On Mon, 2020-07-27 at 18:20 +0200, Nikola Forró wrote:
> On Sat, 2020-07-25 at 01:11 -0600, Jeff Law wrote:
> > So at a high level ar makes a call to lrealpath. That naturally goes through the
> > PLT. The PLT stub loads the value out of the GOT and jumps to it. The problem
> > is the entry in the GOT is *zero* when it should be pointing to the resolver.
> >
> > Now lrealpath is provided by libiberty and a copy is in libbfd.so and the GOT
> > entry in libbfd.so looked right. But by the time the program has hit main, the
> > GOT entry has been reset to zero. Naturally that's happening inside the dynamic
> > linker (as expected, confirmed with a watchpoint). If you've ever had to debug
> > ld.so, you'll know it's an insanely painful experience, but it proved fruitful.
> >
> > The key was finding out that we were not using the libbfd.so linker map to
> > resolve lrealpath, instead we were using the linker map for the main program (ar
> > in this case). So natrually it's time to look a bit more closely at the symbol
> > table for ar.
> >
> > The main symbol table for ar it doesn't mention lrealpath. But that's just a
> > confusing byproduct of having two symbol tables. What matters to ld.so is the
> > *dynamic* symbol table. And ar has lrealpath in its dynamic symbol table. And
> > here's the kicker, it's an absolute symbol with the value 0:
> >
> > 0000000000000000 A lrealpath
> >
> > A symbol in the main program takes precedence over a symbol in a DSO. So the
> > dynamic linker was actually doing the right thing given the input it was
> > provided.
> >
> > Now why (*&@#$ does ar have lrealpath as an absolute symbol? It's got to be
> > related to the fact that when we link ar we pull in another copy of libiberty.
> > In fact, ar links against libiberty twice. Once via -liberty then again against
> > libiberty.a (and kindof a 3rd time indirectly via libbfd). BUt even so that
> > shouldn't be creating an absolute symbol. That's just weird.
> >
> > This smells like a linker bug to me. Not surprisingly if I force the system to
> > use ld.gold, then I don't see the bogus absolute symbol and the resultant ar
> > works just fine.
> >
> > It's late and I'll dig further over the weekend, but right now this looks like a
> > linker bug to me. I may turn off LTO globally or in the various instances of
> > binutils -- I need to sleep on that.
>
> I'm seeing the same behavior with man-db, more specifically with accessdb
> linking to libmandb:
>
> $ nm -D accessdb | grep xmalloc
> 0000000000000000 A xmalloc
>
> Obviously it segfaults, unless I disable LTO.
>
> Is there a bugzilla for that linker bug?
I don't think so. Nick was trying to pull together a simpler testcase and open a
discussion with the other binutils developers on a path forward. He's aware of
the impacts, so I'm sure he's working diligently on it.
In the immediate term, disabling LTO seems reasonable.
%define _lto_cflags %{nil}
We're going to go through all the opt-outs at some point after the mass rebuild,
so we can re-enable once the ld bug is fixed.
jeff
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