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

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Date:   Mon, 12 Nov 2018 06:02:33 +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 12 November 2018 at 05:56, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> On Mon, Nov 12, 2018 at 05:39:38AM +0100, Ard Biesheuvel wrote:
>> On 12 November 2018 at 04:07, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
>> > On Sat, Nov 10, 2018 at 08:09:17AM -0500, Steven Rostedt wrote:
>> >> On Sat, 10 Nov 2018 12:58:08 +0100
>> >> Ard Biesheuvel <ard.biesheuvel@...aro.org> wrote:
>> >>
>> >>
>> >> > > The complaint is on:
>> >> > >
>> >> > >         DEFINE_STATIC_CALL(__tp_func_##name, __tracepoint_iter_##name);
>> >> > >
>> >> > > And the previous definition is on:
>> >> > >
>> >> > >         DECLARE_STATIC_CALL(__tp_func_##name, __tracepoint_iter_##name); \
>> >> > >
>> >> >
>> >> > Does the DECLARE really need the __ADDRESSABLE? Its purpose is to
>> >> > ensure that symbols with static linkage are not optimized away, but
>> >> > since the reference is from a header file, the symbol should have
>> >> > external linkage anyway.
>> >
>> > Yes, DECLARE needs the __ADDRESSABLE.  In the case where DECLARE
>> > is used, but DEFINE is not, GCC strips the symbol.
>> >
>>
>> I assume DECLARE() is intended for use in header files, and DEFINE()
>> for source files, no?
>
> Right.
>
>> Doesn't that mean that whatever symbol __ADDRESSABLE() refers to
>> should have external linkage, in which case it it addressable anyway?
>> Or are we talking about some LTO / --gc-sections use case here?
>
> If the key is declared, but not used, GCC doesn't put the key's ELF
> symbol in the binary's symbol table.  That makes objtool's life harder,
> because if the file has a call site, then objtool has to add the key
> symbol to the symbol table, so that it can create a relocation (in the
> call site table) which references the symbol.
>

Ah, right. So this is specific to objtool rather than a generic issue.

Should we perhaps wrap __ADDRESSABLE() into something else that
resolves to a NOP unless on X86, and give it a clear name?