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[PATCH] ld: Check ELF relocs before allocation

H.J. Lu hjl.tools@gmail.com
Wed Dec 29 16:18:27 GMT 2021 
On Tue, Dec 28, 2021 at 11:46 PM Alan Modra <amodra@gmail.com> wrote:
>
> On Mon, Dec 27, 2021 at 06:48:33PM -0800, H.J. Lu via Binutils wrote:
> > To prepare for DT_RELR implemenation, delay checking ELF relocations
> > before allocation so that all input sections have been mapped to output
> > sections when relocations are checked.  This is only enabled for x86
> > targets.
>
> Please expand on why this is necessary.  "Prepare for DT_RELR
> implementation" doesn't tell me much.

DT_RELR encodes consecutive R_*_RELATIVE relocations in GOT (the global
offset table) in a compact format:

https://groups.google.com/g/generic-abi/c/bX460iggiKg

On some targets, R_*_RELATIVE relocations are counted and the GOT offsets
are allocated when setting the dynamic section sizes after seeing all
relocations.  R_*_RELATIVE relocations are generated while relocating
sections after section layout has been finalized.

To prepare for DT_RELR implementation on these targets, we need to delay
checking ELF relocations before allocation so that when relocations are
checked, all input sections have been mapped to output sections, dynamic
symbols are known and R_*_RELATIVE relocations can be counted.

DT_RELR encoding may be generated in the relax pass when R_*_RELATIVE
relocations and their GOT offsets are known.  A later fix up is needed if
the final GOT vma is different from when DT_RELR encoding was generated.

> Also, it seems to me that you could have made the functional changes
> in this patch by implementing an x86 before_allocation routine rather
> than introducing a new BFD_JUMP_TABLE_LINK entry.  Doing it that way
> would result in a much smaller patch with few if any changes to
> generic ELF linker code.

A new linker function, bfd_link_make_reloc_sections, is added to create
dynamic relocation sections after all input files have been opened if relocation
check is delayed.  Otherwise, the dynamic relocation sections are missing
when input sections are being mapped to output sections.   It is much
cleaner than checking relocations in a before_allocation routine.

-- 
H.J.
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