linux-kernel - Re: [PATCH] word-at-a-time: use the same return type for has

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Message-ID: <CAHk-=wjmWjd+xe88cf14hFGkSK7fYJBSixK8Ym0DLYCa+dTxtg@mail.gmail.com>
Date:   Wed, 2 Aug 2023 10:37:43 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Nathan Chancellor <nathan@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>
Cc:     ndesaulniers@...gle.com, Arnd Bergmann <arnd@...db.de>,
        Tom Rix <trix@...hat.com>, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH] word-at-a-time: use the same return type for has_zero
 regardless of endianness

On Wed, 2 Aug 2023 at 09:16, Nathan Chancellor <nathan@...nel.org> wrote:
>
> We see this warning with ARCH=arm64 defconfig + CONFIG_CPU_BIG_ENDIAN=y.

Oh Christ. I didn't even realize that arm64 allowed a BE config.

The config option goes back to 2013 - are there actually BE user space
implementations around?

People, why do we do that? That's positively crazy. BE is dead and
should be relegated to legacy platforms. There are no advantages to
being different just for the sake of being different - any "security
by obscurity" argument would be far outweighed by the inconvenience to
actual users.

Yes, yes, I know the aarch64 architecture technically allows BE
implementations - and apparently you can even do it by exception
level, which I had to look up. But do any actually exist?

Does the kernel even work right in BE mode? It's really easy to miss
some endianness check when all the actual hardware and use is LE, and
when (for example) instruction encoding and IO is then always LE
anyway.

> With both clang 18.0.0 (tip of tree) and GCC 13.1.0, I don't see any
> actual code generation changes in fs/namei.o with this configuration.

Ok, since the main legacy platform confirmed that, I'll just apply
that patch directly.

I'll also do the powerpc version that Arnd pointed to at the same
time, since it seems silly to pick these off one at a time. It too
should just be 'unsigned long', so that the two values can be bitwise
or'ed together without any questions.

              Linus