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/gvfsfused... https://gitlab.gnome.org/GNOME/gvfs/blob/1.40.0-6-gcbc54396/client/gvfsfused... https://gitlab.gnome.org/GNOME/gvfs/blob/1.40.0-6-gcbc54396/client/gvfsfused...

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