Kirill Smelkov 585724f83b fuse: Add FOPEN_STREAM to use stream_open() ... commit bbd84f3365 upstream. Starting from commit 9c225f2655 ("vfs: atomic f_pos accesses as per POSIX") files opened even via nonseekable_open gate read and write via lock and do not allow them to be run simultaneously. This can create read vs write deadlock if a filesystem is trying to implement a socket-like file which is intended to be simultaneously used for both read and write from filesystem client. See commit 10dce8af34 ("fs: stream_open - opener for stream-like files so that read and write can run simultaneously without deadlock") for details and e.g. commit 581d21a2d0 ("xenbus: fix deadlock on writes to /proc/xen/xenbus") for a similar deadlock example on /proc/xen/xenbus. To avoid such deadlock it was tempting to adjust fuse_finish_open 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. Add another flag (FOPEN_STREAM) for filesystem servers to indicate that the opened handler is having stream-like semantics; does not use file position and thus the kernel is free to issue simultaneous read and write request on opened file handle. This patch together with stream_open() should be added to stable kernels starting from v3.14+. This will allow to patch OSSPD and other FUSE filesystems that provide stream-like files to return FOPEN_STREAM | FOPEN_NONSEEKABLE in open handler and this way avoid the deadlock on all kernel versions. This should work because fuse_finish_open 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. Cc: stable@vger.kernel.org # v3.14+ Signed-off-by: Kirill Smelkov <kirr@nexedi.com> Signed-off-by: Miklos Szeredi <mszeredi@redhat.com> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>		2019-06-11 12:21:51 +02:00
..
asm-generic	License cleanup: add SPDX license identifier to uapi header files with no license	2017-11-02 11:19:54 +01:00
drm	drm/i915: Fix I915_EXEC_RING_MASK	2019-06-11 12:21:50 +02:00
linux	fuse: Add FOPEN_STREAM to use stream_open()	2019-06-11 12:21:51 +02:00
misc	License cleanup: add SPDX license identifier to uapi header files with a license	2017-11-02 11:20:11 +01:00
mtd	License cleanup: add SPDX license identifier to uapi header files with a license	2017-11-02 11:20:11 +01:00
rdma	RDMA/vmw_pvrdma: Return the correct opcode when creating WR	2019-05-16 19:42:29 +02:00
scsi	License cleanup: add SPDX license identifier to uapi header files with a license	2017-11-02 11:20:11 +01:00
sound	ASoC: topology: Add missing clock gating parameter when parsing hw_configs	2018-08-03 07:50:42 +02:00
video	License cleanup: add SPDX license identifier to uapi header files with a license	2017-11-02 11:20:11 +01:00
xen	License cleanup: add SPDX license identifier to uapi header files with a license	2017-11-02 11:20:11 +01:00