man4/fuse.4

   1 .\" Copyright (c) 2016 Julia Computing Inc, Keno Fischer
   2 .\" Description based on include/uapi/fuse.h and code in fs/fuse
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  25 .\"
  26 .TH FUSE 4 2016-12-10 "Linux" "Linux Programmer's Manual"
  27 .SH NAME
  28 /dev/fuse \- Filesystem in Userspace (FUSE) device
  29 .SH SYNOPSIS
  30 .nf
  31 .B #include <linux/fuse.h>
  32 .nf
  33 .SH DESCRIPTION
  34
  35 This device is the primary interface between the FUSE filesystem driver
  36 and a user-space process wishing to provide the filesystem (referred to
  37 in the rest of this manual page as the
  38 .IR "filesystem daemon" ).
  39 This manual page is intended for those
  40 interested in understanding the kernel interface itself.
  41 Those implementing a FUSE filesystem may wish to make use of
  42 a user-space library such as
  43 .I libfuse
  44 that abstracts away the low-level interface.
  45
  46 At its core, FUSE is a simple client-server protocol, in which the Linux
  47 kernel is the client and the daemon is the server.
  48 After obtaining a file descriptor for this device, the daemon may
  49 .BR read (2)
  50 requests from that file descriptor and is expected to
  51 .BR write (2)
  52 back its replies.
  53 It is important to note that a file descriptor is
  54 associated with a unique FUSE filesystem.
  55 In particular, opening a second copy of this device,
  56 will not allow access to resources created
  57 through the first file descriptor (and vice versa).
  58 .\"
  59 .SS The basic protocol
  60 Every message that is read by the daemon begins with a header described by
  61 the following structure:
  62
  63 .in +4n
  64 .nf
  65 struct fuse_in_header {
  66     uint32_t len;       /* Total length of the data,
  67                            including this header */
  68     uint32_t opcode;    /* The kind of operation (see below) */
  69     uint64_t unique;    /* A unique identifier for this request */
  70     uint64_t nodeid;    /* ID of the filesystem object
  71                            being operated on */
  72     uint32_t uid;       /* UID of the requesting process */
  73     uint32_t gid;       /* GID of the requesting process */
  74     uint32_t pid;       /* PID of the requesting process */
  75     uint32_t padding;
  76 };
  77 .fi
  78 .in
  79
  80 The header is followed by a variable-length data portion
  81 (which may be empty) specific to the requested operation
  82 (the requested operation is indicated by
  83 .IR opcode ).
  84
  85 The daemon should then process the request and if applicable send
  86 a reply (almost all operations require a reply; if they do not,
  87 this is documented below), by performing a
  88 .BR write (2)
  89 to the file descriptor.
  90 All replies must start with the following header:
  91
  92 .in +4n
  93 .nf
  94 struct fuse_out_header {
  95     uint32_t len;       /* Total length of data written to
  96                            the file descriptor */
  97     int32_t  error;     /* Any error that occurred (0 if none) */
  98     uint64_t unique;    /* The value from the
  99                            corresponding request */
 100 };
 101 .fi
 102 .in
 103
 104 This header is also followed by (potentially empty) variable-sized
 105 data depending on the executed request.
 106 However, if the reply is an error reply (i.e.,
 107 .I error
 108 is set),
 109 then no further payload data should be sent, independent of the request.
 110 .\"
 111 .SS Exchanged messages
 112 This section should contain documentation for each of the messages
 113 in the protocol.
 114 This manual page is currently incomplete,
 115 so not all messages are documented.
 116 For each message, first the struct sent by the kernel is given,
 117 followed by a description of the semantics of the message.
 118 .TP
 119 .BR FUSE_INIT " ( 25 )"
 120
 121 .in +4n
 122 .nf
 123 struct fuse_init_in {
 124     uint32_t major;
 125     uint32_t minor;
 126     uint32_t max_readahead; /* Since protocol v7.6 */
 127     uint32_t flags;         /* Since protocol v7.6 */
 128 };
 129 .fi
 130 .in
 131
 132 This is the first request sent by the kernel to the daemon.
 133 It is used to negotiate the protocol version and other filesystem parameters.
 134 Note that the protocol version may affect the layout of any structure
 135 in the protocol (including this structure).
 136 The daemon must thus remember the negotiated version
 137 and flags for each session.
 138 As of the writing of this man page,
 139 the highest supported kernel protocol version is
 140 .IR 7.26 .
 141
 142 Users should be aware that the descriptions in this manual page
 143 may be incomplete or incorrect for older or more recent protocol versions.
 144
 145 The reply for this request has the following format:
 146
 147 .in +4n
 148 .nf
 149 struct fuse_init_out {
 150     uint32_t major;
 151     uint32_t minor;
 152     uint32_t max_readahead;   /* Since v7.6 */
 153     uint32_t flags;           /* Since v7.6; some flags bits
 154                                  were introduced later */
 155     uint16_t max_background;  /* Since v7.13 */
 156     uint16_t congestion_threshold;  /* Since v7.13 */
 157     uint32_t max_write;       /* Since v7.5 */
 158     uint32_t time_gran;       /* Since v7.6 */
 159     uint32_t unused[9];
 160 };
 161 .fi
 162 .in
 163
 164 If the major version supported by the kernel is larger than that supported
 165 by the daemon, the reply shall consist of only
 166 .I uint32_t major
 167 (following the usual header),
 168 indicating the largest major version supported by the daemon.
 169 The kernel will then issue a new
 170 .B FUSE_INIT
 171 request conforming to the older version.
 172 In the reverse case, the daemon should
 173 quietly fall back to the kernel's major version.
 174
 175 The negotiated minor version is considered to be the minimum
 176 of the minor versions provided by the daemon and the kernel and
 177 both parties should use the protocol corresponding to said minor version.
 178 .TP
 179 .BR FUSE_GETATTR " ( 3 )"
 180 .\" FIXME It looks like this is for implementing a stat(2) type of
 181 .\" operation. There needs to be a sentence here describing what
 182 .\" this option does.
 183
 184 .in +4n
 185 .nf
 186 struct fuse_getattr_in {
 187     uint32_t getattr_flags;
 188     uint32_t dummy;
 189     uint64_t fh;      /* Set only if
 190                          (getattr_flags & FUSE_GETATTR_FH)
 191 };
 192 .fi
 193 .in
 194
 195 As usual, the filesystem object operated on is indicated by
 196 .IR header\->nodeid .
 197 The daemon should compute the attributes
 198 of this object and reply with the following message:
 199 .in +4n
 200
 201 .nf
 202 struct fuse_attr {
 203     uint64_t ino;
 204     uint64_t size;
 205     uint64_t blocks;
 206     uint64_t atime;
 207     uint64_t mtime;
 208     uint64_t ctime;
 209     uint32_t atimensec;
 210     uint32_t mtimensec;
 211     uint32_t ctimensec;
 212     uint32_t mode;
 213     uint32_t nlink;
 214     uint32_t uid;
 215     uint32_t gid;
 216     uint32_t rdev;
 217     uint32_t blksize;
 218     uint32_t padding;
 219 };
 220
 221 struct fuse_attr_out {
 222     /* Attribute cache duration (seconds + nanoseconds) */
 223     uint64_t attr_valid;
 224     uint32_t attr_valid_nsec;
 225     uint32_t dummy;
 226     struct fuse_attr attr;
 227 };
 228 .fi
 229 .in
 230
 231 The fields of
 232 .I struct fuse_attr
 233 describe the attributes of the required file.
 234 For the interpretation of these fields, see
 235 .BR stat (2).
 236 .TP
 237 .BR FUSE_ACCESS " ( 34 )"
 238
 239 .in +4n
 240 .nf
 241 struct fuse_access_in {
 242     uint32_t mask;
 243     uint32_t padding;
 244 };
 245 .fi
 246 .in
 247
 248 If the
 249 .I default_permissions
 250 mount options is not used, this request may be used for permissions checking.
 251 No reply data is expected, but errors may be indicated
 252 as usual in the reply header (in particular, access denied errors
 253 may be indicated, by setting such field to
 254 .\" FIXME What does "such field" mean? The 'error' field?
 255 .BR \-EACCES ).
 256 .TP
 257 .BR FUSE_OPEN " ( 14 ) and " FUSE_OPENDIR " ( 34 )"
 258 .in +4n
 259 .nf
 260 struct fuse_open_in {
 261     uint32_t flags;     /* The flags that were passed
 262                            to the open(2) */
 263     uint32_t unused;
 264 };
 265 .fi
 266 .in
 267
 268 The requested operation is to open the node indicated by
 269 .IR header\->nodeid .
 270 The exact semantics of what this means will depend on the
 271 filesystem being implemented.
 272 However, at the very least the
 273 filesystem should validate that the requested
 274 .I flags
 275 are valid for the indicated resource and then send a reply with the
 276 following format:
 277
 278 .in +4n
 279 .nf
 280
 281 struct fuse_open_out {
 282     uint64_t fh;
 283     uint32_t open_flags;
 284     uint32_t padding;
 285 };
 286
 287 .fi
 288 .in
 289
 290 The
 291 .I fh
 292 field is an opaque identifier that the kernel will use to refer
 293 to this resource
 294 The
 295 .I open_flags
 296 field is a bit mask of any number of
 297 .B FOPEN_*
 298 flags, which indicate properties of this file handle to the kernel.
 299 .TP
 300 .BR FUSE_READ " ( 15 ) and " FUSE_READDIR " ( 28 )"
 301 .in +4n
 302 .nf
 303
 304 struct fuse_read_in {
 305     uint64_t fh;
 306     uint64_t offset;
 307     uint32_t size;
 308     uint32_t read_flags;
 309     uint64_t lock_owner;
 310     uint32_t flags;
 311     uint32_t padding;
 312 };
 313
 314 .fi
 315 .in
 316
 317 The requested action is to read up to
 318 .I size
 319 bytes of the file or directory, starting at
 320 .IR offset .
 321 .\" FIXME
 322 .\" In the following, what are "out header" and "out structure"?
 323 The bytes should be returned directly following the out header,
 324 with no further special out structure.
 325 .TP
 326 .BR FUSE_INTERRUPT " ( 36 )"
 327 .in +4n
 328 .nf
 329 struct fuse_interrupt_in {
 330     uint64_t unique;
 331 };
 332 .fi
 333 .in
 334
 335 The requested action is to cancel the pending operation indicated by
 336 .IR unique .
 337 This request requires no response.
 338 However, receipt of this message does
 339 not by itself cancel the indicated operation.
 340 The kernel will still expect a reply to said operation (e.g., an
 341 .I EINTR
 342 error or a short read).
 343 At most one
 344 .B FUSE_INTERRUPT
 345 request will be issued for a given operation.
 346 After issuing said operation,
 347 the kernel will wait uninterruptibly for completion of the indicated request.
 348 .TP
 349 .BR FUSE_LOOKUP " ( 1 )"
 350 Directly following the header is a filename to be looked up in the directory
 351 indicated by
 352 .IR header\->nodeid .
 353 The expected reply is of the form:
 354
 355 .in +4n
 356 .nf
 357 struct fuse_entry_out {
 358     uint64_t nodeid;            /* Inode ID */
 359     uint64_t generation;        /* Inode generation */
 360     uint64_t entry_valid;
 361     uint64_t attr_valid;
 362     uint32_t entry_valid_nsec;
 363     uint32_t attr_valid_nsec;
 364     struct fuse_attr attr;
 365 };
 366 .fi
 367 .in
 368
 369 The combination of
 370 .I nodeid
 371 and
 372 .I generation
 373 must be unique for the filesystem's lifetime.
 374
 375 The interpretation of timeouts and
 376 .I attr
 377 is as for
 378 .BR FUSE_GETATTR .
 379 .TP
 380 .BR FUSE_FLUSH " ( 36 )"
 381 .in +4n
 382 .nf
 383 struct fuse_flush_in {
 384     uint64_t fh;
 385     uint32_t unused;
 386     uint32_t padding;
 387     uint64_t lock_owner;
 388 };
 389 .fi
 390 .in
 391
 392 The requested action is to flush any pending changes to the indicated
 393 file handle.
 394 No reply data is expected.
 395 However, an empty reply message
 396 still needs to be issued once the flush operation is complete.
 397 .TP
 398 .BR FUSE_RELEASE " ( 18 ) and " FUSE_RELEASEDIR " ( 29 )"
 399 .in +4n
 400 .nf
 401 struct fuse_release_in {
 402     uint64_t fh;
 403     uint32_t flags;
 404     uint32_t release_flags;
 405     uint64_t lock_owner;
 406 };
 407 .fi
 408 .in
 409
 410 These are the converse of
 411 .BR FUSE_OPEN
 412 and
 413 .BR FUSE_OPENDIR
 414 respectively.
 415 The daemon may now free any resources associated with the
 416 file handle
 417 .I fh
 418 as the kernel will no longer refer to it.
 419 There is no reply data associated with this request,
 420 but a reply still needs to be issued once the request has
 421 been completely processed.
 422 .TP
 423 .BR FUSE_STATFS " ( 17 )"
 424 This operation implements
 425 .BR statfs (2)
 426 for this filesystem.
 427 There is no input data associated with this request.
 428 The expected reply data has the following structure:
 429
 430 .in +4n
 431 .nf
 432 struct fuse_kstatfs {
 433     uint64_t blocks;
 434     uint64_t bfree;
 435     uint64_t bavail;
 436     uint64_t files;
 437     uint64_t ffree;
 438     uint32_t bsize;
 439     uint32_t namelen;
 440     uint32_t frsize;
 441     uint32_t padding;
 442     uint32_t spare[6];
 443 };
 444
 445 struct fuse_statfs_out {
 446     struct fuse_kstatfs st;
 447 };
 448 .fi
 449 .in
 450
 451 For the interpretation of these fields, see
 452 .BR statfs (2).
 453 .SH ERRORS
 454 .TP
 455 .B EPERM
 456 Returned from operations on a
 457 .I /dev/fuse
 458 file descriptor that has not been mounted.
 459 .TP
 460 .B EIO
 461 Returned from
 462 .BR read (2)
 463 operations when the kernel's request is too large for the provided buffer.
 464
 465 .IR Note :
 466 There are various ways in which incorrect use of these interfaces can cause
 467 operations on the provided filesystem's files and directories to fail with
 468 .BR EIO .
 469 Among the possible incorrect uses are
 470 .IP * 3
 471 changing
 472 .I mode & S_IFMT
 473 for an inode that has previously been reported to the kernel; or
 474 .IP *
 475 giving replies to the kernel that are shorter than what the kernel expected.
 476 .TP
 477 .B EINVAL
 478 Returned from
 479 .BR write (2)
 480 if validation of the reply failed.
 481 Not all mistakes in replies will be caught by this validation.
 482 However, basic mistakes, such as short replies or an incorrect
 483 .I unique
 484 value, are detected.
 485 .TP
 486 .B E2BIG
 487 Returned from
 488 .BR read (2)
 489 operations when the kernel's request is too large for the provided buffer
 490 and the request was
 491 .BR FUSE_SETXATTR .
 492 .TP
 493 .B ENODEV
 494 Returned from
 495 .BR read (2)
 496 and
 497 .BR write (2)
 498 if the FUSE filesystem was unmounted.
 499 .SH CONFORMING TO
 500 The FUSE filesystem is Linux-specific.
 501 .SH SEE ALSO
 502 .BR fusermount (1),
 503 .BR mount.fuse (8)