linux-kernel - Re: [PATCH v3 0/6] Static calls

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Message-ID: <F6735FF5-4D62-4C4E-A145-751E6469CE9E@vmware.com>
Date:   Fri, 11 Jan 2019 15:48:59 +0000
From:   Nadav Amit <namit@...are.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
CC:     Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Jason Baron <jbaron@...mai.com>, Jiri Kosina <jkosina@...e.cz>,
        David Laight <David.Laight@...lab.com>,
        Borislav Petkov <bp@...en8.de>,
        Julia Cartwright <julia@...com>, Jessica Yu <jeyu@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Edward Cree <ecree@...arflare.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: Re: [PATCH v3 0/6] Static calls

> On Jan 11, 2019, at 7:15 AM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> 
> On Fri, Jan 11, 2019 at 01:47:01AM +0000, Nadav Amit wrote:
>> Here is an alternative idea (although similar to Steven’s and my code).
>> 
>> Assume that we always clobber R10, R11 on static-calls explicitly, as anyhow
>> should be done by the calling convention (and gcc plugin should allow us to
>> enforce). Also assume that we hold a table with all source RIP and the
>> matching target.
>> 
>> Now, in the int3 handler can you take the faulting RIP and search for it in
>> the “static-calls” table, writing the RIP+5 (offset) into R10 (return
>> address) and the target into R11. You make the int3 handler to divert the
>> code execution by changing pt_regs->rip to point to a new function that does:
>> 
>> 	push R10
>> 	jmp __x86_indirect_thunk_r11
>> 
>> And then you are done. No?
> 
> IIUC, that sounds pretty much like what Steven proposed:
> 
>  https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Flkml.kernel.org%2Fr%2F20181129122000.7fb4fb04%40gandalf.local.home&amp;data=02%7C01%7Cnamit%40vmware.com%7Ce3f0b96a1e83417af48808d677d7a147%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C0%7C0%7C636828165370908292&amp;sdata=PFzrJQzoa21IRYmEuqHSSGYrNZt0zIo8TGOZa3NWbOE%3D&amp;reserved=0

Stupid me. I’ve remembered it slightly different (the caller saving the
target in a register).

> I liked the idea, BUT, how would it work for callee-saved PV ops?  In
> that case there's only one clobbered register to work with (rax).

That’s would be more tricky. How about using a per-CPU trampoline code to
hold a direct call to the target and temporarily disable preemption (which
might be simpler by disabling IRQs):

Static-call modifier:

        1. synchronize_sched() to ensure per-cpu trampoline is not used
	2. Patches the jmp in a per-cpu trampoline (see below)
	3. Saves the call source RIP in [per-cpu scratchpad RIP] (below) 
	4. Configures the int3 handler to use static-call int3 handler
	5. Patches the call target (as it currently does).

Static-call int3 handler:
	1. Changes flags on the stack to keep IRQs disabled on return
	2. Jumps to per-cpu trampoline on return

Per-cpu trampoline:
	push [per-CPU scratchpad RIP]
	sti
	jmp [ target ] (this one is patched)

Note that no IRQ should be possible between the STI and the JMP due to STI
blocking.

What do you say?