linux-kernel - Re: [PATCH 3/3] static_call: Fix static_call

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Date:   Thu, 18 Mar 2021 17:58:38 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     x86@...nel.org, jbaron@...mai.com, rostedt@...dmis.org,
        ardb@...nel.org, linux-kernel@...r.kernel.org,
        sumit.garg@...aro.org, oliver.sang@...el.com, jarkko@...nel.org
Subject: Re: [PATCH 3/3] static_call: Fix static_call_update() sanity check

On Thu, Mar 18, 2021 at 11:13:08AM -0500, Josh Poimboeuf wrote:
> On Thu, Mar 18, 2021 at 12:31:59PM +0100, Peter Zijlstra wrote:
> >  			if (!kernel_text_address((unsigned long)site_addr)) {
> > -				WARN_ONCE(1, "can't patch static call site at %pS",
> > +				/*
> > +				 * This skips patching __exit, which is part of

				  This skips patching built-in __exit, ...
?

> > +				 * init_section_contains() but is not part of
> > +				 * kernel_text_address().
> > +				 *
> > +				 * Skipping __exit is fine since it will never

		+ built-in, again

> > +				 * be executed.
> > +				 */
> > +				WARN_ONCE(!static_call_is_init(site),
> > +					  "can't patch static call site at %pS",
> >  					  site_addr);
> >  				continue;
> >  			}
> 
> It might be good to clarify the situation for __exit in modules in the
> comment and/or changelog, as they both seem to be implicitly talking
> only about __exit in vmlinux.

Correct.

> For CONFIG_MODULE_UNLOAD, the code ends up in the normal text area, so
> static_call_is_init() is false and kernel_text_address() is true.
> 
> For !CONFIG_MODULE_UNLOAD, the code gets discarded during module load,
> so static_call_is_init() and kernel_text_address() are both false.  I
> guess that will trigger a warning?

Oh gawd, more variants.

Afaict MODULE_UNLOAD, by virtue of that #ifdef in
rewrite_section_headers() won't even load the .exit sections. Afaict
that will break: alterative, jump_label and static_call patching all in
one go.