Add partial support for R_BPF_64_NODLYD32 reloc

[Jose E. Marchesi]

> Hi Nick,
>
> Thanks for letting me know of this.
> Indeed, I removed it since I failed to find a use case for it.
> We clearly forgot the use of tools such as strip, when used with objects
> created with clang.
>
> AFAIK, BPF clang support does not allow to link BPF object files, and
> the object files are used directly by libbpf without linking.
> The discussion was that R_BPF_64_NODLYD32 was specific to clang object
> files as I could not find a reason to generate those relocations in gas.
>
> At this moment I am unshure what should be done in the linker for this
> relocations. I promisse to come back to this ASAP.

The R_BPF_64_NODLYD32 is like R_BPF_64_ABS32.  The only difference (and
the reason why it exists) is that it is ignored by the LLVM
ExecutionEngine.

So I think in the GNU tools R_BPF_64_NODLYD32 should be handled in
exactly the same way than R_BPF_64_ABS32.

> Cheers,
> Cupertino
>
>
> Nick Clifton writes:
>
>> Hi Cupertino,
>>
>>   A bug report was recently filed with the Fedora binutils about
>>   the strip program being unable to handle the R_BPF_64_NODLYD32
>>   Relocation:
>>
>> https://bugzilla.redhat.com/show_bug.cgi?id=2245296
>>
>>   It turns out that whilst the GNU tools do not generate this reloc
>>   other tools do, and strip complains when it detects its presence.
>>
>>   I am going to apply the attached patch as a workaround so that
>>   strip will work, but I think that if we want the linker to be able
>>   to cope with bpf object files generated by other tools, then we will
>>   need a better fix.  I am leaving this up to you... :-)
>>
>> Cheers
>>   Nick
>>
>> diff --git a/bfd/ChangeLog b/bfd/ChangeLog
>> index 1b9d13add25..2b12eb35d27 100644
>> --- a/bfd/ChangeLog
>> +++ b/bfd/ChangeLog
>> @@ -1,3 +1,7 @@
>> +2023-10-30  Nick Clifton  <nickc@redhat.com>
>> +
>> +	* bpf-reloc.def (R_BPF_64_NODLD32): Add entry.
>> +
>>  2023-10-16  Nick Clifton  <nickc@redhat.com>
>>
>>  	PR 28910
>> diff --git a/bfd/bpf-reloc.def b/bfd/bpf-reloc.def
>> index 31f761d291d..7e7497892fa 100644
>> --- a/bfd/bpf-reloc.def
>> +++ b/bfd/bpf-reloc.def
>> @@ -87,3 +87,20 @@
>>          0xffff,                /* src_mask */
>>          0xffff,                /* dst_mask */
>>          true)                  /* pcrel_offset */
>> +
>> +  /* R_BPF_64_NODYLD32 is not used by GNU tools - but it is generated by LLVM.
>> +     We provide an entry here so that tools like strip can safely handle BPF
>> +     binaries generated by other tools.  */
>> +  BPF_HOWTO (R_BPF_64_NODYLD32,	/* type */
>> +	 0,			/* rightshift */
>> +	 0,			/* size */
>> +	 0,			/* bitsize */
>> +	 false,			/* pc_relative */
>> +	 0,			/* bitpos */
>> +	 complain_overflow_dont, /* complain_on_overflow */
>> +	 bpf_elf_generic_reloc, /* special_function */
>> +	 "R_BPF_64_NODYLD32",	/* name */
>> +	 false,			/* partial_inplace */
>> +	 0,			/* src_mask */
>> +	 0,			/* dst_mask */
>> +	 false)			/* pcrel_offset */
>> diff --git a/include/ChangeLog b/include/ChangeLog
>> index 90ee73650c3..e0f0cb3edf4 100644
>> --- a/include/ChangeLog
>> +++ b/include/ChangeLog
>> @@ -1,3 +1,8 @@
>> +2023-10-30  Nick Clifton  <nickc@redhat.com>
>> +
>> +	* elf/bpf.h (R_BPF_64_NODYLD32): Add a note that this reloc is
>> +	generated by other tools, eg LLVM.
>> +
>>  2023-09-28  Frederic Cambus  <fred@statdns.com>
>>
>>  	* elf/common.h (PT_OPENBSD_NOBTCFI): Define.
>> diff --git a/include/elf/bpf.h b/include/elf/bpf.h
>> index e4d416290a7..1ad074f3d0e 100644
>> --- a/include/elf/bpf.h
>> +++ b/include/elf/bpf.h
>> @@ -30,8 +30,8 @@ START_RELOC_NUMBERS (elf_bpf_reloc_type)
>>    RELOC_NUMBER (R_BPF_64_64,       		1)
>>    RELOC_NUMBER (R_BPF_64_ABS64,    		2)
>>    RELOC_NUMBER (R_BPF_64_ABS32,    		3)
>> -/* R_BPF_64_NODYLD32 is not used by GNU tools.
>> - * It is kept in this file to remind that the value is already taken. */
>> +/* R_BPF_64_NODYLD32 is not used by GNU tools - but it is generated by LLVM.
>> +   It is kept in this file to remind that the value is already taken.  */
>>    RELOC_NUMBER (R_BPF_64_NODYLD32, 		4)
>>    RELOC_NUMBER (R_BPF_64_32,      		10)
>>    RELOC_NUMBER (R_BPF_GNU_64_16,                256)