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.\" $FreeBSD: head/share/man/man9/hhook.9 267936 2014-06-26 21:44:30Z bapt $
.\"
.Dd June 21, 2013
.Dt HHOOK 9
.Os
.Sh NAME
.Nm hhook ,
.Nm hhook_head_register ,
.Nm hhook_head_deregister ,
.Nm hhook_head_deregister_lookup ,
.Nm hhook_run_hooks ,
.Nm HHOOKS_RUN_IF ,
.Nm HHOOKS_RUN_LOOKUP_IF
.Nd Helper Hook Framework
.Sh SYNOPSIS
.In sys/hhook.h
.Ft typedef int
.Fn "\*(lp*hhook_func_t\*(rp" "int32_t hhook_type" "int32_t hhook_id" \
"void *udata" "void *ctx_data" "void *hdata" "struct osd *hosd"
.Fn "int hhook_head_register" "int32_t hhook_type" "int32_t hhook_id" \
"struct hhook_head **hhh" "uint32_t flags"
.Fn "int hhook_head_deregister" "struct hhook_head *hhh"
.Fn "int hhook_head_deregister_lookup" "int32_t hhook_type" "int32_t hhook_id"
.Fn "void hhook_run_hooks" "struct hhook_head *hhh" "void *ctx_data" \
"struct osd *hosd"
.Fn HHOOKS_RUN_IF "hhh" "ctx_data" "hosd"
.Fn HHOOKS_RUN_LOOKUP_IF "hhook_type" "hhook_id" "ctx_data" "hosd"
.Sh DESCRIPTION
.Nm
provides a framework for managing and running arbitrary hook functions at
defined hook points within the kernel.
The KPI was inspired by
.Xr pfil 9 ,
and in many respects can be thought of as a more generic superset of pfil.
.Pp
The
.Xr khelp 9
and
.Nm
frameworks are tightly integrated.
Khelp is responsible for registering and deregistering Khelp module hook
functions with
.Nm
points.
The KPI functions used by
.Xr khelp 9
to do this are not documented here as they are not relevant to consumers wishing
to instantiate hook points.
.Ss Information for Khelp Module Implementors
Khelp modules indirectly interact with
.Nm
by defining appropriate hook functions for insertion into hook points.
Hook functions must conform to the
.Ft hhook_func_t
function pointer declaration
outlined in the
.Sx SYNOPSIS .
.Pp
The
.Fa hhook_type
and
.Fa hhook_id
arguments identify the hook point which has called into the hook function.
These are useful when a single hook function is registered for multiple hook
points and wants to know which hook point has called into it.
.In sys/hhook.h
lists available
.Fa hhook_type
defines and subsystems which export hook points are responsible for defining
the
.Fa hhook_id
value in appropriate header files.
.Pp
The
.Fa udata
argument will be passed to the hook function if it was specified in the
.Vt struct hookinfo
at hook registration time.
.Pp
The
.Fa ctx_data
argument contains context specific data from the hook point call site.
The data type passed is subsystem dependent.
.Pp
The
.Fa hdata
argument is a pointer to the persistent per-object storage allocated for use by
the module if required.
The pointer will only ever be NULL if the module did not request per-object
storage.
.Pp
The
.Fa hosd
argument can be used with the
.Xr khelp 9
framework's
.Fn khelp_get_osd
function to access data belonging to a different Khelp module.
.Pp
Khelp modules instruct the Khelp framework to register their hook functions with
.Nm
points by creating a
.Vt "struct hookinfo"
per hook point, which contains the following members:
.Bd -literal -offset indent
struct hookinfo {
	hhook_func_t	hook_func;
	struct helper	*hook_helper;
	void		*hook_udata;
	int32_t		hook_id;
	int32_t		hook_type;
};
.Ed
.Pp
Khelp modules are responsible for setting all members of the struct except
.Va hook_helper
which is handled by the Khelp framework.
.Ss Creating and Managing Hook Points
Kernel subsystems that wish to provide
.Nm
points typically need to make four and possibly five key changes to their
implementation:
.Bl -bullet
.It
Define a list of
.Va hhook_id
mappings in an appropriate subsystem header.
.It
Register each hook point with the
.Fn hhook_head_register
function during initialisation of the subsystem.
.It
Select or create a standardised data type to pass to hook functions as
contextual data.
.It
Add a call to
.Fn HHOOKS_RUN_IF
or
.Fn HHOOKS_RUN_IF_LOOKUP
at the point in the subsystem's code where the hook point should be executed.
.It
If the subsystem can be dynamically added/removed at runtime, each hook
point registered with the
.Fn hhook_head_register
function when the subsystem was initialised needs to be deregistered with the
.Fn hhook_head_deregister
or
.Fn hhook_head_deregister_lookup
functions when the subsystem is being deinitialised prior to removal.
.El
.Pp
The
.Fn hhook_head_register
function registers a hook point with the
.Nm
framework.
The
.Fa hook_type
argument defines the high level type for the hook point.
Valid types are defined in
.In sys/hhook.h
and new types should be added as required.
The
.Fa hook_id
argument specifies a unique, subsystem specific identifier for the hook point.
The
.Fa hhh
argument will, if not NULL, be used to store a reference to the
.Vt struct hhook_head
created as part of the registration process.
Subsystems will generally want to store a local copy of the
.Vt struct hhook_head
so that they can use the
.Fn HHOOKS_RUN_IF
macro to instantiate hook points.
The HHOOK_WAITOK flag may be passed in via the
.Fa flags
argument if
.Xr malloc 9
is allowed to sleep waiting for memory to become available.
If the hook point is within a virtualised subsystem (e.g. the network stack),
the HHOOK_HEADISINVNET flag should be passed in via the
.Fa flags
argument so that the
.Vt struct hhook_head
created during the registration process will be added to a virtualised list.
.Pp
The
.Fn hhook_head_deregister
function deregisters a previously registered hook point from the
.Nm
framework.
The
.Fa hhh
argument is the pointer to the
.Vt struct hhook_head
returned by
.Fn hhoook_head_register
when the hook point was registered.
.Pp
The
.Fn hhook_head_deregister_lookup
function can be used instead of
.Fn hhook_head_deregister
in situations where the caller does not have a cached copy of the
.Vt struct hhook_head
and wants to deregister a hook point using the appropriate
.Fa hook_type
and
.Fa hook_id
identifiers instead.
.Pp
The
.Fn hhook_run_hooks
function should normally not be called directly and should instead be called
indirectly via the
.Fn HHOOKS_RUN_IF
macro.
However, there may be circumstances where it is preferable to call the function
directly, and so it is documented here for completeness.
The
.Fa hhh
argument references the
.Nm
point to call all registered hook functions for.
The
.Fa ctx_data
argument specifies a pointer to the contextual hook point data to pass into the
hook functions.
The
.Fa hosd
argument should be the pointer to the appropriate object's
.Vt struct osd
if the subsystem provides the ability for Khelp modules to associate per-object
data.
Subsystems which do not should pass NULL.
.Pp
The
.Fn HHOOKS_RUN_IF
macro is the preferred way to implement hook points.
It only calls the
.Fn hhook_run_hooks
function if at least one hook function is registered for the hook point.
By checking for registered hook functions, the macro minimises the cost
associated with adding hook points to frequently used code paths by reducing to
a simple if test in the common case where no hook functions are registered.
The arguments are as described for the
.Fn hhook_run_hooks
function.
.Pp
The
.Fn HHOOKS_RUN_IF_LOOKUP
macro performs the same function as the
.Fn HHOOKS_RUN_IF
macro, but performs an additional step to look up the
.Vt struct hhook_head
for the specified
.Fa hook_type
and
.Fa hook_id
identifiers.
It should not be used except in code paths which are infrequently executed
because of the reference counting overhead associated with the look up.
.Sh IMPLEMENTATION NOTES
Each
.Vt struct hhook_head
protects its internal list of hook functions with a
.Xr rmlock 9 .
Therefore, anytime
.Fn hhook_run_hooks
is called directly or indirectly via the
.Fn HHOOKS_RUN_IF
or
.Fn HHOOKS_RUN_IF_LOOKUP
macros, a non-sleepable read lock will be acquired and held across the calls to
all registered hook functions.
.Sh RETURN VALUES
.Fn hhook_head_register
returns 0 if no errors occurred.
It returns EEXIST if a hook point with the same
.Fa hook_type
and
.Fa hook_id
is already registered.
It returns EINVAL if the HHOOK_HEADISINVNET flag is not set in
.Fa flags
because the implementation does not yet support hook points in non-virtualised
subsystems (see the
.Sx BUGS
section for details).
It returns ENOMEM if
.Xr malloc 9
failed to allocate memory for the new
.Vt struct hhook_head .
.Pp
.Fn hhook_head_deregister
and
.Fn hhook_head_deregister_lookup
return 0 if no errors occurred.
They return ENOENT if
.Fa hhh
is NULL.
They return EBUSY if the reference count of
.Fa hhh
is greater than one.
.Sh EXAMPLES
A well commented example Khelp module can be found at:
.Pa /usr/share/examples/kld/khelp/h_example.c
.Pp
The
.Xr tcp 4
implementation provides two
.Nm
points which are called for packets sent/received when a connection is in the
established phase.
Search for HHOOK in the following files:
.Pa sys/netinet/tcp_var.h ,
.Pa sys/netinet/tcp_input.c ,
.Pa sys/netinet/tcp_output.c
and
.Pa sys/netinet/tcp_subr.c .
.Sh SEE ALSO
.Xr khelp 9
.Sh ACKNOWLEDGEMENTS
Development and testing of this software were made possible in part by grants
from the FreeBSD Foundation and Cisco University Research Program Fund at
Community Foundation Silicon Valley.
.Sh HISTORY
The
.Nm
framework first appeared in
.Fx 9.0 .
.Pp
The
.Nm
framework was first released in 2010 by Lawrence Stewart whilst studying at
Swinburne University of Technology's Centre for Advanced Internet Architectures,
Melbourne, Australia.
More details are available at:
.Pp
http://caia.swin.edu.au/urp/newtcp/
.Sh AUTHORS
.An -nosplit
The
.Nm
framework was written by
.An Lawrence Stewart Aq Mt lstewart@FreeBSD.org .
.Pp
This manual page was written by
.An David Hayes Aq Mt david.hayes@ieee.org
and
.An Lawrence Stewart Aq Mt lstewart@FreeBSD.org .