linux-kernel - Re: [PATCH v2 4/4] x86/static_call: Add inline static call implementation for x86-64

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Date:   Thu, 29 Nov 2018 10:00:48 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Lutomirski <luto@...nel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>, mhiramat@...nel.org,
        jbaron@...mai.com, Jiri Kosina <jkosina@...e.cz>,
        David.Laight@...lab.com, bp@...en8.de, julia@...com,
        jeyu@...nel.org, Peter Anvin <hpa@...or.com>
Subject: Re: [PATCH v2 4/4] x86/static_call: Add inline static call implementation for x86-64


> On Nov 29, 2018, at 9:50 AM, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
>> On Thu, Nov 29, 2018 at 9:44 AM Steven Rostedt <rostedt@...dmis.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

For C calls, we do indeed know that.  But I guess there could be asm calls.

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

And how do we do that?  With a gcc plugin and some asm magic?

> 
> 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.

That’s not a terrible idea.

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