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Subject[RESEND, PATCH 3/3] fuse: Add FOPEN_STREAM and use stream_open() if filesystem returned that from open handler
Date
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 10dce8af3422 ("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 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 (10dce8af3422) 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 <viro@zeniv.linux.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Yongzhi Pan <panyongzhi@gmail.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: David Vrabel <david.vrabel@citrix.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Kirill Tkhai <ktkhai@virtuozzo.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Julia Lawall <Julia.Lawall@lip6.fr>
Cc: Nikolaus Rath <Nikolaus@rath.org>
Cc: Han-Wen Nienhuys <hanwen@google.com>
Cc: stable@vger.kernel.org # v3.14+
Signed-off-by: Kirill Smelkov <kirr@nexedi.com>
---

( resending the same patch with updated description to reference stream_open
that was landed to master as 10dce8af3422; also added cc stable )

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 06096b60f1df..44de96cb7871 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -178,7 +178,9 @@ void fuse_finish_open(struct inode *inode, struct file *file)

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 2ac598614a8f..26abf0a571c7 100644
--- a/include/uapi/linux/fuse.h
+++ b/include/uapi/linux/fuse.h
@@ -229,11 +229,13 @@ 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
--
2.21.0.765.geec228f530
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