linux-kernel - Re: [PATCH 05/15] x86: Implement function

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Message-ID: <20210416221414.GF22348@zn.tnic>
Date:   Sat, 17 Apr 2021 00:14:14 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Sami Tolvanen <samitolvanen@...gle.com>, X86 ML <x86@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        linux-hardening@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH 05/15] x86: Implement function_nocfi

On Fri, Apr 16, 2021 at 03:06:17PM -0700, Andy Lutomirski wrote:
> On Fri, Apr 16, 2021 at 3:03 PM Borislav Petkov <bp@...en8.de> wrote:
> >
> > On Fri, Apr 16, 2021 at 02:49:23PM -0700, Sami Tolvanen wrote:
> > > __nocfi only disables CFI checking in a function, the compiler still
> > > changes function addresses to point to the CFI jump table, which is
> > > why we need function_nocfi().
> >
> > So call it __func_addr() or get_function_addr() or so, so that at least
> > it is clear what this does.
> >
> 
> This seems backwards to me.  If I do:
> 
> extern void foo(some signature);
> 
> then I would, perhaps naively, expect foo to be the actual symbol that

I'm just reading the patch:

... The function_nocfi macro always returns the address of the
+ * actual function instead.
+ */
+#define function_nocfi(x) ({                                           \
+       void *addr;                                                     \
+       asm("leaq " __stringify(x) "(%%rip), %0\n\t" : "=r" (addr));    \
+       addr;

so it does a rip-relative load into a reg which ends up with the function
address.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette