB1.4070805@gmail.com/ https://lwn.net/Articles/180387 https://lwn.net/Articles/180396 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 examples: 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 a7c1b990f715 ("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: See https://github.com/libfuse/osspd https://lwn.net/Articles/308445 https://github.com/libfuse/osspd/blob/14a9cff0/osspd.c#L1406 https://github.com/libfuse/osspd/blob/14a9cff0/osspd.c#L1438-L1477 https://github.com/libfuse/osspd/blob/14a9cff0/osspd.c#L1479-L1510 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: https://github.com/libfuse/libfuse/blob/fuse-3.4.2-3-ga1bff7d/example/poll.c#L124-L131 https://github.com/libfuse/libfuse/blob/fuse-3.4.2-3-ga1bff7d/example/poll.c#L146-L163 https://github.com/libfuse/libfuse/blob/fuse-3.4.2-3-ga1bff7d/example/poll.c#L209-L216 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: https://lab.nexedi.com/kirr/wendelin.core/blob/f13aa600/wcfs/wcfs.go#L88-169 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 https://codesearch.debian.net/search?q=-%3Enonseekable+%3D https://gitlab.gnome.org/GNOME/gvfs/blob/1.40.0-6-gcbc54396/client/gvfsfusedaemon.c#L1080 https://gitlab.gnome.org/GNOME/gvfs/blob/1.40.0-6-gcbc54396/client/gvfsfusedaemon.c#L1247-1346 https://gitlab.gnome.org/GNOME/gvfs/blob/1.40.0-6-gcbc54396/client/gvfsfusedaemon.c#L1399-1481 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 9c225f2655 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. drivers/input/mousedev.c) Cc: Michael Kerrisk Cc: Yongzhi Pan Cc: Jonathan Corbet Cc: David Vrabel Cc: Juergen Gross Cc: Miklos Szeredi Cc: Tejun Heo Cc: Kirill Tkhai Cc: Arnd Bergmann Cc: Christoph Hellwig Cc: Greg Kroah-Hartman Cc: Julia Lawall Cc: Nikolaus Rath Cc: Han-Wen Nienhuys [ backport to 4.4: actually fixed deadlock on /proc/xen/xenbus as 581d21a2d02a was not backported to 4.4 ] Signed-off-by: Kirill Smelkov Signed-off-by: Linus Torvalds Signed-off-by: Greg Kroah-Hartman

fs: add the fsnotify call to vfs_iter_write

2019-02-06T18:43:06Z

A bug has been discovered when redirecting splice output to regular files on EXT4 and tmpfs. Other filesystems might be affected. This commit fixes the issue for stable series kernel, using one of the change introduced during the rewrite and refactoring of vfs_iter_write in 4.13, specifically in the commit abbb65899aec ("fs: implement vfs_iter_write using do_iter_write"). This issue affects v4.4 and v4.9 stable series of kernels. Without this fix for v4.4 and v4.9 stable, the following upstream commits (and their dependencies would need to be backported): * commit abbb65899aec ("fs: implement vfs_iter_write using do_iter_write") * commit 18e9710ee59c ("fs: implement vfs_iter_read using do_iter_read") * commit edab5fe38c2c ("fs: move more code into do_iter_read/do_iter_write") * commit 19c735868dd0 ("fs: remove __do_readv_writev") * commit 26c87fb7d10d ("fs: remove do_compat_readv_writev") * commit 251b42a1dc64 ("fs: remove do_readv_writev") as well as the following dependencies: * commit bb7462b6fd64 ("vfs: use helpers for calling f_op->{read,write}_iter()") * commit 0f78d06ac1e9 ("vfs: pass type instead of fn to do_{loop,iter}_readv_writev()") * commit 7687a7a4435f ("vfs: extract common parts of {compat_,}do_readv_writev()") In order to reduce the changes, this commit uses only the part of commit abbb65899aec ("fs: implement vfs_iter_write using do_iter_write") that fixes the issue. This issue and the reproducer can be found on https://bugzilla.kernel.org/show_bug.cgi?id=85381 Reported-by: Richard Li Reported-by: Chad Miller Reviewed-by: Stefan Nuernberger Reviewed-by: Frank Becker Signed-off-by: Jimmy Durand Wesolowski

vfs: Return -ENXIO for negative SEEK_HOLE / SEEK_DATA offsets

2017-10-05T07:41:45Z

commit fc46820b27a2d9a46f7e90c9ceb4a64a1bc5fab8 upstream. In generic_file_llseek_size, return -ENXIO for negative offsets as well as offsets beyond EOF. This affects filesystems which don't implement SEEK_HOLE / SEEK_DATA internally, possibly because they don't support holes. Fixes xfstest generic/448. Signed-off-by: Andreas Gruenbacher Signed-off-by: Linus Torvalds Signed-off-by: Greg Kroah-Hartman

new_sync_write(): discard ->ki_pos unless the return value is positive

2015-04-12T02:29:46Z

That allows ->write_iter() instances much more convenient life wrt iocb->ki_pos (and fixes several filesystems with borderline POSIX violations when zero-length write succeeds and changes the current position). Signed-off-by: Al Viro

->aio_read and ->aio_write removed

2015-04-12T02:29:43Z

no remaining users Signed-off-by: Al Viro

kill do_sync_read/do_sync_write

2015-04-12T02:29:42Z

all remaining instances of aio_{read,write} (all 4 of them) have explicit ->read and ->write resp.; do_sync_read/do_sync_write is never called by __vfs_read/__vfs_write anymore and no other users had been left. Signed-off-by: Al Viro

make new_sync_{read,write}() static

2015-04-12T02:29:40Z

All places outside of core VFS that checked ->read and ->write for being NULL or called the methods directly are gone now, so NULL {read,write} with non-NULL {read,write}_iter will do the right thing in all cases. Signed-off-by: Al Viro

export __vfs_read()

2015-04-12T02:29:38Z

Signed-off-by: Al Viro

new helper: __vfs_write()

2015-04-12T02:29:37Z

Signed-off-by: Al Viro

switch {compat_,}do_readv_writev() to {compat_,}import_iovec()

2015-04-12T02:27:12Z

Signed-off-by: Al Viro

user/sven/linux.git/fs/read_write.c, branch v4.4.197

fs: stream_open - opener for stream-like files so that read and write can run simultaneously without deadlock

fs: add the fsnotify call to vfs_iter_write

vfs: Return -ENXIO for negative SEEK_HOLE / SEEK_DATA offsets

new_sync_write(): discard ->ki_pos unless the return value is positive

->aio_read and ->aio_write removed

kill do_sync_read/do_sync_write

make new_sync_{read,write}() static

export __vfs_read()

new helper: __vfs_write()

switch {compat_,}do_readv_writev() to {compat_,}import_iovec()