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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 11 Jan 2019 12:12:30 -0800
Message-ID: <CAHk-=whLu9kG3dqgXUkCrNrTp0NVtdOOWxvonXTfhtq-krXo+Q@mail.gmail.com>
Subject: Re: [PATCH v3 0/6] Static calls
To:     Josh Poimboeuf <jpoimboe@redhat.com>
Cc:     Nadav Amit <namit@vmware.com>, Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        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>,
        Julia Cartwright <julia@ni.com>, Jessica Yu <jeyu@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Edward Cree <ecree@solarflare.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>
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On Fri, Jan 11, 2019 at 12:04 PM Josh Poimboeuf <jpoimboe@redhat.com> wrote:
>
> But really, to me, having to create and manage all those custom
> trampolines still feels a lot more complex than just making a gap on the
> stack.

There are no "all those custom trampolines".

There is literally *one* custom trampoline that you generate as you do
the rewriting.

Well, two, since you need the version with the "sti" before the jmp.

It would be possible to generate the custom trampoline on the fly in
the BP handler itself, and just have a magic flag for that case. But
it's probably simpler to do it in the caller, since you need to
generate that special writable and executable code sequence. You
probably don't want to do that at BP time.

You probably want to use a FIX_TEXT_POKE2 page for the generated
sequence that just maps some generated code executably for a short
while. Or something like that.

              Linus

             Linus