linux-kernel - Re: [POC][RFC][PATCH 1/2] jump_function: Addition of new feature "jump

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Message-Id: <F36C6419-7D92-4357-BDB1-D8B90DE09889@amacapital.net>
Date:   Mon, 8 Oct 2018 09:29:56 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Matthew Helsley <mhelsley@...are.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jason Baron <jbaron@...mai.com>, Jiri Kosina <jkosina@...e.cz>,
        ard.biesheuvel@...aro.org, Andy Lutomirski <luto@...nel.org>
Subject: Re: [POC][RFC][PATCH 1/2] jump_function: Addition of new feature "jump_function"



> On Oct 8, 2018, at 8:57 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
> On Mon, Oct 08, 2018 at 01:33:14AM -0700, Andy Lutomirski wrote:
>>> Can't we hijack the relocation records for these functions before they
>>> get thrown out in the (final) link pass or something?
>> 
>> I could be talking out my arse here, but I thought we could do this,
>> too, then changed my mind.  The relocation records give us the
>> location of the call or jump operand, but they don’t give the address
>> of the beginning of the instruction.
> 
> But that's like 1 byte before the operand, right? We could even double check
> this by reading back that byte and ensuring it is in fact 0xE8 (CALL).
> 
> AFAICT there is only the _1_ CALL encoding, and that is the 5 byte: E8 <PLT32>,
> so if we have the PLT32 location, we also have the instruction location. Or am
> I missing something?

There’s also JMP and Jcc, any of which can be used for rail calls, but those are also one byte. I suppose GCC is unlikely to emit a prefixed form of any of these. So maybe we really can assume they’re all one byte.

But there is a nasty potential special case: anything that takes the function’s address. This includes jump tables, computed gotos, and plain old function pointers. And I suspect that any of these could have one of the rather large number of CALL/JMP/Jcc bytes before the relocation by coincidence.