Hello Keno, On 12/10/2016 08:20 AM, Keno Fischer wrote: > This is my writeup of a basic description of /dev/fuse after playing with > it for a few hours today. It is of course woefully incomplete, and since > I neither have a use case nor am working on this code, I will not be > in a position to expand it in the near future. However, I'm hoping this > could still serve as a handy reference for others looking at this interface. Thanks. This is, as you say, incomplete. But it's a great start that could be grown over time. Thank you! I've placed this into a branch here: http://git.kernel.org/cgit/docs/man-pages/man-pages.git/log/?h=draft_fuse I've made a lot of trivial (I believe obviously correct) edits. Then I've made a few edits that I believe are correct, but that I'd like you to check. Could you please take a look at the following commits to see if they are okay: pick 59c4cca fuse.4: fuse_entry_out: rework discussion of uniqueness of nodeid + generation pick ecc57e6 fuse.4: Repair wording in EINVAL error text pick 00a5527 fuse.4: Small rewording in FUSE_INIT pick 57b63af fuse.4: Repair ENODEV description pick 8c160d3 fuse.4: Add list of FOPEN_* flags pick bd49f64 fuse.4: Add list of undocumented messages In addition, I've dropped a few FIXMEs into the page. Could you take a look at those? For convenience, I've pasted the current page source below. Cheers, Michael .\" Copyright (c) 2016 Julia Computing Inc, Keno Fischer .\" Description based on include/uapi/fuse.h and code in fs/fuse .\" .\" %%%LICENSE_START(VERBATIM) .\" Permission is granted to make and distribute verbatim copies of this .\" manual provided the copyright notice and this permission notice are .\" preserved on all copies. .\" .\" Permission is granted to copy and distribute modified versions of this .\" manual under the conditions for verbatim copying, provided that the .\" entire resulting derived work is distributed under the terms of a .\" permission notice identical to this one. .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date. The author(s) assume no .\" responsibility for errors or omissions, or for damages resulting from .\" the use of the information contained herein. The author(s) may not .\" have taken the same level of care in the production of this manual, .\" which is licensed free of charge, as they might when working .\" professionally. .\" .\" Formatted or processed versions of this manual, if unaccompanied by .\" the source, must acknowledge the copyright and authors of this work. .\" %%%LICENSE_END .\" .TH FUSE 4 2016-12-10 "Linux" "Linux Programmer's Manual" .SH NAME /dev/fuse \- Filesystem in Userspace (FUSE) device .SH SYNOPSIS .nf .B #include <linux/fuse.h> .nf .SH DESCRIPTION This device is the primary interface between the FUSE filesystem driver and a user-space process wishing to provide the filesystem (referred to in the rest of this manual page as the .IR "filesystem daemon" ). This manual page is intended for those interested in understanding the kernel interface itself. Those implementing a FUSE filesystem may wish to make use of a user-space library such as .I libfuse that abstracts away the low-level interface. At its core, FUSE is a simple client-server protocol, in which the Linux kernel is the client and the daemon is the server. After obtaining a file descriptor for this device, the daemon may .BR read (2) requests from that file descriptor and is expected to .BR write (2) back its replies. It is important to note that a file descriptor is associated with a unique FUSE filesystem. In particular, opening a second copy of this device, will not allow access to resources created through the first file descriptor (and vice versa). .\" .SS The basic protocol Every message that is read by the daemon begins with a header described by the following structure: .in +4n .nf struct fuse_in_header { uint32_t len; /* Total length of the data, including this header */ uint32_t opcode; /* The kind of operation (see below) */ uint64_t unique; /* A unique identifier for this request */ uint64_t nodeid; /* ID of the filesystem object being operated on */ uint32_t uid; /* UID of the requesting process */ uint32_t gid; /* GID of the requesting process */ uint32_t pid; /* PID of the requesting process */ uint32_t padding; }; .fi .in The header is followed by a variable-length data portion (which may be empty) specific to the requested operation (the requested operation is indicated by .IR opcode ). The daemon should then process the request and if applicable send a reply (almost all operations require a reply; if they do not, this is documented below), by performing a .BR write (2) to the file descriptor. All replies must start with the following header: .in +4n .nf struct fuse_out_header { uint32_t len; /* Total length of data written to the file descriptor */ int32_t error; /* Any error that occurred (0 if none) */ uint64_t unique; /* The value from the corresponding request */ }; .fi .in This header is also followed by (potentially empty) variable-sized data depending on the executed request. However, if the reply is an error reply (i.e., .I error is set), then no further payload data should be sent, independent of the request. .\" .SS Exchanged messages This section should contain documentation for each of the messages in the protocol. This manual page is currently incomplete, so not all messages are documented. For each message, first the struct sent by the kernel is given, followed by a description of the semantics of the message. .TP .BR FUSE_INIT " ( 25 )" .in +4n .nf struct fuse_init_in { uint32_t major; uint32_t minor; uint32_t max_readahead; /* Since protocol v7.6 */ uint32_t flags; /* Since protocol v7.6 */ }; .fi .in This is the first request sent by the kernel to the daemon. It is used to negotiate the protocol version and other filesystem parameters. Note that the protocol version may affect the layout of any structure in the protocol (including this structure). The daemon must thus remember the negotiated version and flags for each session. As of the writing of this man page, the highest supported kernel protocol version is .IR 7.26 . Users should be aware that the descriptions in this manual page may be incomplete or incorrect for older or more recent protocol versions. The reply for this request has the following format: .in +4n .nf struct fuse_init_out { uint32_t major; uint32_t minor; uint32_t max_readahead; /* Since v7.6 */ uint32_t flags; /* Since v7.6; some flags bits were introduced later */ uint16_t max_background; /* Since v7.13 */ uint16_t congestion_threshold; /* Since v7.13 */ uint32_t max_write; /* Since v7.5 */ uint32_t time_gran; /* Since v7.6 */ uint32_t unused[9]; }; .fi .in If the major version supported by the kernel is larger than that supported by the daemon, the reply shall consist of only .I uint32_t major (following the usual header), indicating the largest major version supported by the daemon. The kernel will then issue a new .B FUSE_INIT request conforming to the older version. In the reverse case, the daemon should quietly fall back to the kernel's major version. The negotiated minor version is considered to be the minimum of the minor versions provided by the daemon and the kernel and both parties should use the protocol corresponding to said minor version. .TP .BR FUSE_GETATTR " ( 3 )" .\" FIXME It looks like this is for implementing a stat(2) type of .\" operation. There needs to be a sentence here describing what .\" this option does. .in +4n .nf struct fuse_getattr_in { uint32_t getattr_flags; uint32_t dummy; uint64_t fh; /* Set only if (getattr_flags & FUSE_GETATTR_FH) }; .fi .in As usual, the filesystem object operated on is indicated by .IR header\->nodeid . The daemon should compute the attributes of this object and reply with the following message: .in +4n .nf struct fuse_attr { uint64_t ino; uint64_t size; uint64_t blocks; uint64_t atime; uint64_t mtime; uint64_t ctime; uint32_t atimensec; uint32_t mtimensec; uint32_t ctimensec; uint32_t mode; uint32_t nlink; uint32_t uid; uint32_t gid; uint32_t rdev; uint32_t blksize; uint32_t padding; }; struct fuse_attr_out { /* Attribute cache duration (seconds + nanoseconds) */ uint64_t attr_valid; uint32_t attr_valid_nsec; uint32_t dummy; struct fuse_attr attr; }; .fi .in The fields of .I struct fuse_attr describe the attributes of the required file. For the interpretation of these fields, see .BR stat (2). .TP .BR FUSE_ACCESS " ( 34 )" .in +4n .nf struct fuse_access_in { uint32_t mask; uint32_t padding; }; .fi .in If the .I default_permissions mount options is not used, this request may be used for permissions checking. No reply data is expected, but errors may be indicated as usual in the reply header (in particular, access denied errors may be indicated, by setting such field to .\" FIXME What does "such field" mean? The 'error' field? .BR \-EACCES ). .TP .BR FUSE_OPEN " ( 14 ) and " FUSE_OPENDIR " ( 34 )" .in +4n .nf struct fuse_open_in { uint32_t flags; /* The flags that were passed to the open(2) */ uint32_t unused; }; .fi .in The requested operation is to open the node indicated by .IR header\->nodeid . The exact semantics of what this means will depend on the filesystem being implemented. However, at the very least the filesystem should validate that the requested .I flags are valid for the indicated resource and then send a reply with the following format: .in +4n .nf struct fuse_open_out { uint64_t fh; uint32_t open_flags; uint32_t padding; }; .fi .in The .I fh field is an opaque identifier that the kernel will use to refer to this resource The .I open_flags field is a bit mask of any number of the flags that indicate properties of this file handle to the kernel: .RS 7 .TP 18 .BR FOPEN_DIRECT_IO Bypass page cache for this open file. .TP .BR FOPEN_KEEP_CACHE Don't invalidate the data cache on open. .TP .BR FOPEN_NONSEEKABLE The file is not seekable. .RE .TP .BR FUSE_READ " ( 15 ) and " FUSE_READDIR " ( 28 )" .in +4n .nf struct fuse_read_in { uint64_t fh; uint64_t offset; uint32_t size; uint32_t read_flags; uint64_t lock_owner; uint32_t flags; uint32_t padding; }; .fi .in The requested action is to read up to .I size bytes of the file or directory, starting at .IR offset . .\" FIXME .\" In the following, what are "out header" and "out structure"? The bytes should be returned directly following the out header, with no further special out structure. .TP .BR FUSE_INTERRUPT " ( 36 )" .in +4n .nf struct fuse_interrupt_in { uint64_t unique; }; .fi .in The requested action is to cancel the pending operation indicated by .IR unique . This request requires no response. However, receipt of this message does not by itself cancel the indicated operation. The kernel will still expect a reply to said operation (e.g., an .I EINTR error or a short read). At most one .B FUSE_INTERRUPT request will be issued for a given operation. After issuing said operation, the kernel will wait uninterruptibly for completion of the indicated request. .TP .BR FUSE_LOOKUP " ( 1 )" Directly following the header is a filename to be looked up in the directory indicated by .IR header\->nodeid . The expected reply is of the form: .in +4n .nf struct fuse_entry_out { uint64_t nodeid; /* Inode ID */ uint64_t generation; /* Inode generation */ uint64_t entry_valid; uint64_t attr_valid; uint32_t entry_valid_nsec; uint32_t attr_valid_nsec; struct fuse_attr attr; }; .fi .in The combination of .I nodeid and .I generation must be unique for the filesystem's lifetime. The interpretation of timeouts and .I attr is as for .BR FUSE_GETATTR . .TP .BR FUSE_FLUSH " ( 36 )" .in +4n .nf struct fuse_flush_in { uint64_t fh; uint32_t unused; uint32_t padding; uint64_t lock_owner; }; .fi .in The requested action is to flush any pending changes to the indicated file handle. No reply data is expected. However, an empty reply message still needs to be issued once the flush operation is complete. .TP .BR FUSE_RELEASE " ( 18 ) and " FUSE_RELEASEDIR " ( 29 )" .in +4n .nf struct fuse_release_in { uint64_t fh; uint32_t flags; uint32_t release_flags; uint64_t lock_owner; }; .fi .in These are the converse of .BR FUSE_OPEN and .BR FUSE_OPENDIR respectively. The daemon may now free any resources associated with the file handle .I fh as the kernel will no longer refer to it. There is no reply data associated with this request, but a reply still needs to be issued once the request has been completely processed. .TP .BR FUSE_STATFS " ( 17 )" This operation implements .BR statfs (2) for this filesystem. There is no input data associated with this request. The expected reply data has the following structure: .in +4n .nf struct fuse_kstatfs { uint64_t blocks; uint64_t bfree; uint64_t bavail; uint64_t files; uint64_t ffree; uint32_t bsize; uint32_t namelen; uint32_t frsize; uint32_t padding; uint32_t spare[6]; }; struct fuse_statfs_out { struct fuse_kstatfs st; }; .fi .in For the interpretation of these fields, see .BR statfs (2). .SH ERRORS .TP .B EPERM Returned from operations on a .I /dev/fuse file descriptor that has not been mounted. .TP .B EIO Returned from .BR read (2) operations when the kernel's request is too large for the provided buffer. .IR Note : There are various ways in which incorrect use of these interfaces can cause operations on the provided filesystem's files and directories to fail with .BR EIO . Among the possible incorrect uses are .IP * 3 changing .I mode & S_IFMT for an inode that has previously been reported to the kernel; or .IP * giving replies to the kernel that are shorter than what the kernel expected. .TP .B EINVAL Returned from .BR write (2) if validation of the reply failed. Not all mistakes in replies will be caught by this validation. However, basic mistakes, such as short replies or an incorrect .I unique value, are detected. .TP .B E2BIG Returned from .BR read (2) operations when the kernel's request is too large for the provided buffer and the request was .BR FUSE_SETXATTR . .TP .B ENODEV Returned from .BR read (2) and .BR write (2) if the FUSE filesystem was unmounted. .SH CONFORMING TO The FUSE filesystem is Linux-specific. .SH NOTES The following messages are not yet documented in this manual page: .in +8n .nf .BR FUSE_BATCH_FORGET .BR FUSE_BMAP .BR FUSE_CREATE .BR FUSE_DESTROY .BR FUSE_FALLOCATE .BR FUSE_FORGET .BR FUSE_FSYNC .BR FUSE_FSYNCDIR .BR FUSE_GETLK .BR FUSE_GETXATTR .BR FUSE_IOCTL .BR FUSE_LINK .BR FUSE_LISTXATTR .BR FUSE_LSEEK .BR FUSE_MKDIR .BR FUSE_MKNOD .BR FUSE_NOTIFY_REPLY .BR FUSE_POLL .BR FUSE_READDIRPLUS .BR FUSE_READLINK .BR FUSE_REMOVEXATTR .BR FUSE_RENAME .BR FUSE_RENAME2 .BR FUSE_RMDIR .BR FUSE_SETATTR .BR FUSE_SETLK .BR FUSE_SETLKW .BR FUSE_SYMLINK .BR FUSE_UNLINK .BR FUSE_WRITE .fi .in .\" It looks like the following are undocumented so far. It probably would .\" be kind to list these in the man page .SH SEE ALSO .BR fusermount (1), .BR mount.fuse (8) -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/ -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
