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SubjectRe: [PATCH v7 6/7] livepatch: Add atomic replace
On Tue, 6 Feb 2018, Petr Mladek wrote:

> From: Jason Baron <jbaron@akamai.com>
>
> Sometimes we would like to revert a particular fix. Currently, this
> is not easy because we want to keep all other fixes active and we
> could revert only the last applied patch.
>
> One solution would be to apply new patch that implemented all
> the reverted functions like in the original code. It would work
> as expected but there will be unnecessary redirections. In addition,
> it would also require knowing which functions need to be reverted at
> build time.
>
> Another problem is when there are many patches that touch the same
> functions. There might be dependencies between patches that are
> not enforced on the kernel side. Also it might be pretty hard to
> actually prepare the patch and ensure compatibility with
> the other patches.
>
> A better solution would be to create cumulative patch and say that
> it replaces all older ones.
>
> This patch adds a new "replace" flag to struct klp_patch. When it is
> enabled, a set of 'nop' klp_func will be dynamically created for all
> functions that are already being patched but that will not longer be
> modified by the new patch. They are temporarily used as a new target
> during the patch transition.
>
> There are used several simplifications:
>
> + nops' structures are generated already when the patch is registered.
> All registered patches are taken into account, even the disabled ones.
> As a result, there might be more nops than are really needed when
> the patch is enabled and some disabled patches were removed before.
> But we are on the safe side and it simplifies the implementation.
> Especially we could reuse the existing init() functions. Also freeing
> is easier because the same nops are created and removed only once.
>
> Alternative solution would be to create nops when the patch is enabled.
> But then we would need to complicated to reuse the init() functions

I cannot parse this.

> and error paths. It would increase the risk of errors because of
> late kobject initialization. It would need tricky waiting for
> freed kobjects when finalizing a reverted enable transaction.
>
> + The replaced patches are removed from the stack and cannot longer
> be enabled directly. Otherwise, we would need to implement a more
> complex logic of handling the stack of patches. It might be hard
> to come with a reasonable semantic.
>
> A fallback is to remove (rmmod) the replaced patches and register
> (insmod) them again.
>
> + Nops are handled like normal function patches. It reduces changes
> in the existing code.
>
> It would be possible to copy internal values when they are allocated
> and make short cuts in init() functions. It would be possible to use
> the fact that old_func and new_func point to the same function and
> do not init new_func and new_size at all. It would be possible to
> detect nop func in ftrace handler and just leave. But all these would
> just complicate the code and maintenance.
>
> + The callbacks from the replaced patches are not called. It would be
> pretty hard to define a reasonable semantic and implement it.
>
> It might even be counter-productive. The new patch is cumulative.
> It is supposed to include most of the changes from older patches.
> In most cases, it will not want to call pre_unpatch() post_unpatch()
> callbacks from the replaced patches. It would disable/break things
> for no good reasons. Also it should be easier to handle various
> scenarios in a single script in the new patch than think about
> interactions caused by running many scripts from older patches.
> No to say that the old scripts even would not expect to be called
> in this situation.
>
> Signed-off-by: Jason Baron <jbaron@akamai.com>
> [pmladek@suse.com: Split, reuse existing code, simplified]
> Signed-off-by: Petr Mladek <pmladek@suse.com>
> Cc: Josh Poimboeuf <jpoimboe@redhat.com>
> Cc: Jessica Yu <jeyu@kernel.org>
> Cc: Jiri Kosina <jikos@kernel.org>
> Cc: Miroslav Benes <mbenes@suse.cz>
> Cc: Petr Mladek <pmladek@suse.com>

[...]

> +/*
> + * This function removes replaced patches from both func_stack
> + * and klp_patches stack.
> + *
> + * We could be pretty aggressive here. It is called in situation
> + * when these structures are not longer accessible. All functions

...are no longer... :)

> + * are redirected using the klp_transition_patch. They use either
> + * a new code or they in the original code because of the special

...or they are... ?

> + * nop function patches.
> + */

[...]

> +static struct klp_func *klp_alloc_func_nop(struct klp_func *old_func,
> + struct klp_object *obj)
> +{
> + struct klp_func *func;
> +
> + func = kzalloc(sizeof(*func), GFP_KERNEL);
> + if (!func)
> + return ERR_PTR(-ENOMEM);
> +
> + if (old_func->old_name) {
> + func->old_name = kstrdup(old_func->old_name, GFP_KERNEL);
> + if (!func->old_name) {
> + kfree(func);
> + return ERR_PTR(-ENOMEM);
> + }
> + }
> + func->old_sympos = old_func->old_sympos;
> + /* NOP func is the same as using the original implementation. */
> + func->new_func = (void *)old_func->old_addr;
> + func->ftype = KLP_FUNC_NOP;
> +
> + return func;
> +}
> +
> +static int klp_add_func_nop(struct klp_object *obj,
> + struct klp_func *old_func)
> +{
> + struct klp_func *func;
> +
> + func = klp_find_func(obj, old_func);
> +
> + if (func)
> + return 0;
> +
> + func = klp_alloc_func_nop(old_func, obj);
> + if (IS_ERR(func))
> + return PTR_ERR(func);
> +
> + klp_init_func_list(obj, func);
> +
> + return 0;
> +}
> +
> +static int klp_add_object_nops(struct klp_patch *patch,
> + struct klp_object *old_obj)
> +{
> + struct klp_object *obj;
> + struct klp_func *old_func;
> + int err = 0;
> +
> + obj = klp_get_or_add_object(patch, old_obj);
> + if (IS_ERR(obj))
> + return PTR_ERR(obj);
> +
> + klp_for_each_func(old_obj, old_func) {
> + err = klp_add_func_nop(obj, old_func);
> + if (err)
> + return err;
> + }
> +
> + return 0;
> +}
> +
> +/*
> + * Add 'nop' functions which simply return to the caller to run
> + * the original function. The 'nop' functions are added to a
> + * patch to facilitate a 'replace' mode
> + *
> + * The nops are generated for all patches on the stack when
> + * the new patch is initialized. It is safe even though some
> + * older patches might get disabled and removed before the
> + * new one is enabled. In the worst case, there might be nops
> + * there will not be really needed. But it does not harm and

s/there/which/ ?

> + * simplifies the implementation a lot. Especially we could
> + * use the init functions as is.
> + */
> +static int klp_add_nops(struct klp_patch *patch)
> +{
> + struct klp_patch *old_patch;
> + struct klp_object *old_obj;
> + int err = 0;
> +
> + if (WARN_ON(!patch->replace))
> + return -EINVAL;
> +
> + list_for_each_entry(old_patch, &klp_patches, list) {
> + klp_for_each_object(old_patch, old_obj) {
> + err = klp_add_object_nops(patch, old_obj);
> + if (err)
> + return err;
> + }
> + }
> +
> + return 0;
> +}

So again only nits. Otherwise I think the patch does what is supposed to.

Acked-by: Miroslav Benes <mbenes@suse.cz>

Miroslav

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