Re: ARM mapping symbols and --strip-unneeded

["Richard Earnshaw (lists)" <Richard dot Earnshaw at arm dot com>]

On 29/06/16 10:26, Andrew Pinski wrote:
> On Wed, Jun 29, 2016 at 2:24 AM, Nick Clifton <nickc@redhat.com> wrote:
>> Hi Richard,
>>
>>> Might be safer to use strncmp (sym, type, 2) since 'extended' mapping
>>> symbols can be produced by some tools.
>>
>> We might as well extract mapping symbol determination into a separate
>> function - I have a feeling that it is going to be needed again in the
>> future.  So what do you think of this patch ?
> 
> We should check aarch64 target as it might have the same issue too as
> it uses $d and $t too.
> 

Although AArch64 ELF defines some mapping symbols, I don't think they
are currently needed beyond being helpers for disassembly (code vs data).

R.

> Thanks,
> Andrew
> 
>>
>> Cheers
>>   Nick
>>
>> diff --git a/bfd/elf32-arm.c b/bfd/elf32-arm.c
>> index 04e776c..105e3cc 100644
>> --- a/bfd/elf32-arm.c
>> +++ b/bfd/elf32-arm.c
>> @@ -18266,6 +18266,28 @@ elf32_arm_copy_special_section_fields (const bfd *ibfd ATTRIBUTE_UNUSED,
>>    return FALSE;
>>  }
>>
>> +/* Returns TRUE if NAME is an ARM mapping symbol.
>> +   Traditionally the symbols $a, $d and $t have been used.
>> +   The ARM ELF standard also defines $x (for A64 code).  It also allows a
>> +   period initiated suffix to be added to the symbol: "$[adtx]\.[:sym_char]+".
>> +   Other tools might also produce $b (Thumb BL), $f, $p, $m and $v, but we do
>> +   not support them here.  $t.x indicates the start of ThumbEE instructions.  */
>> +
>> +static bfd_boolean
>> +is_mapping_symbol (const char * name)
>> +{
>> +  return name != NULL /* Paranoia.  */
>> +    && name[0] == '$' /* Note: if objcopy --prefix-symbols has been used then
>> +                        the mapping symbols could have acquired a prefix.
>> +                        We do not support this here, since such symbols no
>> +                        longer conform to the ARM ELF ABI.  */
>> +    && (name[1] == 'a' || name[1] == 'd' || name[1] == 't' || name[1] == 'x')
>> +    && (name[2] == 0 || name[2] == '.');
>> +  /* FIXME: Strictly speaking the symbol is only a valid mapping symbol if
>> +     any characters that follow the period are legal characters for the body
>> +     of a symbol's name.  For now we just assume that this is the case.  */
>> +}
>> +
>>  /* Make sure that mapping symbols in object files are not removed via the
>>     "strip --strip-unneeded" tool.  These symbols are needed in order to
>>     correctly generate interworking veneers, and for byte swapping code
>> @@ -18276,11 +18298,8 @@ static void
>>  elf32_arm_backend_symbol_processing (bfd *abfd, asymbol *sym)
>>  {
>>    if (((abfd->flags & (EXEC_P | DYNAMIC)) == 0)
>> -      && sym->name != NULL
>>        && sym->section != bfd_abs_section_ptr
>> -      && (strcmp (sym->name, "$a") == 0
>> -         || strcmp (sym->name, "$t") == 0
>> -         || strcmp (sym->name, "$d") == 0))
>> +      && is_mapping_symbol (sym->name))
>>      sym->flags |= BSF_KEEP;
>>  }