On Sat, Jul 25, 2020 at 01:11:25AM -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. Cool bit of investigation, thanks for looking at that :-) Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-top is 'top' for virtual machines. Tiny program with many powerful monitoring features, net stats, disk stats, logging, etc. http://people.redhat.com/~rjones/virt-top _______________________________________________ devel mailing list -- devel@xxxxxxxxxxxxxxxxxxxxxxx To unsubscribe send an email to devel-leave@xxxxxxxxxxxxxxxxxxxxxxx Fedora Code of Conduct: https://docs.fedoraproject.org/en-US/project/code-of-conduct/ List Guidelines: https://fedoraproject.org/wiki/Mailing_list_guidelines List Archives: https://lists.fedoraproject.org/archives/list/devel@xxxxxxxxxxxxxxxxxxxxxxx
