Re: [PATCH v2 4/4] pc-bios/s390-ccw: Don't use __bss_start with the "larl" instruction

[Claudio Imbrenda <imbrenda@linux.ibm.com>]

On Tue, 27 Jun 2023 09:47:03 +0200
Thomas Huth <thuth@redhat.com> wrote:

> start.S currently cannot be compiled with Clang 16 and binutils 2.40:
> 
>  ld: start.o(.text+0x8): misaligned symbol `__bss_start' (0xc1e5) for
>      relocation R_390_PC32DBL
> 
> According to the built-in linker script of ld, the symbol __bss_start
> can actually point *before* the .bss section and does not need to have
> any alignment, so in certain situations (like when using the internal
> assembler of Clang), the __bss_start symbol can indeed be unaligned
> and thus it is not suitable for being used with the "larl" instruction
> that needs an address that is at least aligned to halfwords.
> The problem went unnoticed so far since binutils <= 2.39 did not
> check the alignment, but starting with binutils 2.40, such unaligned
> addresses are now refused.
> 
> Fix it by using the real start address of the .bss section instead.
> 
> Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=2216662
> Reported-by: Miroslav Rezanina <mrezanin@redhat.com>
> Suggested-by: Nick Clifton <nickc@redhat.com>
> Signed-off-by: Thomas Huth <thuth@redhat.com>
> ---
>  pc-bios/s390-ccw/start.S | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/pc-bios/s390-ccw/start.S b/pc-bios/s390-ccw/start.S
> index 111dea261b..a63c4e3ff2 100644
> --- a/pc-bios/s390-ccw/start.S
> +++ b/pc-bios/s390-ccw/start.S
> @@ -18,7 +18,7 @@ _start:
>      larl    %r15,stack + STACK_SIZE - 160   /* Set up stack */
>  
>      /* clear bss */
> -    larl    %r2,__bss_start
> +    larl    %r2,.bss
>      larl    %r3,_end

since we are here, do you have guarantees that _end is always correctly
aligned?

if so:

Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>

>      slgr    %r3,%r2    /* get sizeof bss */
>      ltgr    %r3,%r3    /* bss empty? */