linux-kernel - Re: [PATCH] lib/clz_ctz.c: Fix __clzdi2() and _

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAHk-=wgJzMzPFTCzejWs1WM4=74z2VENyOzySnucrXG3i=ajrw@mail.gmail.com>
Date:   Fri, 25 Aug 2023 18:07:52 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Bill Wendling <morbo@...gle.com>, Helge Deller <deller@....de>,
        Nathan Chancellor <nathan@...nel.org>,
        linux-kernel@...r.kernel.org, linux-parisc@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Chanho Min <chanho.min@....com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kees Cook <keescook@...omium.org>,
        clang-built-linux <llvm@...ts.linux.dev>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>
Subject: Re: [PATCH] lib/clz_ctz.c: Fix __clzdi2() and __ctzdi2() for 32-bit kernels

On Fri, 25 Aug 2023 at 17:52, Nick Desaulniers <ndesaulniers@...gle.com> wrote:
>
> So 2 concerns where "I'll do it in inline asm" can pessimize codegen:
> 1. You alluded to this, but what happens when one of these functions
> is called with a constant?

This is why our headers have a lot of __builtin_constant_p()'s in them..

In this particular case, see the x86 asm/bitops.h code:

    #define ffs(x) (__builtin_constant_p(x) ? __builtin_ffs(x) :
variable_ffs(x))

but this is actually quite a common pattern, and it's often not about
something like __builtin_ffs() at all.

See all the other __builtin_constant_p()'s that we have in that same
file because we basically just use different code sequences for
constants.

And that file isn't even unusual. We use it quite a lot when we care
about code generation for some particular case.

> 2. by providing the definition of a symbol typically provided by libc
> (and then not building with -ffreestanding) pessimizes libcall
> optimization.

.. and this is partly why we often avoid libgcc things, and do certain
things by hand.

The classic rule is "Don't do 64-bit divisions using the C '/' operator".

So in the kernel you have to use do_div() and friends, because the
library versions are often disgusting and might not know that 64/32 is
much much cheaper and is what you want.

And quite often we simply use other names - but then we also do *not*
build with -freestanding, because -freestanding has at least
traditionally meant that the compiler won't optimize the simple and
obvious cases (typically things like "memcpy with a constant size").

So we mix and match and pick the best option.

The kernel really doesn't care about architecture portability, because
honestly, something like "ffs()" is entirely *trivial* to get right,
compared to the real differences between architectures (eg VM and IO
differences etc).

             Linus