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Re: Using linker script assignments in shared object
- From: Alan Modra <amodra at gmail dot com>
- To: Max Dmitrichenko <dmitrmax at gmail dot com>
- Cc: binutils at sourceware dot org
- Date: Thu, 29 May 2014 10:56:39 +0930
- Subject: Re: Using linker script assignments in shared object
- Authentication-results: sourceware.org; auth=none
- References: <CAMRqRqyTbGY60374hnVoc6PhLkWDNNd-1K2WwQ1WS=DFDMPtmQ at mail dot gmail dot com> <20140528141152 dot GI6679 at bubble dot grove dot modra dot org> <CAMRqRqzAe81W+DFP72P8D_i+_JoAtN82yFHZYQX6FFpoSzyGFQ at mail dot gmail dot com>
On Wed, May 28, 2014 at 06:18:19PM +0400, Max Dmitrichenko wrote:
> 2014-05-28 18:11 GMT+04:00 Alan Modra <firstname.lastname@example.org>:
> > On Wed, May 28, 2014 at 05:29:26PM +0400, Max Dmitrichenko wrote:
> >> But when I paste this code into shared object then I got some address
> >> in the middle of memory address space. Though readelf outputs
> >> following for this symbol:
> >> 552: 0000000000000005 0 NOTYPE LOCAL HIDDEN ABS test_ls_value
> > This isn't a problem with ld, but with the dynamic loader ld.so.
> > ld.so shouldn't relocate absolute symbols but does, because some
> > ancient linkers made symbols like _GLOBAL_OFFSET_TABLE_ absolute, and
> > GNU ld followed this bad practise. It's not impossible to fix in a
> > way that keeps backward compatibility (eg. bump EI_ABIVERSION to
> > indicate to ld.so that absolute symbols really are absolute), but no
> > one has cared enough to do the glibc and ld work required.
> So, is there any other way to pass some specified value to the code of
> shared object with the linker script?
You need to somehow stop the linker generating dynamic relocations for
locations where you use the symbol, and I don't think there is a way
to do that. Now that *is* a ld bug, and perhaps I was wrong to blame
ld.so for your testcase because I'll bet the dynamic relocations
emitted don't even mention test_ls_value.. They'll be some sort of
RELATIVE reloc without a symbol.
However, fixing this in ld might prove difficult to do without
breaking many linker scripts. For example, we define symbols like
"_end" outside of a section statement in the standard linker scripts,
which means they are absolute. Yet clearly a symbol like "_end" is
really meant to be section relative, ie. its value ought to change
when you load a shared library at a different location than its
Australia Development Lab, IBM