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Kirill Smelkov 6d654f9d5d 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: Ben Hutchings <ben@decadent.org.uk>		History 2019-07-09 22:04:19 +01:00
..
control.c	fuse: fix control dir setup and teardown	2018-11-20 18:05:06 +00:00
cuse.c	fuse: flush requests on umount	2018-12-16 22:08:25 +00:00
dev.c	fuse: call pipe_buf_release() under pipe lock	2019-05-02 21:41:22 +01:00
dir.c	fuse: continue to send FUSE_RELEASEDIR when FUSE_OPEN returns ENOSYS	2019-02-11 17:54:24 +00:00
file.c	fuse: Add FOPEN_STREAM to use stream_open()	2019-07-09 22:04:19 +01:00
fuse_i.h	fuse: continue to send FUSE_RELEASEDIR when FUSE_OPEN returns ENOSYS	2019-02-11 17:54:24 +00:00
inode.c	fuse: Fix oops at process_init_reply()	2018-12-16 22:08:26 +00:00
Kconfig	fuse: Move CUSE Kconfig entry from fs/Kconfig into fs/fuse/Kconfig	2013-01-17 13:08:45 +01:00
Makefile	CUSE: implement CUSE - Character device in Userspace	2009-06-09 11:24:11 +02:00