linux-kernel - Re: [RFC PATCH 1/3] static_call: Add static call infrastructure

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Date:   Sat, 10 Nov 2018 12:56:33 +0100
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.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>
Subject: Re: [RFC PATCH 1/3] static_call: Add static call infrastructure

On 9 November 2018 at 21:34, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> On Fri, Nov 09, 2018 at 02:57:46PM -0500, Steven Rostedt wrote:
>> On Fri, 9 Nov 2018 13:35:05 -0600
>> Josh Poimboeuf <jpoimboe@...hat.com> wrote:
>>
..
>> > > So what's the reason for skipping init calls?
>> >
>> > This is the runtime changing code (static_call_update).  Presumably the
>> > init sections no longer exist and we shouldn't write to any (former)
>> > call sites there.
>> >
>> > That's probably a dangerous assumption though...  If
>> > static_call_update() were called early, some init code might not get
>> > patched and then call into the wrong function.
>> >
>> > I'm thinking we should just disallow static call sites in init sections.
>> > I can't think of a good reason why they would be needed in init code.
>> > We can WARN when detecting them during boot / module init.
>> >
>>
>> What I would do is to allow init (like ftrace now does). I have
>> ftrace_free_init_mem() that removes all the mcount references for init
>> calls from its list. You could add a static_call_free_init() to
>> kernel_init() in init/main.c too.
>
> That makes sense for ftrace, but I don't see much point in allowing it
> for static calls.  Maybe we could just add support for it later if it
> turns out to be useful.
>

I don't see how you can prevent that. Some arch may use a static call
in its version of some library code which could be used anywhere,
including in .init code.