[RFC] SHF_GNU_RETAIN ELF Section Flag

[Florian Weimer]

* Carlos O'Donell:

> On 9/15/20 8:37 AM, Florian Weimer via Gnu-gabi wrote:
>> * Jozef Lawrynowicz:
>> 
>>> On Tue, Sep 15, 2020 at 02:09:22PM +0200, Florian Weimer wrote:
>>>> * Jozef Lawrynowicz:
>>>>
>>>>> I'd like to propose a new ELF section flag, SHF_GNU_RETAIN, for addition
>>>>> to the GNU gABI.
>>>>>
>>>>> This flag instructs the linker to "retain" the section in the output
>>>>> file, even if garbage collection would remove it because it appears
>>>>> unused.
>>>>
>>>> How does this flag interaction with libraries (.a files)?
>>>
>>> If a section in a library has SHF_GNU_RETAIN set, and that library gets
>>> searched by the linker for some undefined symbol, then the
>>> SHF_GNU_RETAIN section will also be pulled into the program, and
>>> retained in the linked output file.
>> 
>> Sorry, that's not quite what I meant.  What happens if the .o file with
>> SHF_GNU_RETAIN in an .a library is not otherwise referenced and thus
>> never loaded by the link editor?  How would the link editor realize that
>> it is even there?
>
> Why would it be loaded?

Hypothetically: Because the ranlib section tells the link editor to load
it (so more specification updates are needed).

> Why does the link editor need to detect the presence of such a file?

To make SHF_GNU_RETAIN work with libraries.

I think without that, the same effect can be had today with
SHF_GROUP/SHT_GROUP, perhaps with an assembler-only change to implement
the .retain pseudo.

Thanks,
Florian
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