linux-kernel - Re: [PATCH v2] kbuild: treat char as always unsigned

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Date:   Thu, 20 Oct 2022 21:01:32 -0400
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-kbuild@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-toolchains@...r.kernel.org,
        Masahiro Yamada <masahiroy@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        ndesaulniers@...gle.com
Subject: Re: [PATCH v2] kbuild: treat char as always unsigned

On Thu, Oct 20, 2022 at 11:41:29AM -0700, Kees Cook wrote:
> On Wed, Oct 19, 2022 at 05:38:55PM -0700, Linus Torvalds wrote:
> > Having some scripting automation that just notices "this changes code
> > generation in function X" might actually be interesting, and judging
> > by my quick tests might not be *too* verbose.
> 
> On the reproducible build comparison system[1] we use for checking a lot
> of the KSPP work for .text deltas, an allmodconfig finds a fair bit for
> this change. Out of 33900 .o files, 1005 have changes.
> 
> Spot checking matches a lot of what you found already...
> 
>         u64 flags = how->flags;
> 	...
> fs/open.c:1123:
>         int acc_mode = ACC_MODE(flags);
> -    1c86:      movsbl 0x0(%rdx),%edx
> +    1c86:      movzbl 0x0(%rdx),%edx
> 
> #define ACC_MODE(x) ("\004\002\006\006"[(x)&O_ACCMODE])
> 
> Ignoring those, it goes down to 625, and spot checking those is more
> difficult, but looks to be mostly register selection changes dominating
> the delta. The resulting vmlinux sizes are identical, though.
> 
> -Kees
> 
> [1] A fancier version of:
>     https://outflux.net/blog/archives/2022/06/24/finding-binary-differences/

Say, don't we have some way of outputting LLVM IL? I saw some
-fno-discard-value-names floating through a few days ago. Apparently you
can do `make LLVM=1 fs/select.ll`? This might have less noise in it.
I'll play on the airplane tomorrow.

Jason