linux-kernel - Re: [PATCH v2 0/2] link with -z noexecstack --no-warn-rwx-segments

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Date:   Wed, 10 Aug 2022 19:06:00 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     Fangrui Song <maskray@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Nick Clifton <nickc@...hat.com>, brijesh.singh@....com,
        hpa@...or.com, kirill.shutemov@...ux.intel.com,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
        michael.roth@....com, n.schier@....de, nathan@...nel.org,
        sathyanarayanan.kuppuswamy@...ux.intel.com, trix@...hat.com,
        x86@...nel.org
Subject: Re: [PATCH v2 0/2] link with -z noexecstack --no-warn-rwx-segments

On 8/10/22 4:24 PM, Nick Desaulniers wrote:
> Users of GNU ld (BFD) from binutils 2.39+ will observe multiple
> instances of a new warning when linking kernels in the form:
> 
>   ld: warning: vmlinux: missing .note.GNU-stack
>   section implies executable stack
>   ld: NOTE: This behaviour is deprecated and will be removed in a future
>   version of the linker
>   ld: warning: vmlinux has a LOAD segment with RWX permissions
> 
> Generally, we would like to avoid the stack being executable. Because
> there could be a need for the stack to be executable, assembler sources
> have to opt-in to this security feature via explicit creation of the
> .note.GNU-stack feature (which compilers create by default) or command
> line flag --noexecstack. Or we can simply tell the linker the production
> of such sections is irrelevant and to link the stack as --noexecstack.
> 
> LLVM's LLD linker defaults to -z noexecstack, so this flag isn't
> strictly necessary when linking with LLD, only BFD, but it doesn't hurt
> to be explicit here for all linkers IMO. --no-warn-rwx-segments is
> currently BFD specific and only available in the current latest release,
> so it's wrapped in an ld-option check.
> 
> While the kernel makes extensive usage of ELF sections, it doesn't use
> permissions from ELF segments.
> 
> Broken up into 2 patches; one for the top level vmlinux, one x86
> specific since a few places in the x86 build reset KBUILD_LDFLAGS.

For x86-64:

Tested-by: Jens Axboe <axboe@...nel.dk>

-- 
Jens Axboe