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    Subject[PATCH 4.14 35/35] fuse: Add FOPEN_STREAM to use stream_open()
    Date
    From: Kirill Smelkov <kirr@nexedi.com>

    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>


    ---
    fs/fuse/file.c | 4 +++-
    include/uapi/linux/fuse.h | 2 ++
    2 files changed, 5 insertions(+), 1 deletion(-)

    --- a/fs/fuse/file.c
    +++ b/fs/fuse/file.c
    @@ -178,7 +178,9 @@ void fuse_finish_open(struct inode *inod
    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);
    --- a/include/uapi/linux/fuse.h
    +++ b/include/uapi/linux/fuse.h
    @@ -216,10 +216,12 @@ 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

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