Starting from 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 previous patch "fs: stream_open - opener for
stream-like files so that read and write can run simultaneously without
deadlock" for details and e.g. 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 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+ (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 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: Al Viro <v...@zeniv.linux.org.uk>
Cc: Linus Torvalds <torva...@linux-foundation.org>
Cc: Michael Kerrisk <mtk.manpa...@gmail.com>
Cc: Yongzhi Pan <panyong...@gmail.com>
Cc: Jonathan Corbet <cor...@lwn.net>
Cc: David Vrabel <david.vra...@citrix.com>
Cc: Juergen Gross <jgr...@suse.com>
Cc: Tejun Heo <t...@kernel.org>
Cc: Kirill Tkhai <ktk...@virtuozzo.com>
Cc: Arnd Bergmann <a...@arndb.de>
Cc: Christoph Hellwig <h...@lst.de>
Cc: Greg Kroah-Hartman <gre...@linuxfoundation.org>
Cc: Julia Lawall <julia.law...@lip6.fr>
Cc: Nikolaus Rath <nikol...@rath.org>
Cc: Han-Wen Nienhuys <han...@google.com>
Signed-off-by: Kirill Smelkov <k...@nexedi.com>
---
fs/fuse/file.c | 4 +++-
include/uapi/linux/fuse.h | 2 ++
2 files changed, 5 insertions(+), 1 deletion(-)
diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index ffaffe18352a..7ea4099cde16 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -181,7 +181,9 @@ void fuse_finish_open(struct inode *inode, struct file
*file)
file->f_op = &fuse_direct_io_file_operations;
if (!(ff->open_flags & FOPEN_KEEP_CACHE))
invalidate_inode_pages2(inode->i_mapping);
- if (ff->open_flags & FOPEN_NONSEEKABLE)
+ if (ff->open_flags & FOPEN_STREAM)
+ stream_open(inode, file);
+ else if (ff->open_flags & FOPEN_NONSEEKABLE)
nonseekable_open(inode, file);
if (fc->atomic_o_trunc && (file->f_flags & O_TRUNC)) {
struct fuse_inode *fi = get_fuse_inode(inode);
diff --git a/include/uapi/linux/fuse.h b/include/uapi/linux/fuse.h
index b4967d48bfda..93ac72a1e4ff 100644
--- a/include/uapi/linux/fuse.h
+++ b/include/uapi/linux/fuse.h
@@ -226,11 +226,13 @@ struct fuse_file_lock {
* FOPEN_KEEP_CACHE: don't invalidate the data cache on open
* FOPEN_NONSEEKABLE: the file is not seekable
* FOPEN_CACHE_DIR: allow caching this directory
+ * FOPEN_STREAM: the file is stream-like
*/
#define FOPEN_DIRECT_IO (1 << 0)
#define FOPEN_KEEP_CACHE (1 << 1)
#define FOPEN_NONSEEKABLE (1 << 2)
#define FOPEN_CACHE_DIR (1 << 3)
+#define FOPEN_STREAM (1 << 4)
/**
* INIT request/reply flags
--
2.21.0.392.gf8f6787159
