Re: [RFD] How legal is it to delete dynamic tags?

[Nathaniel Smith <njs at pobox dot com>]

On Fri, Apr 15, 2016 at 8:08 AM, Matthew Fortune
<Matthew.Fortune@imgtec.com> wrote:
> I have a bug report from Debian showing that the DT_MIPS_RLD_MAP_REL
> tag (introduced on MIPS to support shared library debug with PIE)
> can be corrupted by a program called chrpath.
>
> https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=818909#43
>
> chrpath is designed to alter or remove DT_RPATH entries. Removal is
> a problem when such an entry precedes DT_MIPS_RLD_MAP_REL as the
> relative offset stored in DT_MIPS_RLD_MAP_REL then points to the
> wrong address.
>
> Firstly, to what extent is it OK to just delete a dynamic tag rather
> than set it to DT_NULL?
>
> Secondly was it a bad decision to create a slot-relative dynamic
> tag? I.e. If I were to fix chrpath to know that DT_MIPS_RLD_MAP_REL
> needs updating... are there likely to be more utilities out there
> that fiddle with dynamic tags in this way?

There's patchelf at least, which is like a fancier version of chrpath:

  https://github.com/NixOS/patchelf

So it probably has the same bug when deleting DT_RPATH / DT_RUNPATH /
DT_NEED entries. Also, some of patchelf's operations add new entries
to the dynamic tag table (e.g. adding a new DT_RUNPATH or DT_NEED
entry), which I think ends up involving larger rearrangements of the
file (e.g. moving the whole table to somewhere else where there's room
to expand it); it's likely that this might cause problems for your
slot-relative tag as well.

-n

-- 
Nathaniel J. Smith -- https://vorpus.org