On Fri, Jul 28, 2017 at 01:25:22PM -0400, Joe Lawrence wrote:
> Add exported API for livepatch modules:
> 
>   klp_shadow_get()
>   klp_shadow_attach()
>   klp_shadow_get_or_attach()
>   klp_shadow_update_or_attach()
>   klp_shadow_detach()
>   klp_shadow_detach_all()

I like the API.

> +Matching parent's lifecycle
> +---------------------------
> +
> +If parent data structures are frequently created and destroyed, it may
> +be easiest to align its shadow variable lifetimes to the same allocation

"its shadow variable lifetimes" -> "their shadow variables' lifetimes" ?

> +void *klp_shadow_attach(void *obj, unsigned long id, void *data,
> +                     size_t size, gfp_t gfp_flags)

[ Note: some of the following comments also apply to
  klp_shadow_get_or_attach and klp_shadow_update_or_attach. ]

> +{
> +     struct klp_shadow *new_shadow;

nit: The "new" is implied, and there's only one shadow struct used in
the function, so maybe just call it "shadow"?

> +     void *shadow_data;
> +     unsigned long flags;
> +
> +     /* Take error exit path if <obj, id> already exists */
> +     shadow_data = klp_shadow_get(obj, id);
> +     if (unlikely(shadow_data))
> +             goto err_exists;

The return value of klp_shadow_get() can be tested directly, no need for
a variable.

> +     /* Allocate a new shadow variable for use inside the lock below */
> +     new_shadow = kzalloc(size + sizeof(*new_shadow), gfp_flags);
> +     if (!new_shadow)
> +             goto err;

The goto destination is just a single line return, so I think it's
clearer to just return NULL here and get rid of the err label.

> +
> +     /* 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 found, throw away allocation and take
> +              * error exit path */

Multi-line comments should be in the kernel coding style:

                /*
                 * Shadow variable found, throw away allocation and take
                 * error exit path
                 */

Also, complete sentences preferred :-)

> +             spin_unlock_irqrestore(&klp_shadow_lock, flags);
> +             kfree(shadow_data);

Shouldn't it free new_shadow instead of shadow_data?

> +             goto err_exists;
> +     }
> +
> +     /* No <obj, id> found, add the newly allocated one */
> +     shadow_data = data;
> +     klp_shadow_set(new_shadow, obj, id, data, size);

To avoid doing extra work with the lock held, the klp_shadow_set() can
be done before getting the lock, after the kzalloc.


-- 
Josh

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