[PATCH v11 16/25] OAv2 merge: merging Object Attributes

[Matthieu Longo]

On 08/01/2026 14:11, Jan Beulich wrote:
> On 31.12.2025 00:05, Matthieu Longo wrote:
>> The Object Ottributes merge process must handle both optional and
>> required subsections. It also treats the first merge of the frozen set
>> specially, as the OAv2 list in the input BFD serves as the accumulator
>> for subsequent merges.
>>
>> ** Optional subsections
>>
>> Optional subsections are processed as if merging two ordered sets — by
>> iterating linearly through both, checking whether an element of a given
>> ordinality is present in the opposite set, and adding it to the
>> accumulator. The added diffuculty with subsections and attributes lies
>> in the fact that missing elements have default values, and these must be
>> merged with existing ones to produce the final value to be stored.
>>
>> ** Required subsections
>>
>> Required subsections are processed slightly differently from the
>> optional subsections, as they cannot be pruned since they are mandatory,
>> hence an error will be raised by the linker if it is not recognized.
>>
>> For now, the subsection for PAuth ABI is the only one use case, and
>> no merge is applied on the values. The values simply need to match.
>> This implementation choice might be challenged in the future if required
>> subsections can have the same diversity as optional subsections. If the
>> case arises, the refactoring to handle this new behavior should consist
>> in adding a new merge policy MERGE-EQUAL, or something similar. Some "if
>> required" should be added in the optional subsections merge logic to
>> error on any missing elements, or mismatch, and messages should also be
>> rephrased to point out that the error is for a required subsection.
>>
>> ** Important note regarding support for testing
>>
>> In order to test this generic logic, AArch64's use cases are not
>> offering enough coverage, so a "GNU testing namespace" which corresponds
>> to the name of the subsection was introduced. It follows the following
>> pattern:
>>    gnu_testing_<XXXXXX>_MERGE_<POLICY>
>> with:
>>    - <XXXXXX>: an arbitrary name for your testing subsection.
>>    - <POLICY>: the name of the merging policy to apply on the values in
>>      the subsection. The currently supported merge policy are:
>>        * _MERGE_AND: bitwise AND applied on numerical values.
>>        * _MERGE_OR: bitwise OR applied on numerical values.
>>        * _MERGE_ADD: concatenates strings together with a '+' in-between.
>>      Note: "_MERGE_ADD" does not make really sense, and will very likely
>>      never be used for a real merge. Its only purpose is to test the
>>      correct handling of merges with strings.
>> Any subsection name matching neither names supported by the backend, nor
>> following the pattern corresponding GNU testing namespace will be considered
>> unknown and its status set to obj_attr_subsection_v2_unknown. This will
>> have for consequence the pruning of this subsection.
>>
>> Additionally, the first two tags in gnu_testing namespace, GNUTestTag_0
>> and GNUTestTag_1, are known, and so have a name and can be initialized
>> to the default value ('0' or NULL) depending on the encoding specified
>> on the subsection. Any tags above 1 will be considered unknown, so will
>> be default-initialized in the same way but its status will be set to
>> obj_attr_v2_unknown. This behavior of the testing tags allows to test
>> the pruning of unknown attributes.
>> ---
>>   bfd/elf-attrs.c    | 874 ++++++++++++++++++++++++++++++++++++++++++++-
>>   bfd/elf-attrs.h    |  24 ++
>>   bfd/elf-bfd.h      |   9 +
>>   bfd/elfxx-target.h |   8 +
>>   4 files changed, 904 insertions(+), 11 deletions(-)
> 
> Okay with, again, ...
> 
>> +/* Append a new default-initialized attribute with the same key as AREF to the
>> +   given subsection.  */
>> +static void
>> +oav2_subsection_append_attr_default (const struct bfd_link_info *info,
>> +				     obj_attr_subsection_v2_t *s_abfd_missing,
>> +				     const obj_attr_v2_t *a_ref)
>> +{
>> +  obj_attr_v2_t *new_attr = oav2_attr_default (info, s_abfd_missing, a_ref);
>> +  LINKED_LIST_APPEND(obj_attr_v2_t) (s_abfd_missing, new_attr);
> 
> the style issue here (and elsewhere) addressed.
> 
> Jan

Fixed.

Matthieu