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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 29 Nov 2018 09:50:28 -0800
Message-ID: <CAHk-=wgnZobye-KNaBVO-SdqX-5Xkz+i=aJ_kOFZ4RpZFT0HqQ@mail.gmail.com>
Subject: Re: [PATCH v2 4/4] x86/static_call: Add inline static call
 implementation for x86-64
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Andy Lutomirski <luto@amacapital.net>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Andrew Lutomirski <luto@kernel.org>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, mhiramat@kernel.org,
        jbaron@akamai.com, Jiri Kosina <jkosina@suse.cz>,
        David.Laight@aculab.com, bp@alien8.de, julia@ni.com,
        jeyu@kernel.org, Peter Anvin <hpa@zytor.com>
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On Thu, Nov 29, 2018 at 9:44 AM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> Well, the current method (as Jiri mentioned) did get the OK from at
> least Intel (and that was with a lot of arm twisting to do so).

Guys, when the comparison is to:

 - create a huge honking security hole by screwing up the stack frame

or

 - corrupt random registers because we "know" they aren't in use

then it really sounds pretty safe to just say "ok, just make it
aligned and update the instruction with an atomic cmpxchg or
something".

Of course, another option is to just say "we don't do the inline case,
then", and only ever do a call to a stub that does a "jmp"
instruction.

Problem solved, at the cost of some I$. Emulating a "jmp" is trivial,
in ways emulating a "call" is not.

              Linus