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

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Date:   Fri, 26 Jan 2018 10:54:21 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     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 10:23 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.

             Linus