linux/kernel/livepatch/shadow.c
Petr Mladek 3b2c77d000 livepatch: Allow to call a custom callback when freeing shadow variables
We might need to do some actions before the shadow variable is freed.
For example, we might need to remove it from a list or free some data
that it points to.

This is already possible now. The user can get the shadow variable
by klp_shadow_get(), do the necessary actions, and then call
klp_shadow_free().

This patch allows to do it a more elegant way. The user could implement
the needed actions in a callback that is passed to klp_shadow_free()
as a parameter. The callback usually does reverse operations to
the constructor callback that can be called by klp_shadow_*alloc().

It is especially useful for klp_shadow_free_all(). There we need to do
these extra actions for each found shadow variable with the given ID.

Note that the memory used by the shadow variable itself is still released
later by rcu callback. It is needed to protect internal structures that
keep all shadow variables. But the destructor is called immediately.
The shadow variable must not be access anyway after klp_shadow_free()
is called. The user is responsible to protect this any suitable way.

Be aware that the destructor is called under klp_shadow_lock. It is
the same as for the contructor in klp_shadow_alloc().

Signed-off-by: Petr Mladek <pmladek@suse.com>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Acked-by: Miroslav Benes <mbenes@suse.cz>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
2018-04-17 13:42:48 +02:00

312 lines
9.3 KiB
C

/*
* shadow.c - Shadow Variables
*
* Copyright (C) 2014 Josh Poimboeuf <jpoimboe@redhat.com>
* Copyright (C) 2014 Seth Jennings <sjenning@redhat.com>
* Copyright (C) 2017 Joe Lawrence <joe.lawrence@redhat.com>
*
* This program is free software; you can redistribute it and/or
* modify it under the terms of the GNU General Public License
* as published by the Free Software Foundation; either version 2
* of the License, or (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, see <http://www.gnu.org/licenses/>.
*/
/**
* DOC: Shadow variable API concurrency notes:
*
* The shadow variable API provides a simple relationship between an
* <obj, id> pair and a pointer value. It is the responsibility of the
* caller to provide any mutual exclusion required of the shadow data.
*
* Once a shadow variable is attached to its parent object via the
* klp_shadow_*alloc() API calls, it is considered live: any subsequent
* call to klp_shadow_get() may then return the shadow variable's data
* pointer. Callers of klp_shadow_*alloc() should prepare shadow data
* accordingly.
*
* The klp_shadow_*alloc() API calls may allocate memory for new shadow
* variable structures. Their implementation does not call kmalloc
* inside any spinlocks, but API callers should pass GFP flags according
* to their specific needs.
*
* The klp_shadow_hash is an RCU-enabled hashtable and is safe against
* concurrent klp_shadow_free() and klp_shadow_get() operations.
*/
#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
#include <linux/hashtable.h>
#include <linux/slab.h>
#include <linux/livepatch.h>
static DEFINE_HASHTABLE(klp_shadow_hash, 12);
/*
* klp_shadow_lock provides exclusive access to the klp_shadow_hash and
* the shadow variables it references.
*/
static DEFINE_SPINLOCK(klp_shadow_lock);
/**
* struct klp_shadow - shadow variable structure
* @node: klp_shadow_hash hash table node
* @rcu_head: RCU is used to safely free this structure
* @obj: pointer to parent object
* @id: data identifier
* @data: data area
*/
struct klp_shadow {
struct hlist_node node;
struct rcu_head rcu_head;
void *obj;
unsigned long id;
char data[];
};
/**
* klp_shadow_match() - verify a shadow variable matches given <obj, id>
* @shadow: shadow variable to match
* @obj: pointer to parent object
* @id: data identifier
*
* Return: true if the shadow variable matches.
*/
static inline bool klp_shadow_match(struct klp_shadow *shadow, void *obj,
unsigned long id)
{
return shadow->obj == obj && shadow->id == id;
}
/**
* klp_shadow_get() - retrieve a shadow variable data pointer
* @obj: pointer to parent object
* @id: data identifier
*
* Return: the shadow variable data element, NULL on failure.
*/
void *klp_shadow_get(void *obj, unsigned long id)
{
struct klp_shadow *shadow;
rcu_read_lock();
hash_for_each_possible_rcu(klp_shadow_hash, shadow, node,
(unsigned long)obj) {
if (klp_shadow_match(shadow, obj, id)) {
rcu_read_unlock();
return shadow->data;
}
}
rcu_read_unlock();
return NULL;
}
EXPORT_SYMBOL_GPL(klp_shadow_get);
static void *__klp_shadow_get_or_alloc(void *obj, unsigned long id,
size_t size, gfp_t gfp_flags,
klp_shadow_ctor_t ctor, void *ctor_data,
bool warn_on_exist)
{
struct klp_shadow *new_shadow;
void *shadow_data;
unsigned long flags;
/* Check if the shadow variable already exists */
shadow_data = klp_shadow_get(obj, id);
if (shadow_data)
goto exists;
/*
* Allocate a new shadow variable. Fill it with zeroes by default.
* More complex setting can be done by @ctor function. But it is
* called only when the buffer is really used (under klp_shadow_lock).
*/
new_shadow = kzalloc(size + sizeof(*new_shadow), gfp_flags);
if (!new_shadow)
return NULL;
/* Look for <obj, id> again under the lock */
spin_lock_irqsave(&klp_shadow_lock, flags);
shadow_data = klp_shadow_get(obj, id);
if (unlikely(shadow_data)) {
/*
* Shadow variable was found, throw away speculative
* allocation.
*/
spin_unlock_irqrestore(&klp_shadow_lock, flags);
kfree(new_shadow);
goto exists;
}
new_shadow->obj = obj;
new_shadow->id = id;
if (ctor) {
int err;
err = ctor(obj, new_shadow->data, ctor_data);
if (err) {
spin_unlock_irqrestore(&klp_shadow_lock, flags);
kfree(new_shadow);
pr_err("Failed to construct shadow variable <%p, %lx> (%d)\n",
obj, id, err);
return NULL;
}
}
/* No <obj, id> found, so attach the newly allocated one */
hash_add_rcu(klp_shadow_hash, &new_shadow->node,
(unsigned long)new_shadow->obj);
spin_unlock_irqrestore(&klp_shadow_lock, flags);
return new_shadow->data;
exists:
if (warn_on_exist) {
WARN(1, "Duplicate shadow variable <%p, %lx>\n", obj, id);
return NULL;
}
return shadow_data;
}
/**
* klp_shadow_alloc() - allocate and add a new shadow variable
* @obj: pointer to parent object
* @id: data identifier
* @size: size of attached data
* @gfp_flags: GFP mask for allocation
* @ctor: custom constructor to initialize the shadow data (optional)
* @ctor_data: pointer to any data needed by @ctor (optional)
*
* Allocates @size bytes for new shadow variable data using @gfp_flags.
* The data are zeroed by default. They are further initialized by @ctor
* function if it is not NULL. The new shadow variable is then added
* to the global hashtable.
*
* If an existing <obj, id> shadow variable can be found, this routine will
* issue a WARN, exit early and return NULL.
*
* This function guarantees that the constructor function is called only when
* the variable did not exist before. The cost is that @ctor is called
* in atomic context under a spin lock.
*
* Return: the shadow variable data element, NULL on duplicate or
* failure.
*/
void *klp_shadow_alloc(void *obj, unsigned long id,
size_t size, gfp_t gfp_flags,
klp_shadow_ctor_t ctor, void *ctor_data)
{
return __klp_shadow_get_or_alloc(obj, id, size, gfp_flags,
ctor, ctor_data, true);
}
EXPORT_SYMBOL_GPL(klp_shadow_alloc);
/**
* klp_shadow_get_or_alloc() - get existing or allocate a new shadow variable
* @obj: pointer to parent object
* @id: data identifier
* @size: size of attached data
* @gfp_flags: GFP mask for allocation
* @ctor: custom constructor to initialize the shadow data (optional)
* @ctor_data: pointer to any data needed by @ctor (optional)
*
* Returns a pointer to existing shadow data if an <obj, id> shadow
* variable is already present. Otherwise, it creates a new shadow
* variable like klp_shadow_alloc().
*
* This function guarantees that only one shadow variable exists with the given
* @id for the given @obj. It also guarantees that the constructor function
* will be called only when the variable did not exist before. The cost is
* that @ctor is called in atomic context under a spin lock.
*
* Return: the shadow variable data element, NULL on failure.
*/
void *klp_shadow_get_or_alloc(void *obj, unsigned long id,
size_t size, gfp_t gfp_flags,
klp_shadow_ctor_t ctor, void *ctor_data)
{
return __klp_shadow_get_or_alloc(obj, id, size, gfp_flags,
ctor, ctor_data, false);
}
EXPORT_SYMBOL_GPL(klp_shadow_get_or_alloc);
static void klp_shadow_free_struct(struct klp_shadow *shadow,
klp_shadow_dtor_t dtor)
{
hash_del_rcu(&shadow->node);
if (dtor)
dtor(shadow->obj, shadow->data);
kfree_rcu(shadow, rcu_head);
}
/**
* klp_shadow_free() - detach and free a <obj, id> shadow variable
* @obj: pointer to parent object
* @id: data identifier
* @dtor: custom callback that can be used to unregister the variable
* and/or free data that the shadow variable points to (optional)
*
* This function releases the memory for this <obj, id> shadow variable
* instance, callers should stop referencing it accordingly.
*/
void klp_shadow_free(void *obj, unsigned long id, klp_shadow_dtor_t dtor)
{
struct klp_shadow *shadow;
unsigned long flags;
spin_lock_irqsave(&klp_shadow_lock, flags);
/* Delete <obj, id> from hash */
hash_for_each_possible(klp_shadow_hash, shadow, node,
(unsigned long)obj) {
if (klp_shadow_match(shadow, obj, id)) {
klp_shadow_free_struct(shadow, dtor);
break;
}
}
spin_unlock_irqrestore(&klp_shadow_lock, flags);
}
EXPORT_SYMBOL_GPL(klp_shadow_free);
/**
* klp_shadow_free_all() - detach and free all <*, id> shadow variables
* @id: data identifier
* @dtor: custom callback that can be used to unregister the variable
* and/or free data that the shadow variable points to (optional)
*
* This function releases the memory for all <*, id> shadow variable
* instances, callers should stop referencing them accordingly.
*/
void klp_shadow_free_all(unsigned long id, klp_shadow_dtor_t dtor)
{
struct klp_shadow *shadow;
unsigned long flags;
int i;
spin_lock_irqsave(&klp_shadow_lock, flags);
/* Delete all <*, id> from hash */
hash_for_each(klp_shadow_hash, i, shadow, node) {
if (klp_shadow_match(shadow, shadow->obj, id))
klp_shadow_free_struct(shadow, dtor);
}
spin_unlock_irqrestore(&klp_shadow_lock, flags);
}
EXPORT_SYMBOL_GPL(klp_shadow_free_all);