linux-kernel - Re: [RFC PATCH 25/28] x86: Use PIE codegen for the core kernel

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Message-ID: <3bbb85ae-8ba5-4777-999f-d20705c386e7@zytor.com>
Date: Sat, 5 Oct 2024 16:36:48 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Uros Bizjak <ubizjak@...il.com>
Cc: Ard Biesheuvel <ardb@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ard Biesheuvel <ardb+git@...gle.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org, Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, Dennis Zhou <dennis@...nel.org>,
        Tejun Heo <tj@...nel.org>, Christoph Lameter <cl@...ux.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky
 <boris.ostrovsky@...cle.com>,
        Greg Kroah-Hartman
 <gregkh@...uxfoundation.org>,
        Arnd Bergmann <arnd@...db.de>, Masahiro Yamada <masahiroy@...nel.org>,
        Kees Cook <kees@...nel.org>, Nathan Chancellor <nathan@...nel.org>,
        Keith Packard <keithp@...thp.com>,
        Justin Stitt <justinstitt@...gle.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>, Jiri Olsa <jolsa@...nel.org>,
        Ian Rogers <irogers@...gle.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Kan Liang <kan.liang@...ux.intel.com>, linux-doc@...r.kernel.org,
        linux-pm@...r.kernel.org, kvm@...r.kernel.org,
        xen-devel@...ts.xenproject.org, linux-efi@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-sparse@...r.kernel.org,
        linux-kbuild@...r.kernel.org, linux-perf-users@...r.kernel.org,
        rust-for-linux@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [RFC PATCH 25/28] x86: Use PIE codegen for the core kernel

On 10/5/24 01:31, Uros Bizjak wrote:
>>
>> movq $sym to leaq sym(%rip) which you said ought to be smaller (and in
>> reality appears to be the same size, 7 bytes) seems like a no-brainer
>> and can be treated as a code quality issue -- in other words, file bug
>> reports against gcc and clang.
> 
> It is the kernel assembly source that should be converted to
> rip-relative form, gcc (and probably clang) have nothing with it.
> 

Sadly, that is not correct; neither gcc nor clang uses lea:

	-hpa


gcc version 14.2.1 20240912 (Red Hat 14.2.1-3) (GCC)

hpa@...enda:/tmp$ cat foo.c
int foobar;

int *where_is_foobar(void)
{
         return &foobar;
}

hpa@...enda:/tmp$ gcc -mcmodel=kernel -O2 -c -o foo.o foo.c
hpa@...enda:/tmp$ objdump -dr foo.o

foo.o:     file format elf64-x86-64


Disassembly of section .text:

0000000000000000 <where_is_foobar>:
    0:   48 c7 c0 00 00 00 00    mov    $0x0,%rax
                         3: R_X86_64_32S foobar
    7:   c3                      ret

clang version 18.1.8 (Fedora 18.1.8-1.fc40)

hpa@...enda:/tmp$ clang -mcmodel=kernel -O2 -c -o foo.o foo.c
hpa@...enda:/tmp$ objdump -dr foo.o

foo.o:     file format elf64-x86-64


Disassembly of section .text:

0000000000000000 <where_is_foobar>:
    0:   48 c7 c0 00 00 00 00    mov    $0x0,%rax
                         3: R_X86_64_32S foobar
    7:   c3                      ret