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Re: Undo dynamic symbol state after regular object sym type mismatch
- From: "Maciej W. Rozycki" <macro at imgtec dot com>
- To: Alan Modra <amodra at gmail dot com>
- Cc: <binutils at sourceware dot org>, <libc-alpha at sourceware dot org>
- Date: Mon, 24 Jul 2017 12:22:34 +0100
- Subject: Re: Undo dynamic symbol state after regular object sym type mismatch
- Authentication-results: sourceware.org; auth=none
- References: <20170417140540.GX24006@bubble.grove.modra.org> <alpine.DEB.email@example.com> <20170715121356.GC14520@bubble.grove.modra.org>
CC-ing `libc-alpha' for any feedback on the dynamic loader issue.
> > In an attempt to prevent new MIPS target failures in preparation for the
> > 2.29 release I have now looked into it and what the MIPS backend produces
> > looks correct to me according to the MIPS psABI.
> > Specifically, while linking the `indirect5a' executable, in the
> > `indirect5a.o' object supplied we have a locally-defined global `bar'
> > symbol referred to -- in the case of the o32 ABI -- with an R_MIPS_GOT16
> > relocation, which is absolutely fine for a MIPS SVR4 psABI executable.
> > In response the MIPS backend creates the mandatory global GOT entry and
> That's fine.
If we want to prevent some global symbols from being exported, then not
according to the SVR4 MIPS psABI, which requires each global GOT entry to
have a corresponding global dynamic symbol table entry.
> > its corresponding dynsym entry for `bar'.
> But in my opinion this is a bug. A symbol in an executable shouldn't
> be made dynamic unless it is referenced by a dynamic object, or by
> --export-dynamic or other similar options. Since "bar" in the
> executable is a variable and "bar" in the dynamic library a function,
> the dynamic library can't be referencing the executable "bar". The
> executable GOT entry for "bar" will need a dynamic relocation in a
> PIE, but that can be a relative relocation not needing a symbol.
The MIPS SVR4 psABI uses implicitly relocated GOT, there are no dynamic
GOT relocations defined (the only dynamic reloc is R_MIPS_REL32, for
static references to global symbols) and dynamic GOT relocation is
performed according to the DT_MIPS_GOTSYM dynamic entry: all entries whose
index is lower are relocated by the base address and the entries that
follow are relocated according to the external part of the dynamic symbol
table, which has to be identity mapped to the external part of GOT. So an
external GOT entry by definition is associated with an external dynamic
So if we want to prevent a global symbol from being exported, then (as
long as it is locally referenced) it has to be assigned to the local part
of the GOT, which in turn means either giving it a local dynamic symbol
table entry or (IMO preferably) none at all. However this will cause
trouble for undefined weak symbols in PIE executables, as making these
symbols local will make the corresponding GOT entries relocated by the
base address and consequently any NULL pointer checks which typically
guard the use of such symbols will not work anymore, as already noted in
Note that in the SVR4 MIPS psABI there is no way defined for a GOT entry
to remain unchanged in dynamic loading: each is either relocated by the
base address or refers to a global dynamic symbol table entry.
Contrariwise static references to global symbols are not an issue as
R_MIPS_REL32 relocations can be simply omitted, leaving the reference
unrelocated in the dynamic load.
The only solution to PR ld/21375 I have come up with so far is using a
global absolute symbol for GOT entries used for any local references to
undefined weak symbols, either by tweaking the attributes of the original
symbol (setting SHN_ABS/STB_GLOBAL combined with STV_HIDDEN or
STV_INTERNAL; but that breaks strict gABI compliance) or by redirecting to
a special reserved symbol, say `__gnu_absolute_zero', whose value is
globally zero (and SHN_ABS/STB_GLOBAL/STV_DEFAULT). The latter approach
has the advantage of reducing the number of GOT entries (which is always
advantageous) where multiple undefined weak symbols are present.
However this merely shifts the issue with PIE binaries to the dynamic
loader, which currently does not tell absolute and regular symbols apart
and relocates both kinds by the base address, as noted in glibc BZ 19818.
This is PR ld/21805 now, for tracking purposes. I have experimented with
a preliminary fix, and in addition to cleaning indirect.exp test results
it indeed makes a number of other test cases crash, that work fine right
now. In the course of this investigation I have also identified what I
think are a generic ELF linker bug and an issue in the Cris backend
respectively, both of which I'll be sending patches for soon.
So I think the correct way to address the problem with the MIPS backend
1. Fix glibc BZ 19818, the dynamic loader's absolute symbol bug.
2. Bump ABIVERSION up in `ld.so' for correct SHN_ABS support.
3. Fix PR ld/21375, the local undefined weak symbol bug.
4. Document `--export-dynamic' as the default with MIPS PIE binaries.
5. Fix PR ld/21805, this issue, by making `--no-export-dynamic' work for
SVR4 executables in the first place and then with a PIE executable
produce SHN_ABS symbol GOT references for undefined weak symbols and
make the executable request the corresponding dynamic loader's
ABIVERSION for processing, possibly only where such a SHN_ABS symbol
has indeed been produced.
6. Set the defaults for `--export-dynamic'/`--no-export-dynamic' as
-- in this order.
I am yet reviewing past discussions about our handling of undefined weak
symbols in PIE executables, in particular PR ld/2218 and the corresponding
thread at: <https://sourceware.org/ml/binutils/2006-01/msg00193.html> and
then <https://sourceware.org/ml/binutils/2006-02/msg00023.html>, because
it seems we're a bit inconsistent here among targets as far as making them
global or local is concerned, and this is not covered by the ELF gABI
Personally to keep their handling consistent between the static link and
the dynamic load I am leaning towards thinking that weak symbol references
from the main executable that are not satisfied at the static link time
ought to resolve to zero with the original reference removed, and ones
that are satisfied by a dynamic dependency ought to be converted to a
global symbol reference.
Questions or comments? I will appreciate further input. This is
obviously too complex an issue for the pending 2.29 binutils release.