linux-kernel - Re: [PATCH] x86/retpoline/entry: Disable the entire SYSCALL64 fast path with retpolines on

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Date:   Thu, 25 Jan 2018 13:31:11 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alan Cox <alan@...ux.intel.com>, Jann Horn <jannh@...gle.com>,
        Samuel Neves <samuel.c.p.neves@...il.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH] x86/retpoline/entry: Disable the entire SYSCALL64 fast
 path with retpolines on

On Thu, Jan 25, 2018 at 1:20 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Thu, Jan 25, 2018 at 1:08 PM, Andy Lutomirski <luto@...nel.org> wrote:
>>
>> With retpoline, the retpoline in the trampoline sucks.  I don't need
>> perf for that -- I've benchmarked it both ways.  It sucks.  I'll fix
>> it, but it'll be kind of complicated.
>
> Ahh, I'd forgotten about that (and obviously didn't see it in the profiles).
>
> But yeah, that is fixable even if it does require a page per CPU. Or
> did you have some clever scheme in mind?

Nothing clever.  I was going to see if I could get actual
binutils-generated relocations to work in the trampoline.  We already
have code to parse ELF relocations and turn them into a simple table,
and it shouldn't be *that* hard to run a separate pass on the entry
trampoline.

Another potentially useful if rather minor optimization would be to
rejigger the SYSCALL_DEFINE macros a bit.  Currently we treat all
syscalls like this:

long func(long arg0, long arg1, long arg2, long arg3, long arg4, long arg5);

I wonder if we'd be better off doing:

long func(const struct pt_regs *regs);

and autogenerating:

static long SyS_read(const struct pt_regs *regs)
{
   return sys_reg(regs->di, ...);
}