Re: RFA: Add a new gynamic tag: DT_GNU_GOT_PLT_END

[Florian Weimer <fweimer@redhat.com>]

On 06/22/2018 11:28 PM, Cary Coutant wrote:
> Third: Adding an "end of section" address is unusual for the dynamic
> table. You've been quite explicit to say "the address of the first
> byte beyond the end of the .got.plt", but it's actually much less
> error prone to give the size of the section rather than its ending
> address. You could have DT_PLTGOTSZ, which would parallel other
> dynamic tags.

With the size, it's also possible to express that the section ends at 
the top of the address space.  (Although this doesn't really matter in 
practice.)

> Fourth: Isn't the size of .got.plt inferrable from DT_PLTRELSZ? Divide
> that by the size of each relocation (DT_PLTREL == DT_REL ? DT_RELENT :
> DT_RELAENT), then multiply by the size of the PLTGOT entry (4 or 8),
> then add the three reserved entries. There's a one-to-one relationship
> between the PLT and the PLTGOT, and every PLTGOT entry must have a
> relocation.

The PLT count wouldn't include the padding at the end of the section, so 
it doesn't confer the required information.

> Fifth: If all you're trying to say is "the .got.plt section is
> isolated on its own pages", couldn't you just use one of the DT_FLAGS
> or DT_FLAGS_1? Letting the presence of an END or SZ entry signal this
> condition is a bit risky -- what if someone decides they'd like to
> record the size of .got.plt even when it's not isolated on its own
> pages?

Well, isn't this a problem no matter what we specify?  That someone 
might provide incorrect information?  If we use a number, at least 
disagreement about the run-time page size isn't a source of potential 
issues.

Thanks,
Florian