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Date: Mon, 17 Jul 2017 17:29:41 +0200 (CEST)
From: Miroslav Benes <mbenes@...e.cz>
To: Joe Lawrence <joe.lawrence@...hat.com>
cc: live-patching@...r.kernel.org, linux-kernel@...r.kernel.org,
Josh Poimboeuf <jpoimboe@...hat.com>,
Jessica Yu <jeyu@...hat.com>, Jiri Kosina <jikos@...nel.org>,
Petr Mladek <pmladek@...e.com>
Subject: Re: [PATCH v2 1/2] livepatch: introduce shadow variable API
On Wed, 28 Jun 2017, Joe Lawrence wrote:
> +Brief API summary
> +-----------------
> +
> +See the full API usage docbook notes in the livepatch/shadow.c
> +implementation.
> +
> +An in-kernel hashtable references all of the shadow variables. These
> +references are stored/retrieved through a <obj, num> key pair.
> +
> +* The klp_shadow variable data structure encapsulates both tracking
> +meta-data and shadow-data:
> + - meta-data
> + - obj - pointer to original data
> + - num - numerical description of new data
> + - new_data[] - storage for shadow data
> +
> +* klp_shadow_attach() - allocate and add a new shadow variable:
> + - allocate a new shadow variable
> + - push a <obj, num> key pair into hashtable
> +
> +* klp_shadow_get() - retrieve a shadow variable new_data pointer
> + - search hashtable for <obj, num> key pair
> +
> +* klp_shadow_get_or_attach() - get existing or attach a new shadow variable
> + - search hashtable for <obj, num> key pair
> + - if not found, call klp_shadow_attach()
> +
> +* klp_shadow_detach() - detach and free a <obj, num> shadow variable
> + - find and remove any <obj, num> references from hashtable
> + - if found, release shadow variable
> +
> +* klp_shadow_detach() - detach and free all <*, num> shadow variables
> + - find and remove any <*, num> references from hashtable
> + - if found, release shadow variable
I think that the second one should be klp_shadow_detach_all(), shouldn't
it?
[...]
> +static DEFINE_HASHTABLE(klp_shadow_hash, 12);
Is there a reason, why you pick 12? I'm just curious.
> +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 original data
> + * @num: numerical description of new data
Josh proposed better description. Could we also have a note somewhere in
the documentation what this member is practically for? I mean versioning
and ability to attach new members to a data structure if live patches are
stacked.
> + * @new_data: new data area
> + */
> +struct klp_shadow {
> + struct hlist_node node;
> + struct rcu_head rcu_head;
> + void *obj;
> + unsigned long num;
> + char new_data[];
> +};
What is the reason to change 'void *new_data' to 'char new_data[]'? I
assume it is related to API changes below...
[...]
> +/**
> + * _klp_shadow_attach() - allocate and add a new shadow variable
> + * @obj: pointer to original data
> + * @num: numerical description of new data
> + * @new_data: pointer to new data
> + * @new_size: size of new data
> + * @gfp_flags: GFP mask for allocation
> + * @lock: take klp_shadow_lock during klp_shadow_hash operations
I am not sure about lock argument. Do we need it? Common practice is to
have function foo() which takes a lock, and function __foo() which does
not.
In klp_shadow_get_or_attach(), you use it as I'd expect. You take the
spinlock, call this function and release the spinlock. Is it possible
to do the same in klp_shadow_attach() and have __klp_shadow_attach()
without lock argument?
> + *
> + * Note: allocates @new_size space for shadow variable data and copies
> + * @new_size bytes from @new_data into the shadow varaible's own @new_data
> + * space. If @new_data is NULL, @new_size is still allocated, but no
> + * copy is performed.
I must say I'm not entirely happy with this. I don't know if this is what
Petr had in mind (I'm sure he'll get to the patch set soon). Calling
memcpy instead of a simple assignment in v1 seems worse.
I'll take another look tomorrow and will think about it.
Miroslav
> + * Return: the shadow variable new_data element, NULL on failure.
> + */
> +static void *_klp_shadow_attach(void *obj, unsigned long num, void *new_data,
> + size_t new_size, gfp_t gfp_flags,
> + bool lock)
> +{
> + struct klp_shadow *shadow;
> + unsigned long flags;
> +
> + shadow = kzalloc(new_size + sizeof(*shadow), gfp_flags);
> + if (!shadow)
> + return NULL;
> +
> + shadow->obj = obj;
> + shadow->num = num;
> + if (new_data)
> + memcpy(shadow->new_data, new_data, new_size);
> +
> + if (lock)
> + spin_lock_irqsave(&klp_shadow_lock, flags);
> + hash_add_rcu(klp_shadow_hash, &shadow->node, (unsigned long)obj);
> + if (lock)
> + spin_unlock_irqrestore(&klp_shadow_lock, flags);
> +
> + return shadow->new_data;
> +}
> +
> +/**
> + * klp_shadow_attach() - allocate and add a new shadow variable
> + * @obj: pointer to original data
> + * @num: numerical description of new num
> + * @new_data: pointer to new data
> + * @new_size: size of new data
> + * @gfp_flags: GFP mask for allocation
> + *
> + * Return: the shadow variable new_data element, NULL on failure.
> + */
> +void *klp_shadow_attach(void *obj, unsigned long num, void *new_data,
> + size_t new_size, gfp_t gfp_flags)
> +{
> + return _klp_shadow_attach(obj, num, new_data, new_size,
> + gfp_flags, true);
> +}
> +EXPORT_SYMBOL_GPL(klp_shadow_attach);
> +
> +/**
> + * klp_shadow_get_or_attach() - get existing or attach a new shadow variable
> + * @obj: pointer to original data
> + * @num: numerical description of new data
> + * @new_data: pointer to new data
> + * @new_size: size of new data
> + * @gfp_flags: GFP mask used to allocate shadow variable metadata
> + *
> + * Note: if memory allocation is necessary, it will do so under a spinlock,
> + * so @gfp_flags should include GFP_NOWAIT, or GFP_ATOMIC, etc.
> + *
> + * Return: the shadow variable new_data element, NULL on failure.
> + */
> +void *klp_shadow_get_or_attach(void *obj, unsigned long num, void *new_data,
> + size_t new_size, gfp_t gfp_flags)
> +{
> + void *nd;
> + unsigned long flags;
> +
> + nd = klp_shadow_get(obj, num);
> +
> + if (!nd) {
> + spin_lock_irqsave(&klp_shadow_lock, flags);
> + nd = klp_shadow_get(obj, num);
> + if (!nd)
> + nd = _klp_shadow_attach(obj, num, new_data, new_size,
> + gfp_flags, false);
> + spin_unlock_irqrestore(&klp_shadow_lock, flags);
> + }
> +
> + return nd;
> +
> +}
> +EXPORT_SYMBOL_GPL(klp_shadow_get_or_attach);
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