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Date:   Mon, 8 Oct 2018 12:31:40 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Andy Lutomirski <luto@amacapital.net>,
        linux-kernel@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Matthew Helsley <mhelsley@vmware.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
        David Woodhouse <dwmw2@infradead.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Jason Baron <jbaron@akamai.com>, Jiri Kosina <jkosina@suse.cz>,
        ard.biesheuvel@linaro.org, Andy Lutomirski <luto@kernel.org>
Subject: Re: [POC][RFC][PATCH 1/2] jump_function: Addition of new feature
 "jump_function"
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On Mon, 8 Oct 2018 17:57:57 +0200
Peter Zijlstra <peterz@infradead.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?

Yes, this is exactly what I was thinking of doing. All we need to do is
have objtool (or a modification of whatever we come up with), to find
the call sites of a specific function (we can have a table to look up
for), that creates a section listing all these call sites, and on boot
up, we can confirm that they are indeed calls (e8 operations).

-- Steve