[LLVMdev] ORC and relocations

[Eugene Rozenfeld]

Hi Lang,

Can you please let me know you think it would be right to modify RuntimeDyldImpl::resolveExternalSymbols to allow resolvers to return 0 addresses? Something like this would be ideal for me:

void RuntimeDyldImpl::resolveExternalSymbols() {
  while (!ExternalSymbolRelocations.empty()) {
    StringMap<RelocationList>::iterator i = ExternalSymbolRelocations.begin();

    StringRef Name = i->first();
    if (Name.size() == 0) {
      // This is an absolute symbol, use an address of zero.
      DEBUG(dbgs() << "Resolving absolute relocations."
                   << "\n");
      RelocationList &Relocs = i->second;
      resolveRelocationList(Relocs, 0);
    } else {
      uint64_t Addr = 0;
      RTDyldSymbolTable::const_iterator Loc = GlobalSymbolTable.find(Name);
      if (Loc == GlobalSymbolTable.end()) {
        // This is an external symbol, try to get its address from the symbol
        // resolver.
        Addr = Resolver.findSymbol(Name.data()).getAddress();
        // The call to getSymbolAddress may have caused additional modules to
        // be loaded, which may have added new entries to the
        // ExternalSymbolRelocations map.  Consquently, we need to update our
        // iterator.  This is also why retrieval of the relocation list
        // associated with this symbol is deferred until below this point.
        // New entries may have been added to the relocation list.
        i = ExternalSymbolRelocations.find(Name);
      } else {
        // We found the symbol in our global table.  It was probably in a
        // Module that we loaded previously.
        const auto &SymInfo = Loc->second;
        Addr = getSectionLoadAddress(SymInfo.getSectionID()) +
               SymInfo.getOffset();
      }

      if (Addr) {
        DEBUG(dbgs() << "Resolving relocations Name: " << Name << "\t"
          << format("0x%lx", Addr) << "\n");
        // This list may have been updated when we called getSymbolAddress, so
        // don't change this code to get the list earlier.
        RelocationList &Relocs = i->second;
        resolveRelocationList(Relocs, Addr);
      }
    }

    ExternalSymbolRelocations.erase(i);
  }
}

Thanks,

Eugene
From: Eugene Rozenfeld
Sent: Wednesday, June 24, 2015 4:47 PM
To: 'Lang Hames'
Cc: llvmdev at cs.uiuc.edu
Subject: RE: ORC and relocations

Hi Lang,

Thank you for your reply. I tried to supply a custom Resolver that just returns 0 for the symbols I want to handle manually, but that results in a fatal error below. If that can be changed, then yes,
it would be sufficient for my use case.

Thanks,

Eugene

void RuntimeDyldImpl::resolveExternalSymbols() {
  while (!ExternalSymbolRelocations.empty()) {
    StringMap<RelocationList>::iterator i = ExternalSymbolRelocations.begin();

    StringRef Name = i->first();
    if (Name.size() == 0) {
      // This is an absolute symbol, use an address of zero.
      DEBUG(dbgs() << "Resolving absolute relocations."
                   << "\n");
      RelocationList &Relocs = i->second;
      resolveRelocationList(Relocs, 0);
    } else {
      uint64_t Addr = 0;
      RTDyldSymbolTable::const_iterator Loc = GlobalSymbolTable.find(Name);
      if (Loc == GlobalSymbolTable.end()) {
        // This is an external symbol, try to get its address from the symbol
        // resolver.
        Addr = Resolver.findSymbol(Name.data()).getAddress();
        // The call to getSymbolAddress may have caused additional modules to
        // be loaded, which may have added new entries to the
        // ExternalSymbolRelocations map.  Consquently, we need to update our
        // iterator.  This is also why retrieval of the relocation list
        // associated with this symbol is deferred until below this point.
        // New entries may have been added to the relocation list.
        i = ExternalSymbolRelocations.find(Name);
      } else {
        // We found the symbol in our global table.  It was probably in a
        // Module that we loaded previously.
        const auto &SymInfo = Loc->second;
        Addr = getSectionLoadAddress(SymInfo.getSectionID()) +
               SymInfo.getOffset();
      }

      // FIXME: Implement error handling that doesn't kill the host program!
      if (!Addr) {
        report_fatal_error("Program used external function '" + Name +
                           "' which could not be resolved!");
      }

      DEBUG(dbgs() << "Resolving relocations Name: " << Name << "\t"
        << format("0x%lx", Addr) << "\n");
      // This list may have been updated when we called getSymbolAddress, so
      // don't change this code to get the list earlier.
      RelocationList &Relocs = i->second;
      resolveRelocationList(Relocs, Addr);
    }

    ExternalSymbolRelocations.erase(i);
  }
}


From: Lang Hames [mailto:lhames at gmail.com]
Sent: Wednesday, June 24, 2015 4:42 PM
To: Eugene Rozenfeld
Cc: llvmdev at cs.uiuc.edu<mailto:llvmdev at cs.uiuc.edu>
Subject: Re: ORC and relocations

Hi Eugene,

There's no way to 'skip' application of a relocation.

You could use the NotifyLoadedFtor and libObject to scan the object ahead of time and record the relocations that you'd like to override, then you could supply a custom Resolver that just returns '0' for the symbols that you want to handle manually.

There's no way to handle distinct relocations for the same symbol differently: For performance reasons we only look up each symbol once and re-use the supplied address to apply every relocation for that symbol.

Is that sufficient to support your use-case?

Cheers,
Lang.

On Wed, Jun 24, 2015 at 12:03 AM, Eugene Rozenfeld <Eugene.Rozenfeld at microsoft.com<mailto:Eugene.Rozenfeld at microsoft.com>> wrote:
Hello,

I’m working on LLILC (a jit for the CoreCLR built on ORC), in particular, on using LLILC as an ngen jit. I would like to have an ability to be notified of relocations that ObjectLinkingLayer  is applying and to be able to tell the linking layer not to resolve certain relocations for external symbols (so that the client can do some custom resolutions later). The only way I found of looking at relocations in the client is via NotifyLoadedFtor notifications but I couldn’t find a way of blocking a relocation resolution.

One way to achieve that is to change the Resolver::findSymbol api to allow clients to indicate that the relocations for the symbol shouldn’t be resolved and update RuntimeDyldImpl::resolveExternalSymbols accordingly. Would this be a reasonable approach?

Thanks,

Eugene


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