Clang __bos vs loop unrolling (was Re: [PATCH] ALSA: asihpi: work around clang-17+ false positive fortify-string warning)

From: Kees Cook <keescook@chromium.org>
To: Arnd Bergmann <arnd@arndb.de>, Bill Wendling <morbo@google.com>,
	Nathan Chancellor <nathan@kernel.org>
Cc: Takashi Iwai <tiwai@suse.de>, Arnd Bergmann <arnd@kernel.org>,
	Jaroslav Kysela <perex@perex.cz>, Takashi Iwai <tiwai@suse.com>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Justin Stitt <justinstitt@google.com>,
	linux-sound@vger.kernel.org, linux-kernel@vger.kernel.org,
	llvm@lists.linux.dev
Subject: Clang __bos vs loop unrolling (was Re: [PATCH] ALSA: asihpi: work around clang-17+ false positive fortify-string warning)
Date: Wed, 28 Feb 2024 09:39:30 -0800	[thread overview]
Message-ID: <202402280925.5B709A60F@keescook> (raw)
In-Reply-To: <1265517d-b98a-4ec6-9215-10cd988ff78f@app.fastmail.com>

On Wed, Feb 28, 2024 at 04:03:56PM +0100, Arnd Bergmann wrote:
> My first thought was that clang warns about it here because
> the 'u16 adapter' declaration limits the index to something
> smaller than an 'int' or 'long', but changing the type
> did not get rid of the warning.

Our current theory is that Clang has a bug with
__builtin_object_size/__builtin_dynamic_object_size when doing loop
unrolling (or other kinds of execution flow splitting). Some examples:
https://github.com/ClangBuiltLinux/linux/issues?q=label%3A%22loop+unroller%22+

Which is perhaps related to __bos miscalculations:
https://github.com/ClangBuiltLinux/linux/issues?q=label%3A%22%5B__bos%5D+miscalculation%22+

-Kees

-- 
Kees Cook