[Bug ld/20828] [MIPS] produces invalid dynamic symbol table when --gc-sections is used since PR ld/13177 fix

[james410 at cowgill dot org.uk]

https://sourceware.org/bugzilla/show_bug.cgi?id=20828

--- Comment #16 from James Cowgill  ---
(In reply to Maciej W. Rozycki from comment #15)
> NB while I'll implement the missing dynsym ordering case for the MIPS
> backend so that it follows the ELF gABI even if weird output is somehow
> produced by the generic BFD ELF linker I maintain that dropping symbols
> that have been GC-ed (but not other forced local ones) is the right
> solution.

So you you think the second patch is wrong (since it drops all forced local
symbols)?

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[Bug ld/20828] [MIPS] produces invalid dynamic symbol table when --gc-sections is used since PR ld/13177 fix

[ma...@linux-mips.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=20828

Maciej W. Rozycki  changed:

   What|Removed |Added

 Status|NEW |ASSIGNED

--- Comment #15 from Maciej W. Rozycki  ---
I can't comment on individual distribution policies here, this is not the
right place to discuss them.

As per the milestone set this will be fixed with 2.28.  If this does not
satisfy your distribution's requirements, then you have several options
to choose from, including but possibly not limited to these:

1. Propose a change suitable for inclusion with our sources; I have
   outlined one earlier on although there may be other acceptable ways to
   do it too.

2. Temporarily use one of the quick fixes proposed here.

3. Work the problem around by disabling linker GC.

It is then up to individual distribution's maintainers to decide which of
these options suits them best; in particular submitting a change that can
be accepted right away is always welcome and guarantees quick processing.

NB while I'll implement the missing dynsym ordering case for the MIPS
backend so that it follows the ELF gABI even if weird output is somehow
produced by the generic BFD ELF linker I maintain that dropping symbols
that have been GC-ed (but not other forced local ones) is the right
solution.

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[Bug ld/20828] [MIPS] produces invalid dynamic symbol table when --gc-sections is used since PR ld/13177 fix

[glaubitz at physik dot fu-berlin.de]

https://sourceware.org/bugzilla/show_bug.cgi?id=20828

--- Comment #14 from John Paul Adrian Glaubitz  ---
Hi Maciej!

(In reply to Maciej W. Rozycki from comment #12)
> There's been no further feedback from other people and myself I haven't
> made up my mind about this bug yet, so if you are blocked with your
> work, then for the time being please use one of the proposals posted
> here as a temporary workaround.

The problem with this approach is that our toolchain maintainers are unwilling
to merge a patch which has not been fully approved upstream for a supported
target architecture.

This means, the binutils package in Debian is still broken on all MIPS targets
(mips32 big-endian, mips32 little-endian, mips64 little-endian) in Debian and
if the issue is not resolved within the near future, the upcoming Debian
Stretch release may be shipped without any official support for MIPS [1].

So, from the Debian point-of-view, fixing this bug has urgent priority unless
we want to waive for MIPS support.

Thanks,
Adrian

> [1] https://release.debian.org/stretch/arch_qualify.html

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[Bug ld/20828] [MIPS] produces invalid dynamic symbol table when --gc-sections is used since PR ld/13177 fix

[glaubitz at physik dot fu-berlin.de]

https://sourceware.org/bugzilla/show_bug.cgi?id=20828

John Paul Adrian Glaubitz  changed:

   What|Removed |Added

 CC||glaubitz at physik dot 
fu-berlin.d
   ||e

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[Bug ld/20828] [MIPS] produces invalid dynamic symbol table when --gc-sections is used since PR ld/13177 fix

[pochu27 at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=20828

--- Comment #13 from Emilio Pozuelo Monfort  ---
This has been broken in the stable binutils-2_27-branch for a while now, which
is rather unfortunate. Granted I could just take this patch myself, but a
better solution would be to commit one of the fixes (both to master and the
2.27 and 2.28 branches). A thorough fix is nice, but 'perfect is the enemy of
good'.

Thanks for working on binutils btw.

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[Bug ld/20828] [MIPS] produces invalid dynamic symbol table when --gc-sections is used since PR ld/13177 fix

[ma...@linux-mips.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=20828

--- Comment #12 from Maciej W. Rozycki  ---
There's been no further feedback from other people and myself I haven't
made up my mind about this bug yet, so if you are blocked with your
work, then for the time being please use one of the proposals posted
here as a temporary workaround.

I will come up with a proper dynsym sorting implementation (which I
think we ought to have for general robustness, and which I think I will
implement by splitting GGA_NONE into two subclasses, so that we don't
need any extra passes over the dynsym table to get it sorted) before
2.28, and then whether we want to keep local dynamic symbols or not can
be decided independently; for consistency I'd be leaning towards doing
exactly the same what a linker version script does with symbols that
end up forced local, and the details of which I don't remember offhand.

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[Bug ld/20828] [MIPS] produces invalid dynamic symbol table when --gc-sections is used since PR ld/13177 fix

[pochu27 at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=20828

--- Comment #11 from Emilio Pozuelo Monfort  ---
So do we want to sort the symbols, or drop them altogether?

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[Bug ld/20828] [MIPS] produces invalid dynamic symbol table when --gc-sections is used since PR ld/13177 fix

[ma...@linux-mips.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=20828

Maciej W. Rozycki  changed:

   What|Removed |Added

   Assignee|unassigned at sourceware dot org   |ma...@linux-mips.org
   Target Milestone|--- |2.28

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[Bug ld/20828] [MIPS] produces invalid dynamic symbol table when --gc-sections is used since PR ld/13177 fix

[james410 at cowgill dot org.uk]

https://sourceware.org/bugzilla/show_bug.cgi?id=20828

--- Comment #10 from James Cowgill  ---
(In reply to Maciej W. Rozycki from comment #8)
> By the MIPS psABI's definition you shall not have external GGA_NONE
> symbols as any GGA_NONE symbols will be assigned to the local GOT part,
> whose entries are only relocated by the base address at the load time.
> Symbols in the GGA_NONE class will normally not have dynsym entries,
> except from selected section symbols.

The psABI says:
The dynamic symbol table, like all ELF symbol tables, is divided into local and
global parts. The global part of the dynamic symbol table is further divided
into two parts: symbols that do not have GOT entries associated with them and
symbols that do have GOT entries associated with them.

Surly this implies that it is legitimate for an external symbol to be GGA_NONE?
The case illustrated here where a symbol is defined but not used within a
shared library seems like a good use case for this.

Since LOCAL symbols must be GGA_NONE, the local symbol handler in my patch
could probably be simplified a little.

[...]
> I agree the presence of (non-section) local symbols in the dynsym table
> is a generic issue, and given that `elf_gc_sweep_symbol' appears to be
> the only place where they are created I think that rather than
> adjusting this piece of code to assign these symbols to the GGA_NONE
> class it will make sense to discard them altogether.  Unless there is
> an actual need for them, that is, which I yet need to be told about.
> FAOD it is not incorrect to have them -- it is just useless.

It breaks the ld-elf/provide-hidden testcase, but that can be adjusted to not
look for local dynamic symbols. This patch implements what you suggest
(dropping symbols defined in the linker script dynamic symbol table if they are
definitely local) and on its own fixes this bug as well.

--- a/bfd/elflink.c
+++ b/bfd/elflink.c
@@ -669,7 +669,8 @@ bfd_elf_record_link_assignment (bfd *output_bfd,
|| h->ref_dynamic
|| bfd_link_dll (info)
|| elf_hash_table (info)->is_relocatable_executable)
-  && h->dynindx == -1)
+  && h->dynindx == -1
+  && h->forced_local == 0)
 {
   if (! bfd_elf_link_record_dynamic_symbol (info, h))
return FALSE;

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[Bug ld/20828] [MIPS] produces invalid dynamic symbol table when --gc-sections is used since PR ld/13177 fix

[james410 at cowgill dot org.uk]

https://sourceware.org/bugzilla/show_bug.cgi?id=20828

--- Comment #9 from James Cowgill  ---
I added some debug code ontop of my patch and it seems that at the time the
dynamic symbol table indexes are allocated, all the symbols are GGA_NONE.

debug: _fdata, LOCAL, GGA_NONE
debug: _ftext, LOCAL, GGA_NONE
debug: __bss_start, LOCAL, GGA_NONE
debug: _edata, LOCAL, GGA_NONE
debug: _end, LOCAL, GGA_NONE
debug: _fbss, LOCAL, GGA_NONE
debug: psi, GLOBAL, GGA_NONE
debug: omega, GLOBAL, GGA_NONE

Even when --gc-sections is removed all the symbols are GGA_NONE (although they
are all now GLOBAL). Clearly at least psi and omega are external symbols.

If I adjust stubs.c a bit, it seems a symbol is only ever GGA_NORMAL if it
actually needs a GOT entry (eg if I call psi from omega, psi becomes GGA_NORMAL
but omega remains GGA_NONE).

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[Bug ld/20828] [MIPS] produces invalid dynamic symbol table when --gc-sections is used since PR ld/13177 fix

[ma...@linux-mips.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=20828

--- Comment #8 from Maciej W. Rozycki  ---
By the MIPS psABI's definition you shall not have external GGA_NONE
symbols as any GGA_NONE symbols will be assigned to the local GOT part,
whose entries are only relocated by the base address at the load time.
Symbols in the GGA_NONE class will normally not have dynsym entries,
except from selected section symbols.

Any external symbols belong to the GGA_NORMAL and GGA_RELOC_ONLY
classes -- depending on whether there are any GOT relocations implying
signed 16-bit GP-relative access referring to them or not -- and they
will be assigned to the global GOT part.  The indices of those symbols
will be mapped between the GOT and the dynsym table, as mandated by the
MIPS psABI, according to the DT_MIPS_LOCAL_GOTNO and DT_MIPS_GOTSYM
dynamic tags, such that individual entry's indices relative to the
beginning of the global part of both tables will be the same.

NB GGA stands for Global GOT Area.

I agree the presence of (non-section) local symbols in the dynsym table
is a generic issue, and given that `elf_gc_sweep_symbol' appears to be
the only place where they are created I think that rather than
adjusting this piece of code to assign these symbols to the GGA_NONE
class it will make sense to discard them altogether.  Unless there is
an actual need for them, that is, which I yet need to be told about.
FAOD it is not incorrect to have them -- it is just useless.

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[Bug ld/20828] [MIPS] produces invalid dynamic symbol table when --gc-sections is used since PR ld/13177 fix

[james410 at cowgill dot org.uk]

https://sourceware.org/bugzilla/show_bug.cgi?id=20828

--- Comment #7 from James Cowgill  ---
(In reply to Maciej W. Rozycki from comment #6)
> Do we need to have these hidden symbols in the dynsym table at all in
> the first place?  It looks like unnecessary clutter to me, they won't
> ever be used for anything as their references have been gc-ed.  Or am
> I missing anything -- can someone confirm my conclusion or contradict
> it by providing a use case?

Possibly not, although it looks like a generic issue - these symbols appear
when the test is run on x86_64.

> If we do need these symbols in the dynsym table, then I think we just
> need to set GGA_NONE for them correctly as they're being hidden and the
> existing dynsym index selection code will DTRT.

I don't think doing this will ensure they always appear before the other
GGA_NONE global symbols.

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[Bug ld/20828] [MIPS] produces invalid dynamic symbol table when --gc-sections is used since PR ld/13177 fix

[ma...@linux-mips.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=20828

--- Comment #6 from Maciej W. Rozycki  ---
Do we need to have these hidden symbols in the dynsym table at all in
the first place?  It looks like unnecessary clutter to me, they won't
ever be used for anything as their references have been gc-ed.  Or am
I missing anything -- can someone confirm my conclusion or contradict
it by providing a use case?

If we do need these symbols in the dynsym table, then I think we just
need to set GGA_NONE for them correctly as they're being hidden and the
existing dynsym index selection code will DTRT.

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[Bug ld/20828] [MIPS] produces invalid dynamic symbol table when --gc-sections is used since PR ld/13177 fix

[pochu27 at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=20828

--- Comment #5 from Emilio Pozuelo Monfort  ---
(In reply to Alan Modra from comment #4)
> On second thoughts, don't look at elf_link_output_extsym.  bind_local there
> can't be right since it's too late to make any decision different to that
> done by _bfd_elf_link_renumber_dynsyms.

So does the patch look sane? Or do you still have any comments?

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[Bug ld/20828] [MIPS] produces invalid dynamic symbol table when --gc-sections is used since PR ld/13177 fix

[amodra at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=20828

--- Comment #4 from Alan Modra  ---
On second thoughts, don't look at elf_link_output_extsym.  bind_local there
can't be right since it's too late to make any decision different to that done
by _bfd_elf_link_renumber_dynsyms.

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[Bug ld/20828] [MIPS] produces invalid dynamic symbol table when --gc-sections is used since PR ld/13177 fix

[amodra at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=20828

--- Comment #3 from Alan Modra  ---
> Is checking forced_local enough?
See elflink.c:elf_link_output_extsym local_bind.

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[Bug ld/20828] [MIPS] produces invalid dynamic symbol table when --gc-sections is used since PR ld/13177 fix

[mh-sourceware at glandium dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=20828

Mike Hommey  changed:

   What|Removed |Added

 CC||mh-sourceware at glandium dot 
org

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[Bug ld/20828] [MIPS] produces invalid dynamic symbol table when --gc-sections is used since PR ld/13177 fix

[james410 at cowgill dot org.uk]

https://sourceware.org/bugzilla/show_bug.cgi?id=20828

--- Comment #2 from James Cowgill  ---
Created attachment 9649
  --> https://sourceware.org/bugzilla/attachment.cgi?id=9649=edit
patch attempt 1

So I had a go at fixing this. The patch basically does an extra pass to assign
indexes to all the forced_local symbols first and the original code handles the
rest of the symbols. It fixes my testcase and the original mesa bug in Debian.

As I'm not too familiar with the internals of binutils, I'm not totally sure
this is right. Is checking forced_local enough?

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[Bug ld/20828] [MIPS] produces invalid dynamic symbol table when --gc-sections is used since PR ld/13177 fix

[pochu27 at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=20828

Emilio Pozuelo Monfort  changed:

   What|Removed |Added

 CC||pochu27 at gmail dot com

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[Bug ld/20828] [MIPS] produces invalid dynamic symbol table when --gc-sections is used since PR ld/13177 fix

[amodra at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=20828

Alan Modra  changed:

   What|Removed |Added

 Status|UNCONFIRMED |NEW
   Last reconfirmed||2016-11-17
 CC||amodra at gmail dot com
 Ever confirmed|0   |1

--- Comment #1 from Alan Modra  ---
mips_elf_sort_hash_table_f takes no notice of local dynamic symbols, so that's
why you have local and global symbols out of order.

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[Bug ld/20828] [MIPS] produces invalid dynamic symbol table when --gc-sections is used since PR ld/13177 fix

[ma...@linux-mips.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=20828

Maciej W. Rozycki  changed:

   What|Removed |Added

 CC||ma...@linux-mips.org

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