linux-hardening - Re: [PATCH] lib/test_string.c: Add test for strlen()

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Message-ID: <202202031242.45A7F5704@keescook>
Date:   Thu, 3 Feb 2022 12:45:36 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Guenter Roeck <linux@...ck-us.net>,
        Peter Rosin <peda@...ntia.se>,
        Andy Shevchenko <andy@...nel.org>,
        Matteo Croce <mcroce@...rosoft.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        llvm@...ts.linux.dev, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] lib/test_string.c: Add test for strlen()

On Thu, Feb 03, 2022 at 11:50:34AM -0800, Nick Desaulniers wrote:
> The kernel does not link against a libc; but it does provide many
> symbols that libc would provide, with the same or similar enough
> semantics that I would strongly recommend we _don't_ use
> -ffreestanding in order to get such libcall optimizations (there are a
> lot; see https://github.com/llvm/llvm-project/blob/main/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
> for some examples) and simply use -fno-builtin-* when necessary, or
> fix the kernel implementations individually.

Right, we really don't want -ffreestanding. Rather, we want to not link
against libgcc. This is mostly true already, though some of the smaller
architectures still do:

$ git grep print-libgcc
arch/arc/Makefile:LIBGCC        = $(shell $(CC) $(cflags-y) --print-libgcc-file-name)
arch/csky/Makefile:     $(shell $(CC) $(KBUILD_CFLAGS) $(KCFLAGS) -print-libgcc-file-name)
arch/h8300/boot/compressed/Makefile:LIBGCC := $(shell $(CROSS-COMPILE)$(CC) $(KBUILD_CFLAGS) -print-libgcc-file-name 2>/dev/null)
arch/nios2/Makefile:LIBGCC         := $(shell $(CC) $(KBUILD_CFLAGS) $(KCFLAGS) -print-libgcc-file-name)
arch/openrisc/Makefile:LIBGCC           := $(shell $(CC) $(KBUILD_CFLAGS) -print-libgcc-file-name)
arch/parisc/Makefile:LIBGCC             := $(shell $(CC) -print-libgcc-file-name)
arch/xtensa/Makefile:LIBGCC := $(shell $(CC) $(KBUILD_CFLAGS) -print-libgcc-file-name)
arch/xtensa/boot/boot-redboot/Makefile:LIBGCC := $(shell $(CC) $(KBUILD_CFLAGS) -print-libgcc-file-name)

-- 
Kees Cook