[RFC] SHF_GNU_RETAIN ELF Section Flag

[Carlos O'Donell]

On 9/15/20 9:29 AM, Jozef Lawrynowicz wrote:
> On Tue, Sep 15, 2020 at 02:55:05PM +0200, Florian Weimer wrote:
>> * 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).
>>
> 
> An SHF_GNU_RETAIN section would only be kept if it's containing object
> was loaded in the first place, and the section was therefore considered for
> garbage collection. So no, SHF_GNU_RETAIN is not intended to be used to
> force inclusion of sections which the linker would not have otherwise
> seen.
> SHF_GNU_RETAIN can be thought of to essentially "turn off" garbage
> collection for that section, rather than change the fundamental linking
> behavior for that section or containing object.
> 
> Carlos' ammendment to the definition is accurate:
> 
>> SHF_GNU_RETAIN
>>   When an object file containing such a marked section is included in
>>   the link, the section should not be garbage collected by the linker,
>>   even if it appears unused.
> 
>>> 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.
>>
> 
> Perhaps, but without making any further extensions, wouldn't the
> assembler need to know of a section which will definitiley be kept in
> the output file? Only the linker can truly know this, by looking at the
> entry point (when there is one).
> 
> A new bit in GRP_MASKOS could define that sections in a group with this
> flag must always be kept, but that seems like a more round about way of
> using a new section flag.

Florian's point here, and let me reiterate it to see if I understood it,
is that SHF_GROUP / SHT_GROUP is the right mechanic here because:

(a) *Something* needs the section that would otherwise have been garbage
    collected.

(b) Expressing the "depends on" relationship could be achieved with
    a SHF_GROUP / SHT_GROUP and a new bit GRP_DEPENDS to indicate that
    a group of sections depend upon eachother (k-connected dependency).

Then the linker during garbage collection must either be able to discard
all sections in the group or none of them.

The underlying idea here is that SHF_GNU_RETAIN is really an expression
of "depended upon by something" with no further information about the
dependee or other related dependents.

How would "depends on" (GRP_DEPENDS) be expressed to the developer?

They would have to put code, and data, and other things into the this
new group to make a collection of things that depend upon eachother
in some non-"symbol dependency" way.

In the end you have to define the collection of things that would go
into the section group.

-- 
Cheers,
Carlos.