Reserving specified size of RUNPATH entry in the dynamic section during linking

[Jacob Kroon]

On 11/28/21 12:44, Florian Weimer wrote:
> * Jacob Kroon via Gcc-help:
> 
>> As part of an effort to make binaries reproducible regardless of their
>> build path, I need to enforce the same size of the RUNPATH entry in the
>> dynamic section during linking, even though I don't fill it completely.
>> Is it possible to give some flag to gnu ld that allows me to set it to a
>> specific size ? Or is there a way to patch the elf file after linking,
>> so that the entry has a specified size ?
> 
> Do you want to allocate specific size so that you can patch in a
> different value later?
> 

Yes exactly, but see below.

> The RUNPATH strings are in the string table, so it's necessary to
> allocate space there, and be able to find it during patching.
> 
> Solaris offers this mechanism:
> 
> | DT_SUNW_STRPAD
> | 
> |     The total size, in bytes, of the unused reserved space at the end of
> |     the dynamic string table. If DT_SUNW_STRPAD is not present in an
> |     object, no reserved space is available.
> 
> Would that help in your case as well?
> 

The problem is that for two different build I pass two different
-Wl,--rpath=<path>, and they are of different length. So I'd like to
reserve a maximum size, at link-time, which becomes the same in both
builds, so that when I later remove the rpath's, the binaries become
identical.

Jacob