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

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Message-ID: <CALCETrX1bF4ECNT7sJeg6ai3BGkHGDgwcf_6rppDbAaf1DatQA@mail.gmail.com>
Date:   Fri, 26 Jan 2018 11:02:54 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Alan Cox <alan@...ux.intel.com>,
        David Laight <David.Laight@...lab.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        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 Fri, Jan 26, 2018 at 10:54 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Fri, Jan 26, 2018 at 10:23 AM, Andy Lutomirski <luto@...nel.org> wrote:
>>
>> The issue is that doing it this way gives us, effectively:
>>
>> long sys_foo(int a, int b)
>> {
>>   body here;
>> }
>>
>> long SyS_foo(const struct pt_regs *regs)
>> {
>>   return sys_foo(regs->di, regs->si);
>> }
>>
>> whereas what we want is *static* long sys_foo(...).
>
> How about just marking 'sys_foo()' as being always_inline (but still
> not static)? Because the case that _matters_ is that SyS_foo(), thing
> when this is enabled.
>
> Sure, you'll get two copies of the code (one in SyS_foo(), the other
> being the callable-from C 'sys_foo()' that is exported and almost
> never used). But that seems a fairly small price to pay. We could
> leave it for later to try to get rid of the unused copies entirely.
>

I could do that, but Josh Triplett will yell at me.

Anyway, I'll fiddle with it.  This isn't exactly high priority.