| From bbd84f33652f852ce5992d65db4d020aba21f882 Mon Sep 17 00:00:00 2001 |
| From: Kirill Smelkov <kirr@nexedi.com> |
| Date: Wed, 24 Apr 2019 07:13:57 +0000 |
| Subject: fuse: Add FOPEN_STREAM to use stream_open() |
| |
| 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 |
| @@ -179,7 +179,9 @@ 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 |
| @@ -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 |