linux-kernel - Re: [PATCH] x86: assemble with -Wa,--noexecstack to avoid BFD 2.39 warning

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Date:   Mon, 8 Aug 2022 18:36:53 -0700
From:   Fangrui Song <maskray@...gle.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Nick Clifton <nickc@...hat.com>, Jens Axboe <axboe@...nel.dk>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Tom Rix <trix@...hat.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Nicolas Schier <n.schier@....de>,
        Brijesh Singh <brijesh.singh@....com>,
        Michael Roth <michael.roth@....com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
        Ben Dooks <ben-linux@...ff.org>,
        Heinrich Schuchardt <heinrich.schuchardt@...onical.com>
Subject: Re: [PATCH] x86: assemble with -Wa,--noexecstack to avoid BFD 2.39
 warning

On 2022-08-08, Linus Torvalds wrote:
>On Mon, Aug 8, 2022 at 1:32 PM Nick Desaulniers <ndesaulniers@...gle.com> wrote:
>>
>> Thanks for the link; I recall Ard mentioning the ELF segments are
>> ignored by the kernels loader in another thread.
>
>Yeah, the kernel loader is not some generic ELF loader thing.
>
>> Perhaps we should be adding
>> --no-warn-execstack
>> wrapped in an ld-option check to KBUILD_LDFLAGS at the top level?
>
>I think both the linker flag and the assembler flag are the "correct"
>thing to do.
>
>And yes, I don't think there is anything architecture-specific about
>it, and the top-level Makefile is likely the right thing to modify.
>
>I'm a tiny bit worried about "what versions of as/ld accept those
>flags", though.
>
>              Linus
>

If most ports don't need executable stacks, I think using `-z
noexecstack` is better than the binutils 2.39 specific
--no-warn-execstack (not recognized by lld and older GNU ld, so a
configure check will be needed).

Then -Wa,--noexecstack is not necessary: if the built relocatable files
are destined to be used with ld with -z noexecstack, the input
.note.GNU-stack sections are really redundant. It would be a difficult
story if the relocatable files can "leak" to the outside world where a
GNU ld without -z noexecstack may be used. But IMO this is not the case
for the kernel.