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

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Date: Wed, 28 Feb 2024 09:39:30 -0800
From: Kees Cook <keescook@...omium.org>
To: Arnd Bergmann <arnd@...db.de>, Bill Wendling <morbo@...gle.com>,
	Nathan Chancellor <nathan@...nel.org>
Cc: Takashi Iwai <tiwai@...e.de>, Arnd Bergmann <arnd@...nel.org>,
	Jaroslav Kysela <perex@...ex.cz>, Takashi Iwai <tiwai@...e.com>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>, linux-sound@...r.kernel.org,
	linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Clang __bos vs loop unrolling (was Re: [PATCH] ALSA: asihpi: work
 around clang-17+ false positive fortify-string warning)

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