linux-hardening - Re: mainline build failure of powerpc allmodconfig for prom_init

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Date:   Mon, 18 Jul 2022 12:06:52 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Sudip Mukherjee <sudipm.mukherjee@...il.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Kees Cook <keescook@...omium.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-hardening@...r.kernel.org
Subject: Re: mainline build failure of powerpc allmodconfig for prom_init_check

On Sun, Jul 17, 2022 at 9:41 PM Michael Ellerman <mpe@...erman.id.au> wrote:
>
> >         li 4,254                 #,
>
> Here we load 254 into r4, which is the 2nd parameter to memset (c).

I love how even powerpc people know that "4" is bogus, and have to
make it clear that it means "r4".

I don't understand why the powerpc assembler is so messed up, and uses
random integer constants for register "names".

And it gets even worse, when you start mixing FP, vector and integer "names".

I've seen many bad assemblers (in fact, I have *written* a couple of
bad assemblers myself), but I have never seen anything quite that
broken on any other architecture.

Oddities, yes ("$" as a prefix for register? Alpha asm is also very
odd), but nothing *quite* as broken as "simple constants have entirely
different meanings depending on the exact instruction and argument
position".

It's not even an IBM thing. S390 uses perfectly sane register syntax,
and calls things '%r4" etc.

The human-written asm files have those #define's in headers just to
make things slightly more legible, because apparently the assembler
doesn't even *accept* the sane names. So it's not even a "the compiler
generates this abbreviated illegible mess". It's literally that the
assembler is so horrid.

Why do people put up with that?

               Linus