Re: [PATCH kvm-unit-tests] realmode: load above stack

[Thomas Huth <thuth@redhat.com>]

On 04/06/2024 16.35, Paolo Bonzini wrote:
> The bottom 32K of memory are generally reserved for use by the BIOS;
> for example, traditionally the boot loader is placed at 0x7C00 and
> the stack grows below that address.
> 
> It turns out that with some versions of clang, realmode.flat has
> become big enough that it overlaps the stack used by the multiboot
> option ROM loader.  The result is that a couple instructions are
> overwritten.  Typically one or two tests fail and that's it...
> 
> Move the code above the forbidden region, in real 90s style.
> 
> Reported-by: Thomas Huth <thuth@redhat.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>   x86/realmode.lds | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/x86/realmode.lds b/x86/realmode.lds
> index 0ed3063b..e4782a98 100644
> --- a/x86/realmode.lds
> +++ b/x86/realmode.lds
> @@ -1,6 +1,6 @@
>   SECTIONS
>   {
> -    . = 16K;
> +    . = 32K;
>       stext = .;
>       .text : { *(.init) *(.text) }
>       . = ALIGN(4K);

This fails for me with:

ld -m elf_i386 -nostdlib -o x86/realmode.elf \
       -T /home/thuth/devel/kvm-unit-tests/x86/realmode.lds x86/realmode.o
x86/realmode.o: in function `init_inregs':
/home/thuth/devel/kvm-unit-tests/x86/realmode.c:176:(.text+0x79): relocation 
truncated to fit: R_386_16 against `.bss'
/home/thuth/devel/kvm-unit-tests/x86/realmode.c:176:(.text+0x82): relocation 
truncated to fit: R_386_16 against `.bss'
/home/thuth/devel/kvm-unit-tests/x86/realmode.c:176:(.text+0x8b): relocation 
truncated to fit: R_386_16 against `.bss'
/home/thuth/devel/kvm-unit-tests/x86/realmode.c:176:(.text+0x94): relocation 
truncated to fit: R_386_16 against `.bss'
/home/thuth/devel/kvm-unit-tests/x86/realmode.c:176:(.text+0x9d): relocation 
truncated to fit: R_386_16 against `.bss'
/home/thuth/devel/kvm-unit-tests/x86/realmode.c:176:(.text+0xa6): relocation 
truncated to fit: R_386_16 against `.bss'
/home/thuth/devel/kvm-unit-tests/x86/realmode.c:176:(.text+0xaf): relocation 
truncated to fit: R_386_16 against `.bss'
/home/thuth/devel/kvm-unit-tests/x86/realmode.c:176:(.text+0xb8): relocation 
truncated to fit: R_386_16 against `.bss'
/home/thuth/devel/kvm-unit-tests/x86/realmode.c:176:(.text+0xc1): relocation 
truncated to fit: R_386_16 against `.bss'
/home/thuth/devel/kvm-unit-tests/x86/realmode.c:176:(.text+0xca): relocation 
truncated to fit: R_386_16 against `.bss'
/home/thuth/devel/kvm-unit-tests/x86/realmode.c:180:(.text+0xda): additional 
relocation overflows omitted from the output
make: *** [/home/thuth/devel/kvm-unit-tests/x86/Makefile.common:107: 
x86/realmode.elf] Error 1

  Thomas