RFC Adding a section group flag of 0

[Florian Weimer]

* Nick Clifton:

> Hi Guys,
>
>   Currently the ELF standard for section groups requires that the first word 
>   of data in the group section be a flag, with only certain values recognised:
>
>     The following flags are currently defined:
>     Figure 4-13: Section Group Flags
>
>     Name 		Value
>     GRP_COMDAT 		0x1
>     GRP_MASKOS 		0x0ff00000
>     GRP_MASKPROC 	0xf0000000
>
>     [...]
>
>   I would like to propose adding a new value to this list.  A value of 0.
>   This value would indicate that the section group has no special processing
>   semantics, and that the contents of the group, unless discarded via garbage
>   collection, should be linked as normal.
>
>   This matches the current behaviour of the bfd and gold linkers, and it would
>   also help to resolve a problem for the lld linker:
>
> https://reviews.llvm.org/D56437
>
>   This is not a theoretical problem, as binaries containing section groups with
>   a flag value of zero already exist:
>
> https://bugzilla.redhat.com/show_bug.cgi?id=1658942
>
>   The maintainers of lld have stated, quite reasonably, that they do not want to
>   add support for an unofficial flag value.  Hence I would like to make the value
>   official.

According to my reading of the specification, the behavior of flag 0 is
already specified.  Setting GRP_COMDAT merely enables deduplication of
the section group based on its name.  With flag 0, the name is not used
and no deduplication happens.

Thanks,
Florian