linux-kernel - Re: [PATCH 4/4] x86: fix function types in COND

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Message-ID: <CABCJKufLKfd9RjtdAjCN4eccVhL1ztTOAc2nX4D+zzPhvM_c9g@mail.gmail.com>
Date:   Tue, 17 Sep 2019 15:44:05 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Kees Cook <keescook@...omium.org>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/4] x86: fix function types in COND_SYSCALL

On Fri, Sep 13, 2019 at 5:28 PM Andy Lutomirski <luto@...capital.net> wrote:
> Ah, I get it. Doesn’t this cause a little bit of code bloat, though?

A little bit yes, a few extra functions for syscalls that are not
otherwise implemented.

> What if you made __x86_ni_syscall, etc (possibly using the *DEFINE_SYSCALL0 macros) and then generate weak aliases to those?

That would be convenient, but COND_SYSCALL is used in kernel/sys_ni.c,
and we can't create an alias to a function defined elsewhere:

$ cat test.c
long b(void);
long a(void) __attribute__((alias("b")));
$ gcc -c test.c
test.c:2:6: error: ‘a’ aliased to undefined symbol ‘b’
 long a(void) __attribute__((alias("b")));
      ^

Curiously, when we use inline assembly to create the alias (similarly
to the current cond_syscall), gcc just quietly drops the alias if the
function is not defined.

Sami