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| author | Kirill Smelkov <kirr@nexedi.com> | 2019-04-24 10:13:57 +0300 |
|---|---|---|
| committer | Greg Kroah-Hartman <gregkh@linuxfoundation.org> | 2019-05-25 19:23:40 +0300 |
| commit | fea685000caf196f676fc5a813599f63634c2c60 (patch) | |
| tree | 03d551cac514add6b98585bbc870f932fd39aee0 /include/uapi | |
| parent | f9eccf6ca1e01a0d2a76d4cd9ee5da9725c75561 (diff) | |
| download | linux-fea685000caf196f676fc5a813599f63634c2c60.tar.xz | |
fuse: Add FOPEN_STREAM to use stream_open()
commit bbd84f33652f852ce5992d65db4d020aba21f882 upstream.
Starting from commit 9c225f2655e3 ("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 10dce8af3422 ("fs: stream_open - opener for
stream-like files so that read and write can run simultaneously without
deadlock") for details and e.g. commit 581d21a2d02a ("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>
Diffstat (limited to 'include/uapi')
| -rw-r--r-- | include/uapi/linux/fuse.h | 2 |
1 files changed, 2 insertions, 0 deletions
diff --git a/include/uapi/linux/fuse.h b/include/uapi/linux/fuse.h index 92fa24c24c92..2170e58a2a97 100644 --- a/include/uapi/linux/fuse.h +++ b/include/uapi/linux/fuse.h @@ -219,10 +219,12 @@ struct fuse_file_lock { * FOPEN_DIRECT_IO: bypass page cache for this open file * FOPEN_KEEP_CACHE: don't invalidate the data cache on open * FOPEN_NONSEEKABLE: the file is not seekable + * FOPEN_STREAM: the file is stream-like (no file position at all) */ #define FOPEN_DIRECT_IO (1 << 0) #define FOPEN_KEEP_CACHE (1 << 1) #define FOPEN_NONSEEKABLE (1 << 2) +#define FOPEN_STREAM (1 << 4) /** * INIT request/reply flags |