Amd64 relocation R_X86_64_32S in a static lib

[Philip Guenther]

On Wed, 6 Nov 2013, Torbjorn Granlund wrote:
...
> The change makes be quite worried, since it *seems* to be done without 
> proper understanding of the issues involved.

Appearances can be deceiving.


> Furthermore, the ABI change has not been properly announced,

I guess the release announcement wasn't enough?


> and the new ABI seems to be undocumented.

Please file that bug upstream with GNU binutils, as they are the ones that 
have set the behavior of the -pie option.


> That in turn makes me expect further ABI changes in future OpenBSD 
> releases, which will likely break our code again.

Looking at the total ABI, including such things as types definitions and 
kernel syscalls, I can _guarantee_ that it'll change.  Indeed, for 5.5 the 
type of time_t, clock_t, and ino_t has been changed to be long long on all 
archs, changing the size of many structures and rolling new syscall 
numbers.

But I don't think that that's the level you're talking about.  The 
low-level ABI of register use and accepted relocations changes much more 
rarely.  We'll probably get TLS support hammered out in a way we're happy 
with at some point, which will affect the %fs register of course, but 
that's about the only thing at that level I can think of in the pipeline, 
and it's doesn't affect existing code anyway.


>   Yes, you can.  If you look at libc.a, for example, you'll see it contains
>   R_X86_64_GOTPCREL and R_X86_64_PLT32 relocations.  Those work both when
>   building a PIE executable and when building a fully static non-PIE 
>   executable.  In the latter case they are resolved by the linker when 
>   generating the executable.
>   
> Are you telling me that the most future-proof code would be all-PIC in
> the *static* GMP build?  Should we perhaps stay away from R_X86_64_64
> (also in a static lib)?

Yes, and yes.


> In the absense of documentation, could you tell which of the relocs on 
> page 71 of the official AMD64 ELF ABI are to be supported on OpenBSD 
> from now and on?

I would love to see that information from the binutils people as well.


Philip