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From:   Ard Biesheuvel <ard.biesheuvel@linaro.org>
Date:   Sun, 11 Nov 2018 14:42:55 +0100
Message-ID: <CAKv+Gu_y2vH3eu7rMt7d7Hzum_MvYy7=30sNe1fhnB2QLe5i5A@mail.gmail.com>
Subject: Re: [PATCH RFC 0/3] Static calls
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Ingo Molnar <mingo@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Jason Baron <jbaron@akamai.com>, Jiri Kosina <jkosina@suse.cz>,
        David Laight <David.Laight@aculab.com>,
        Borislav Petkov <bp@alien8.de>
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On 11 November 2018 at 00:20, Peter Zijlstra <peterz@infradead.org> wrote:
> On Fri, Nov 09, 2018 at 02:50:27PM +0100, Ard Biesheuvel wrote:
>> On 9 November 2018 at 08:28, Ingo Molnar <mingo@kernel.org> wrote:
>> >> - I'm not sure about the objtool approach.  Objtool is (currently)
>> >>   x86-64 only, which means we have to use the "unoptimized" version
>> >>   everywhere else.  I may experiment with a GCC plugin instead.
>> >
>> > I'd prefer the objtool approach. It's a pretty reliable first-principles
>> > approach while GCC plugin would have to be replicated for Clang and any
>> > other compilers, etc.
>> >
>>
>> I implemented the GCC plugin approach here for arm64
>
> I'm confused; I though we only needed objtool for variable instruction
> length architectures, because we can't reliably decode our instruction
> stream. Otherwise we can fairly trivially use the DWARF relocation data,
> no?

How would that work? We could build vmlinux with --emit-relocs, filter
out the static jump/call relocations and resolve the symbol names to
filter the ones associated with calls to trampolines. But then, we
have to build the static_call_sites section and reinject it back into
the image in some way, which is essentially objtool, no?