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navytuxgregkh
navytux
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gregkh
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fuse: Add FOPEN_STREAM to use stream_open()
commit bbd84f3 upstream. Starting from commit 9c225f2 ("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 10dce8a ("fs: stream_open - opener for stream-like files so that read and write can run simultaneously without deadlock") for details and e.g. commit 581d21a ("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>
1 parent f9eccf6 commit fea6850

2 files changed

Lines changed: 5 additions & 1 deletion

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‎fs/fuse/file.c‎

Lines changed: 3 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -179,7 +179,9 @@ void fuse_finish_open(struct inode *inode, struct file *file)
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file->f_op = &fuse_direct_io_file_operations;
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if (!(ff->open_flags & FOPEN_KEEP_CACHE))
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invalidate_inode_pages2(inode->i_mapping);
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if (ff->open_flags & FOPEN_NONSEEKABLE)
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if (ff->open_flags & FOPEN_STREAM)
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stream_open(inode, file);
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else if (ff->open_flags & FOPEN_NONSEEKABLE)
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nonseekable_open(inode, file);
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if (fc->atomic_o_trunc && (file->f_flags & O_TRUNC)) {
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struct fuse_inode *fi = get_fuse_inode(inode);

‎include/uapi/linux/fuse.h‎

Lines changed: 2 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -219,10 +219,12 @@ struct fuse_file_lock {
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* FOPEN_DIRECT_IO: bypass page cache for this open file
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* FOPEN_KEEP_CACHE: don't invalidate the data cache on open
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* FOPEN_NONSEEKABLE: the file is not seekable
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* FOPEN_STREAM: the file is stream-like (no file position at all)
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*/
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#define FOPEN_DIRECT_IO (1 << 0)
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#define FOPEN_KEEP_CACHE (1 << 1)
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#define FOPEN_NONSEEKABLE (1 << 2)
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#define FOPEN_STREAM (1 << 4)
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/**
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* INIT request/reply flags

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