Re: [PATCH v3 7/7] pc-bios/s390-ccw: Don't use __bss_start with the "larl" instruction

[Thomas Huth <thuth@redhat.com>]

On 29/06/2023 12.58, Claudio Imbrenda wrote:
> On Thu, 29 Jun 2023 12:48:21 +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 loading the address indirectly instead.
> 
> what are the advantages of this solution compared to your previous one
> (i.e. align .bss) ?

__bss_start is supposed to point to an address that is before all bss-like 
segments. There are also segments like .sbss and .bss.plt on other 
architectures, see https://bugzilla.redhat.com/show_bug.cgi?id=2216662#c11 .
Seems like we don't have them on s390x yet, so currently my previous patch 
is fine, too. But in case there will ever be an extension to the s390x ABI 
that introduces such additional segments, we have to switch back to 
__bss_start again. So it sounds slightly more future-proof to me to keep 
__bss_start here, even if we need a slightly more complex startup code here now.

  Thomas