linux-kernel - Re: [PATCH v5] livepatch: introduce shadow variable API

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Date:   Thu, 31 Aug 2017 16:54:21 +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 v5] livepatch: introduce shadow variable API

On Mon, 21 Aug 2017, Joe Lawrence wrote:

> diff --git a/Documentation/livepatch/shadow-vars.txt b/Documentation/livepatch/shadow-vars.txt
> new file mode 100644
> index 000000000000..d7558a1e4797
> --- /dev/null
> +++ b/Documentation/livepatch/shadow-vars.txt
> @@ -0,0 +1,191 @@
> +
> +Matching parent's lifecycle
> +---------------------------
> +
> +If parent data structures are frequently created and destroyed, it may
> +be easiest to align their shadow variables lifetimes to the same
> +allocation and release functions.  In this case, the parent data
> +structure is typically allocated, initialized, then registered in some
> +manner.  Shadow variable allocation and setup can then be considered
> +part of the parent's initialization and should be completed before the
> +parent "goes live" (ie, any shadow variable get-API requests are made
> +for this <obj, id> pair.

')' is missing.

> diff --git a/kernel/livepatch/shadow.c b/kernel/livepatch/shadow.c
> new file mode 100644
> index 000000000000..83ca71454584
> --- /dev/null
> +++ b/kernel/livepatch/shadow.c
> @@ -0,0 +1,295 @@
> +/*
> + * shadow.c - Shadow Variables
> + *
> + * Copyright (C) 2014 Josh Poimboeuf <jpoimboe@...hat.com>
> + * Copyright (C) 2014 Seth Jennings <sjenning@...hat.com>
> + * Copyright (C) 2017 Joe Lawrence <joe.lawrence@...hat.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.

I think that the last sentence should appear somewhere in the 
documentation.

> +void *__klp_shadow_get_or_alloc(void *obj, unsigned long id, void *data,
> +		       size_t size, gfp_t gfp_flags,
> +		       enum klp_shadow_existing_handling existing_handling)
> +{
> +	struct klp_shadow *new_shadow;
> +	void *shadow_data;
> +	unsigned long flags;
> +
> +	/* Check if the shadow variable if <obj, id> already exists */
> +	shadow_data = klp_shadow_get(obj, id);
> +	if (shadow_data)
> +		goto exists;
> +
> +	/* Allocate a new shadow variable for use inside the lock below */
> +	new_shadow = kzalloc(size + sizeof(*new_shadow), gfp_flags);
> +	if (!new_shadow)
> +		return NULL;
> +
> +	new_shadow->obj = obj;
> +	new_shadow->id = id;
> +
> +	/* Initialize the shadow variable if data provided */
> +	if (data)
> +		memcpy(new_shadow->data, data, size);
> +
> +	/* 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 and update/return the existing one.
> +		 */

The comment is outdated.

Miroslav