linux-kernel - Re: [PATCH 11/15] static_call: Add inline static call infrastructure

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Message-ID: <37CFAEC1-6D36-4A6D-8C44-F85FCFA053AA@vmware.com>
Date:   Thu, 6 Jun 2019 22:24:17 +0000
From:   Nadav Amit <namit@...are.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     the arch/x86 maintainers <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Andy Lutomirski <luto@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Jason Baron <jbaron@...mai.com>, Jiri Kosina <jkosina@...e.cz>,
        David Laight <David.Laight@...LAB.COM>,
        Borislav Petkov <bp@...en8.de>,
        Julia Cartwright <julia@...com>, Jessica Yu <jeyu@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Edward Cree <ecree@...arflare.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH 11/15] static_call: Add inline static call infrastructure

> On Jun 5, 2019, at 6:08 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
> From: Josh Poimboeuf <jpoimboe@...hat.com>
> 
> Add infrastructure for an arch-specific CONFIG_HAVE_STATIC_CALL_INLINE
> option, which is a faster version of CONFIG_HAVE_STATIC_CALL.  At
> runtime, the static call sites are patched directly, rather than using
> the out-of-line trampolines.
> 
> Compared to out-of-line static calls, the performance benefits are more
> modest, but still measurable.  Steven Rostedt did some tracepoint
> measurements:

[ snip ]

> +static void static_call_del_module(struct module *mod)
> +{
> +	struct static_call_site *start = mod->static_call_sites;
> +	struct static_call_site *stop = mod->static_call_sites +
> +					mod->num_static_call_sites;
> +	struct static_call_site *site;
> +	struct static_call_key *key, *prev_key = NULL;
> +	struct static_call_mod *site_mod;
> +
> +	for (site = start; site < stop; site++) {
> +		key = static_call_key(site);
> +		if (key == prev_key)
> +			continue;
> +		prev_key = key;
> +
> +		list_for_each_entry(site_mod, &key->site_mods, list) {
> +			if (site_mod->mod == mod) {
> +				list_del(&site_mod->list);
> +				kfree(site_mod);
> +				break;
> +			}
> +		}
> +	}

I think that for safety, when a module is removed, all the static-calls
should be traversed to check that none of them calls any function in the
removed module. If that happens, perhaps it should be poisoned.

> +}
> +
> +static int static_call_module_notify(struct notifier_block *nb,
> +				     unsigned long val, void *data)
> +{
> +	struct module *mod = data;
> +	int ret = 0;
> +
> +	cpus_read_lock();
> +	static_call_lock();
> +
> +	switch (val) {
> +	case MODULE_STATE_COMING:
> +		module_disable_ro(mod);
> +		ret = static_call_add_module(mod);
> +		module_enable_ro(mod, false);

Doesn’t it cause some pages to be W+X ? Can it be avoided?

> +		if (ret) {
> +			WARN(1, "Failed to allocate memory for static calls");
> +			static_call_del_module(mod);

If static_call_add_module() succeeded in changing some of the calls, but not
all, I don’t think that static_call_del_module() will correctly undo
static_call_add_module(). The code transformations, I think, will remain.