NetBSD/share/man/man4/puffs.4

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.Dd November 17, 2006
.Dt PUFFS 4
.Os
.Sh NAME
.Nm puffs
.Nd Pass-to-Userspace Framework File System
.Sh SYNOPSIS
.Cd "file-system PUFFS"
.Sh DESCRIPTION
.Em IMPORTANT NOTE!
This document describes interfaces which are not yet guaranteed to be
stable.
In case you update your system sources, please recompile everything
and fix complation errors.
If your sources are out-of-sync, incorrect operation may result.
.Pp
.Nm
provides a framework for creating file systems as userspace servers.
The in-kernel VFS attachment is controlled through a special device
node,
.Pa /dev/puffs .
This document describes the operations on the device.
People looking to implement file systems should prefer using the
system through the convenience library described in
.Xr puffs 3 .
Users wanting to access the device node directly should include
the header
.Pa sys/fs/puffs/puffs_msgif.h
for relevant definitions.
.Ss Mounting
The
.Nm
device node should be opened once per file system instance (i.e. mount).
The device itself is a cloning node, so the same node can be opened
a practically unlimited number of times.
Once the device is open, the file system can be mounted the normal
way using the
.Xr mount 2
system call and using the argument structure to control mount options:
.Bd -literal -offset indent
struct puffs_args {
	int		pa_vers;
	int		pa_fd;
	unsigned int	pa_flags;
	size_t		pa_maxreqlen;
	char		pa_name[PUFFSNAMESIZE];
};
.Ed
.Pp
The member
.Va pa_vers
is currently always 0 and ignored.
The
.Va pa_fd
member is the file descriptor number from opening the device node.
.Va pa_flags
controls some operations specific to puffs:
.Bl -tag -width "PUFFSFLAG_ALLOWCTL"
.It Dv PUFFSFLAG_ALLOWCTL
Allow file system fcntl and ioctl operations.
Allowing these has security implications as the file system can
technically read anything out of a calling processes address space.
This flag may additionally be enforced by the kernel security policy.
.It Dv PUFFSFLAG_NOCACHE
Do not store data in the page cache.
This causes operations to always consult the user server instead of
consulting the page cache.
This makes sense in situations where there is relatively little
bulk data to be transferred and the user server does not want to take
part in complex cache management routines in case the file system data
can be modified through routes other than the file system interface.
.El
.Pp
The
.Va pa_maxreqlen
member signifies the length of the incoming data buffer in userspace.
A good value is
.Dv PUFFS_REQ_MAXSIZE ,
which is the maximum the kernel will use.
A minimum value is also enforced, so the value of this field should
be checked after the mount operation to determine the correct buffer
size.
During operation, in case request fetch is attempted with a buffer
too short, the error
.Er E2BIG
will be returned.
.Pp
After a succesful mount system call, the the ioctl
.Dv PUFFSMOUNTOP
must be issued through the open device node.
The parameter for this ioctl is the following structure:
.Bd -literal -offset indent
struct puffs_startreq {
	void    *psr_cookie; fsid_t  psr_fsidx; };
.Ed
.Pp
The member
.Va psr_cookie
should be set before calling.
This signals the cookie value of the root node of the file system
(see
.Xr puffs 3
for more details on cookie strategies).
The value of
.Va psr_fsidx
will be filled by the kernel and should be recorded for future use.
After successfully executing this operation the file system is
active.
.Ss Operation
Operations must be queried from the kernel using the ioctl
.Dv PUFFSGETOP ,
processed, and the results pushed back to the kernel using
.Dv PUFFSPUTOP .
Normally the system will block until an event is available for
.Dv PUFFSGETOP ,
but it is possible to set the file descriptor into non-blocking
mode, in which case
.Er EWOULDBLOCK
is returned if no event is available.
Asynchoronous I/O calls (i.e.,
.Xr select 2 ,
.Xr poll 2 ,
and
.Xr kevent 2 )
can be issued to be notified of events.
.Pp
As the argument both get and push use the following structure:
.Bd -literal -offset indent
struct puffs_req {
	uint64_t        preq_id; uint8_t         preq_opclass;
	uint8_t         preq_optype; void            *preq_cookie;

	int		preq_rv;

	void		*preq_aux;
	size_t		preq_auxlen;
};
.Ed
.Pp
The member
.Va preq_id
is used as an identifier in the reply.
It should not be modified during the processing of a
.Dv PUFFSGETOP -
.Dv PUFFSPUTOP
sequence.
The members
.Va preq_opclass
and
.Va preq_optype
identify the request; they also are used for typing the data
pointed to by
.Va preq_aux .
Currently the mapping between these two is only documented in
code in
.Pa src/lib/libpuffs/puff.c:puffcall() .
The handling of this will very likely change in the future towards
a more automatic direction.
The length of the buffer given to
.Dv PUFFSGETOP
is described by
.Va preq_auxlen
and will be modified by the kernel to indicate how much data
actually was transmitted.
This is for the benefit of calls such as write, which transmit a
variable amount of data.
Similarly, the user server should fill in the amount of data the
kernel must copy for
.Dv PUFFSPUTOP ;
most of the time this will be constant for a given operation, but
operations such as read want to adjust it dynamically.
Finally,
.Va preq_rv
is used by the userspace server to fill in the success value of the
operation in question.
.Pp
In case the macro
.Fn PUFFSOP_WANTREPLY
returns false for
.Va preq_opclass ,
a return value is not wanted and
.Dv PUFFSPUTOP
should not be issued.
.Pp
Additionally, an operation of type
.Dv PUFFSSIZEOP
is supported, but it is only used by the ioctl and fcntl operations
and will likely go away in the future.
It is not described here.
.Ss Termination
The file system can be unmounted regularly using
.Xr umount 8 .
It will automatically be unmounted in case the userspace server is
killed or the control file descriptor closed, but in this case the
userspace server will not be separately requested to unmount itself
and this may result in data loss.
.Sh SEE ALSO
.Xr ioctl 2 ,
.Xr mount 2 ,
.Xr puffs 3 ,
.Xr umount 8
.Sh HISTORY
.Nm
first appeared in
.Nx 4.0 .
.Sh AUTHORS
.An Antti Kantee Aq pooka@iki.fi
.Sh BUGS
.Nm
is currently more like a souffle than puff pastry.