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

["H. Peter Anvin" <hpa@xxxxxxxxx>]

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

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

hpa@tazenda:/tmp$ gcc -mcmodel=kernel -O2 -c -o foo.o foo.c
hpa@tazenda:/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@tazenda:/tmp$ clang -mcmodel=kernel -O2 -c -o foo.o foo.c
hpa@tazenda:/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