SYSCTL(9) BSD Kernel Developer's Manual SYSCTL(9)NAME
sysctl — system variable control interfaces
SYNOPSIS
#include <sys/param.h>
#include <sys/sysctl.h>
Primary external interfaces:
void
sysctl_init(void);
int
sysctl_lock(struct lwp *l, void *oldp, size_t savelen);
int
sysctl_dispatch(const int *name, u_int namelen, void *oldp,
size_t *oldlenp, const void *newp, size_t newlen, const int *oname,
struct lwp *l, const struct sysctlnode *rnode);
void
sysctl_unlock(struct lwp *l);
int
sysctl_createv(struct sysctllog **log, int cflags,
const struct sysctlnode **rnode, const struct sysctlnode **cnode,
int flags, int type, const char *namep, const char *desc,
sysctlfn func, u_quad_t qv, void *newp, size_t newlen, ...);
int
sysctl_destroyv(struct sysctlnode *rnode, ...);
void
sysctl_free(struct sysctlnode *rnode);
void
sysctl_teardown(struct sysctllog **);
int
old_sysctl(int *name, u_int namelen, void *oldp, size_t *oldlenp,
void *newp, size_t newlen, struct lwp *l);
Core internal functions:
int
sysctl_locate(struct lwp *l, const int *name, u_int namelen,
const struct sysctlnode **rnode, int *nip);
int
sysctl_lookup(const int *name, u_int namelen, void *oldp,
size_t *oldlenp, const void *newp, size_t newlen, const int *oname,
struct lwp *l, const struct sysctlnode *rnode);
int
sysctl_create(const int *name, u_int namelen, void *oldp,
size_t *oldlenp, const void *newp, size_t newlen, const int *oname,
struct lwp *l, const struct sysctlnode *rnode);
int
sysctl_destroy(const int *name, u_int namelen, void *oldp,
size_t *oldlenp, const void *newp, size_t newlen, const int *oname,
struct lwp *l, const struct sysctlnode *rnode);
int
sysctl_query(const int *name, u_int namelen, void *oldp, size_t *oldlenp,
const void *newp, size_t newlen, const int *oname, struct lwp *l,
const struct sysctlnode *rnode);
Simple “helper” functions:
int
sysctl_needfunc(const int *name, u_int namelen, void *oldp,
size_t *oldlenp, const void *newp, size_t newlen, const int *oname,
struct lwp *l, const struct sysctlnode *rnode);
int
sysctl_notavail(const int *name, u_int namelen, void *oldp,
size_t *oldlenp, const void *newp, size_t newlen, const int *oname,
struct lwp *l, const struct sysctlnode *rnode);
int
sysctl_null(const int *name, u_int namelen, void *oldp, size_t *oldlenp,
const void *newp, size_t newlen, const int *oname, struct lwp *l,
const struct sysctlnode *rnode);
DESCRIPTION
The SYSCTL subsystem instruments a number of kernel tunables and other
data structures via a simple MIB-like interface, primarily for consump‐
tion by userland programs, but also for use internally by the kernel.
LOCKING
All operations on the SYSCTL tree must be protected by acquiring the main
SYSCTL lock. The only functions that can be called when the lock is not
held are sysctl_lock(), sysctl_createv(), sysctl_destroyv(), and
old_sysctl(). All other functions require the tree to be locked. This
is to prevent other users of the tree from moving nodes around during an
add operation, or from destroying nodes or subtrees that are actively
being used. The lock is acquired by calling sysctl_lock() with a pointer
to the process's lwp l (NULL may be passed to all functions as the lwp
pointer if no lwp is appropriate, though any changes made via
sysctl_create(), sysctl_destroy(), sysctl_lookup(), or by any helper
function will be done with effective superuser privileges).
The oldp and savelen arguments are a pointer to and the size of the mem‐
ory region the caller will be using to collect data from SYSCTL. These
may also be NULL and 0, respectively.
The memory region will be locked via uvm_vslock() if it is a region in
userspace. The address and size of the region are recorded so that when
the SYSCTL lock is to be released via sysctl_unlock(), only the lwp
pointer l is required.
LOOKUPS
Once the lock has been acquired, it is typical to call sysctl_dispatch()
to handle the request. sysctl_dispatch() will examine the contents of
name, an array of integers at least namelen long, which is to be located
in kernel space, in order to determine which function to call to handle
the specific request.
The following algorithm is used by sysctl_dispatch() to determine the
function to call:
· Scan the tree using sysctl_locate().
· If the node returned has a “helper” function, call it.
· If the requested node was found but has no function, call
sysctl_lookup().
· If the node was not found and name specifies one of
sysctl_query(), sysctl_create(), or sysctl_destroy(), call the
appropriate function.
· If none of these options applies and no other error was yet
recorded, return EOPNOTSUPP.
The oldp and oldlenp arguments to sysctl_dispatch(), as with all the
other core functions, describe an area into which the current or
requested value may be copied. oldp may or may not be a pointer into
userspace (as dictated by whether l is NULL or not). oldlenp is a
non-NULL pointer to a size_t. newp and newlen describe an area where the
new value for the request may be found; newp may also be a pointer into
userspace. The oname argument is a non-NULL pointer to the base of the
request currently being processed. By simple arithmetic on name,
namelen, and oname, one can easily determine the entire original request
and namelen values, if needed. The rnode value, as passed to
sysctl_dispatch() represents the root of the tree into which the current
request is to be dispatched. If NULL, the main tree will be used.
The sysctl_locate() function scans a tree for the node most specific to a
request. If the pointer referenced by rnode is not NULL, the tree indi‐
cated is searched, otherwise the main tree will be used. The address of
the most relevant node will be returned via rnode and the number of MIB
entries consumed will be returned via nip, if it is not NULL.
The sysctl_lookup() function takes the same arguments as
sysctl_dispatch() with the caveat that the value for namelen must be zero
in order to indicate that the node referenced by the rnode argument is
the one to which the lookup is being applied.
CREATION AND DESTRUCTION OF NODES
New nodes are created and destroyed by the sysctl_create() and
sysctl_destroy() functions. These functions take the same arguments as
sysctl_dispatch() with the additional requirement that the namelen argu‐
ment must be 1 and the name argument must point to an integer valued
either CTL_CREATE or CTL_CREATESYM when creating a new node, or
CTL_DESTROY when destroying a node.
The newp and newlen arguments should point to a copy of the node to be
created or destroyed. If the create or destroy operation was successful,
a copy of the node created or destroyed will be placed in the space indi‐
cated by oldp and oldlenp. If the create operation fails because of a
conflict with an existing node, a copy of that node will be returned
instead.
In order to facilitate the creation and destruction of nodes from a given
tree by kernel subsystems, the functions sysctl_createv() and
sysctl_destroyv() are provided. These functions take care of the over‐
head of filling in the contents of the create or destroy request, dealing
with locking, locating the appropriate parent node, etc.
The arguments to sysctl_createv() are used to construct the new node. If
the log argument is not NULL, a sysctllog structure will be allocated and
the pointer referenced will be changed to address it. The same log may
be used for any number of nodes, provided they are all inserted into the
same tree. This allows for a series of nodes to be created and later
removed from the tree in a single transaction (via sysctl_teardown())
without the need for any record keeping on the caller's part.
The cflags argument is currently unused and must be zero. The rnode
argument must either be NULL or a valid pointer to a reference to the
root of the tree into which the new node must be placed. If it is NULL,
the main tree will be used. It is illegal for rnode to refer to a NULL
pointer. If the cnode argument is not NULL, on return it will be
adjusted to point to the address of the new node.
The flags and type arguments are combined into the sysctl_flags field,
and the current value for SYSCTL_VERSION is added in. The following
types are defined:
CTLTYPE_NODE A node intended to be a parent for other nodes.
CTLTYPE_INT A signed integer.
CTLTYPE_STRING A NUL-terminated string.
CTLTYPE_QUAD An unsigned 64-bit integer.
CTLTYPE_STRUCT A structure.
CTLTYPE_BOOL A boolean.
The namep argument is copied into the sysctl_name field and must be less
than SYSCTL_NAMELEN characters in length. The string indicated by desc
will be copied if the CTLFLAG_OWNDESC flag is set, and will be used as
the node's description.
Two additional remarks:
1. The CTLFLAG_PERMANENT flag can only be set from SYSCTL setup
routines (see SETUP FUNCTIONS) as called by sysctl_init().
2. If sysctl_destroyv() attempts to delete a node that does not
own its own description (and is not marked as permanent), but
the deletion fails, the description will be copied and
sysctl_destroyv() will set the CTLFLAG_OWNDESC flag.
The func argument is the name of a “helper” function (see HELPER
FUNCTIONS AND MACROS). If the CTLFLAG_IMMEDIATE flag is set, the qv
argument will be interpreted as the initial value for the new “bool”,
“int” or “quad” node. This flag does not apply to any other type of
node. The newp and newlen arguments describe the data external to SYSCTL
that is to be instrumented. One of func, qv and the CTLFLAG_IMMEDIATE
flag, or newp and newlen must be given for nodes that instrument data,
otherwise an error is returned.
The remaining arguments are a list of integers specifying the path
through the MIB to the node being created. The list must be terminated
by the CTL_EOL value. The penultimate value in the list may be
CTL_CREATE if a dynamic MIB entry is to be made for this node.
sysctl_createv() specifically does not support CTL_CREATESYM, since setup
routines are expected to be able to use the in-kernel ksyms(4) interface
to discover the location of the data to be instrumented. If the node to
be created matches a node that already exists, a return code of 0 is
given, indicating success.
When using sysctl_destroyv() to destroy a given node, the rnode argument,
if not NULL, is taken to be the root of the tree from which the node is
to be destroyed, otherwise the main tree is used. The rest of the argu‐
ments are a list of integers specifying the path through the MIB to the
node being destroyed. If the node being destroyed does not exist, a suc‐
cessful return code is given. Nodes marked with the CTLFLAG_PERMANENT
flag cannot be destroyed.
HELPER FUNCTIONS AND MACROS
Helper functions are invoked with the same common argument set as
sysctl_dispatch() except that the rnode argument will never be NULL. It
will be set to point to the node that corresponds most closely to the
current request. Helpers are forbidden from modifying the node they are
passed; they should instead copy the structure if changes are required in
order to effect access control or other checks. The “helper” prototype
and function that needs to ensure that a newly assigned value is within a
certain range (presuming external data) would look like the following:
static int sysctl_helper(SYSCTLFN_PROTO);
static int
sysctl_helper(SYSCTLFN_ARGS)
{
struct sysctlnode node;
int t, error;
t = *(int *)rnode->sysctl_data;
node = *rnode;
node.sysctl_data = &t;
error = sysctl_lookup(SYSCTLFN_CALL(&node));
if (error || newp == NULL)
return (error);
if (t < 0 || t > 20)
return (EINVAL);
*(int *)rnode->sysctl_data = t;
return (0);
}
The use of the SYSCTLFN_PROTO, SYSCTLFN_ARGS, and SYSCTLFN_CALL
macros ensure that all arguments are passed properly. The single argu‐
ment to the SYSCTLFN_CALL macro is the pointer to the node being exam‐
ined.
Three basic helper functions are available for use. sysctl_needfunc()
will emit a warning to the system console whenever it is invoked and pro‐
vides a simplistic read-only interface to the given node.
sysctl_notavail() will forward “queries” to sysctl_query() so that sub‐
trees can be discovered, but will return EOPNOTSUPP for any other condi‐
tion. sysctl_null() specifically ignores any arguments given, sets the
value indicated by oldlenp to zero, and returns success.
SETUP FUNCTIONS
Though nodes can be added to the SYSCTL tree at any time, in order to add
nodes during the kernel bootstrap phase, a proper “setup” function must
be used. Setup functions are declared using the SYSCTL_SETUP macro,
which takes the name of the function and a short string description of
the function as arguments. (See the SYSCTL_DEBUG_SETUP kernel
configuration in options(4).) The address of the function is added to a
list of functions that sysctl_init() traverses during initialization.
Setup functions do not have to add nodes to the main tree, but can set up
their own trees for emulation or other purposes. Emulations that require
use of a main tree but with some nodes changed to suit their own purposes
can arrange to overlay a sparse private tree onto their main tree by mak‐
ing the e_sysctlovly member of their struct emul definition point to the
overlaid tree.
Setup functions should take care to create all nodes from the root down
to the subtree they are creating, since the order in which setup func‐
tions are called is arbitrary (the order in which setup functions are
called is only determined by the ordering of the object files as passed
to the linker when the kernel is built).
MISCELLANEOUS FUNCTIONSsysctl_init() is called early in the kernel bootstrap process. It ini‐
tializes the SYSCTL lock, calls all the registered setup functions, and
marks the tree as permanent.
sysctl_free() will unconditionally delete any and all nodes below the
given node. Its intended use is for the deletion of entire trees, not
subtrees. If a subtree is to be removed, sysctl_destroy() or
sysctl_destroyv() should be used to ensure that nodes not owned by the
sub-system being deactivated are not mistakenly destroyed. The SYSCTL
lock must be held when calling this function.
sysctl_teardown() unwinds a sysctllog and deletes the nodes in the oppo‐
site order in which they were created.
old_sysctl() provides an interface similar to the old SYSCTL implementa‐
tion, with the exception that access checks on a per-node basis are per‐
formed if the l argument is non-NULL. If called with a NULL argument,
the values for newp and oldp are interpreted as kernel addresses, and
access is performed as for the superuser.
NOTES
It is expected that nodes will be added to (or removed from) the tree
during the following stages of a machine's lifetime:
· initialization -- when the kernel is booting
· autoconfiguration -- when devices are being probed at boot time
· “plug and play” device attachment -- when a PC-Card, USB, or other
device is plugged in or attached
· module initialization -- when a module is being loaded
· “run-time” -- when a process creates a node via the sysctl(3) inter‐
face
Nodes marked with CTLFLAG_PERMANENT can only be added to a tree during
the first or initialization phase, and can never be removed. The ini‐
tialization phase terminates when the main tree's root is marked with the
CTLFLAG_PERMANENT flag. Once the main tree is marked in this manner, no
nodes can be added to any tree that is marked with CTLFLAG_READONLY at
its root, and no nodes can be added at all if the main tree's root is so
marked.
Nodes added by device drivers, modules, and at device insertion time can
be added to (and removed from) “read-only” parent nodes.
Nodes created by processes can only be added to “writable” parent nodes.
See sysctl(3) for a description of the flags that are allowed to be used
by when creating nodes.
SEE ALSOsysctl(3)HISTORY
The dynamic SYSCTL implementation first appeared in NetBSD 2.0.
AUTHORS
Andrew Brown ⟨atatat@NetBSD.org⟩ designed and implemented the dynamic
SYSCTL implementation.
BSD December 4, 2011 BSD
