Re: [PATCH 2/2] symbols/x86: don't use symbols-dummy

[Jan Beulich <jbeulich@xxxxxxxx>]

On 25.11.2025 18:36, Roger Pau Monné wrote:
> On Tue, Nov 25, 2025 at 03:14:27PM +0100, Jan Beulich wrote:
>> In particular when linking with lld, which converts hidden symbols to
>> local ones, the ELF symbol table can change in unhelpful ways between the
>> first two linking passes, resulting in the .rodata contributions to change
>> between the 2nd and 3rd pass. That, however, renders our embedded symbol
>> table pretty much unusable; the recently introduced self-test may then
>> also fail. (Another difference between compiling a C file and assembling
>> the generated ones is that - with -fdata-sections in use - the .rodata
>> contributions move between passes 1 and 2, when we'd prefer them not to.)
>>
>> Make tools/symbols capable of producing an "empty" assembly file, such
>> that unwanted differences between passes 1 and 2 go away when then using
>> the corresponding objects in place of common/symbols-dummy.o.
>>
>> Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> Reported-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> LGTM, not sure whether you want to extend to other arches in this
> same patch, or simply guard the build of symbols-dummy.o for non-x86
> arches.

I think I'd prefer to mirror it to other arch-es, but in separate
patches (so they can go in independently). Then once all are in, ...

>> ---
>> May want mirroring to other arch-es.
>>
>> --- a/xen/arch/x86/Makefile
>> +++ b/xen/arch/x86/Makefile
>> @@ -134,8 +134,10 @@ $(TARGET): $(TARGET)-syms $(efi-y) $(obj
>>  CFLAGS-$(XEN_BUILD_EFI) += -DXEN_BUILD_EFI
>>  
>>  $(TARGET)-syms: $(objtree)/prelink.o $(obj)/xen.lds
>> +    $(objtree)/tools/symbols $(all_symbols) --empty > $(dot-target).0.S
>> +    $(MAKE) $(build)=$(@D) $(dot-target).0.o
>>      $(LD) $(XEN_LDFLAGS) -T $(obj)/xen.lds $< $(build_id_linker) \
>> -        $(objtree)/common/symbols-dummy.o -o $(dot-target).0
> 
> It would be good if we could now stop building symbols-dummy.o as part
> of extra-y.  Maybe you could guard it with ifneq ($(CONFIG_X86),y) in
> the Makefile?
> 
> Or otherwise remove this from all arches thus removing
> common/symbols-dummy.c.

... I'd remove symbols-dummy altogether. I don't think there's a need
to transiently disable building of symbols-dummy.o for just some of the
arch-es (like foe x86 right here).

>> --- a/xen/tools/symbols.c
>> +++ b/xen/tools/symbols.c
>> @@ -672,7 +672,10 @@ int main(int argc, char **argv)
>>                              warn_dup = true;
>>                      else if (strcmp(argv[i], "--error-dup") == 0)
>>                              warn_dup = error_dup = true;
>> -                    else if (strcmp(argv[i], "--xensyms") == 0)
>> +                    else if (strcmp(argv[i], "--empty") == 0) {
>> +                            write_src();
>> +                            return 0;
> 
> Oh, that was easier than I was expecting for symbols to generate a
> working empty assembly file.

Yeah, I also expected it to be more intrusive. The file isn't exactly,
though - two of the tables (the ones with 256 entries) are emitted
nevertheless. I didn't consider this an issue, though.

Jan