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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Sami Tolvanen <samitolvanen@google.com>,
        Borislav Petkov <bp@alien8.de>
Cc:     X86 ML <x86@kernel.org>, Kees Cook <keescook@chromium.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Sedat Dilek <sedat.dilek@gmail.com>,
        linux-hardening@vger.kernel.org,
        LKML <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>
Subject: Re: [PATCH 05/15] x86: Implement function_nocfi
In-Reply-To: <CABCJKud8TvzhcjHCpsrtCJ4B50ZUfaL48F42EhZ2zWKLteAc0Q@mail.gmail.com>
References: <20210416203844.3803177-1-samitolvanen@google.com> <20210416203844.3803177-6-samitolvanen@google.com> <20210416211855.GD22348@zn.tnic> <CABCJKud8TvzhcjHCpsrtCJ4B50ZUfaL48F42EhZ2zWKLteAc0Q@mail.gmail.com>
Date:   Sat, 17 Apr 2021 00:13:29 +0200
Message-ID: <87mttxubcm.ffs@nanos.tec.linutronix.de>
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On Fri, Apr 16 2021 at 14:49, Sami Tolvanen wrote:
> On Fri, Apr 16, 2021 at 2:18 PM Borislav Petkov <bp@alien8.de> wrote:
>> In file included from ./include/linux/ftrace.h:22:0,
>>                  from ./include/linux/init_task.h:9,
>>                  from init/init_task.c:2:
>> ./include/linux/ftrace.h: In function =E2=80=98ftrace_init_nop=E2=80=99:
>> ./arch/x86/include/asm/ftrace.h:9:40: error: implicit declaration of fun=
ction =E2=80=98function_nocfi=E2=80=99 [-Werror=3Dimplicit-function-declara=
tion]
>
> This is defined in linux-next, but I do see another issue, which I'll
> fix in v2. Note that CFI_CLANG itself cannot be selected on 32-bit
> x86.

Sure and because of that it's overrated to make sure that it does not
break the build. I know, sekurity ...

But aside of that when looking at the rest of the series, then I really
have to ask whether the only way to address this is to make a large
amount of code unreadable like this:

-	wrmsrl(MSR_LSTAR, (unsigned long)entry_SYSCALL_64);
+	wrmsrl(MSR_LSTAR, (unsigned long)function_nocfi(entry_SYSCALL_64));

plus a gazillion of similar changes.

This is unreadable garbage.

Thanks,

        tglx