[llvm-dev] [lld] ObjFile::createRegular is oblivious of PendingComdat

Thu Jun 7 15:20:57 PDT 2018

Hi Reid, Thanks for the heads up.  I'll try to compile everything with
Clang + LLD.

Peter, I am using a MinGW target, so Clang invokes ld.lld instead of
lld-link.  But the MinGW driver doesn't seem to support --reproduce.

On Thu, Jun 7, 2018 at 2:52 PM Peter Collingbourne <peter at pcc.me.uk> wrote:

> Can you upload a reproducer? You can create it using the /linkrepro flag.
>
> Peter
>
> On Thu, Jun 7, 2018 at 2:50 PM, Reid Kleckner via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> GCC does comdats completely differently from the spec. Since you
>> contacted me about this off of the mailing list, I started investigating
>> what they do, and it is completely different. It's basically an alternative
>> comdat ABI. I guess because GCC never considered it important to implement
>> C++ ABI compatibility, they just did their own thing with comdats, since
>> they aren't really part of the C ABI.
>>
>> Anyway, if we want to be C++ ABI compatible with GCC+binutils on Windows,
>> there will be a lot of work to do. It's not a small patch here or there, it
>> requires more research. We should discuss it in full.
>>
>> I support recovering from invalid COFF files to avoid crashes in any way
>> we can, though, so this patch might still be good.
>>
>> On Thu, Jun 7, 2018 at 2:44 PM Pirama Arumuga Nainar via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>> I encountered a segfault when using lld to cross-compile for Windows
>>> (+MinGW) from Linux.  The problem happens with objects built by gcc.  The
>>> problem is that ObjFile::CreateRegular considers a PendingComdat to be
>>> valid (and later causes an illegal pointer dereference).  The following
>>> patch fixes the crash:
>>>
>>> diff --git a/COFF/InputFiles.cpp b/COFF/InputFiles.cpp
>>> index 9e2345b0a..f47d612df 100644
>>> --- a/COFF/InputFiles.cpp
>>> +++ b/COFF/InputFiles.cpp
>>> @@ -229,11 +229,11 @@ Symbol *ObjFile::createRegular(COFFSymbolRef Sym) {
>>>    if (Sym.isExternal()) {
>>>      StringRef Name;
>>>      COFFObj->getSymbolName(Sym, Name);
>>> -    if (SC)
>>> +    if (SC && SC != PendingComdat)
>>>        return Symtab->addRegular(this, Name, Sym.getGeneric(), SC);
>>>      return Symtab->addUndefined(Name, this, false);
>>>    }
>>> -  if (SC)
>>> +  if (SC && SC != PendingComdat)
>>>      return make<DefinedRegular>(this, /*Name*/ "", false,
>>>                                  /*IsExternal*/ false, Sym.getGeneric(),
>>> SC);
>>>    return nullptr;
>>>
>>> I can upload this to Phabricator in case this is the right fix.  If not,
>>> I'll upload a reproducer for further investigation.  (I omitted it to avoid
>>> manually packaging a whole set of object files - I think
>>> --reproduce=<repro> doesn't work on Windows yet).
>>>
>>> For reference, here's the segfault:
>>> #0 0x0000000000500999 llvm::sys::PrintStackTrace(llvm::raw_ostream&)
>>> /ssd2/pirama/ll$
>>> m-upstream/llvm/lib/Support/Unix/Signals.inc:488:11
>>>
>>> #1 0x0000000000500b49 PrintStackTraceSignalHandler(void*)
>>> /ssd2/pirama/llvm-upstream$
>>> llvm/lib/Support/Unix/Signals.inc:552:1
>>> #2 0x00000000004ff1d6 llvm::sys::RunSignalHandlers()
>>> /ssd2/pirama/llvm-upstream/llvm$
>>> lib/Support/Signals.cpp:66:5
>>> #3 0x0000000000500eb7 SignalHandler(int)
>>> /ssd2/pirama/llvm-upstream/llvm/lib/Support$
>>> Unix/Signals.inc:351:1
>>> #4 0x00007fc7bfaff0c0 __restore_rt
>>> (/lib/x86_64-linux-gnu/libpthread.so.0+0x110c0)
>>> #5 0x00000000005fe370 lld::coff::DefinedRegular::getChunk() const
>>> /ssd2/pirama/llvm-$
>>> pstream/llvm/tools/lld/COFF/Symbols.h:169:43
>>>
>>> #6 0x00000000005fe082 lld::coff::Defined::getChunk()
>>> /ssd2/pirama/llvm-upstream/llvm$
>>> tools/lld/COFF/Symbols.h:381:5
>>>
>>> #7 0x0000000000656114 (anonymous
>>> namespace)::Writer::createSymbol(lld::coff::Defined$
>>> ) /ssd2/pirama/llvm-upstream/llvm/tools/lld/COFF/Writer.cpp:617:42
>>>
>>> #8 0x000000000064c7df (anonymous
>>> namespace)::Writer::createSymbolAndStringTable() /s$
>>> d2/pirama/llvm-upstream/llvm/tools/lld/COFF/Writer.cpp:0:43
>>>
>>> #9 0x00000000006499f9 (anonymous namespace)::Writer::run()
>>> /ssd2/pirama/llvm-upstrea$
>>> /llvm/tools/lld/COFF/Writer.cpp:342:3
>>>
>>> #10 0x0000000000649843 lld::coff::writeResult()
>>> /ssd2/pirama/llvm-upstream/llvm/tool$
>>> /lld/COFF/Writer.cpp:232:31
>>>
>>> #11 0x00000000005d1268 lld::coff::LinkerDriver::link(llvm::ArrayRef<char
>>> const*>) /s$
>>> d2/pirama/llvm-upstream/llvm/tools/lld/COFF/Driver.cpp:1536:3
>>> #12 0x00000000005c8687 lld::coff::link(llvm::ArrayRef<char const*>,
>>> bool, llvm::raw_$
>>> stream&) /ssd2/pirama/llvm-upstream/llvm/tools/lld/COFF/Driver.cpp:75:7
>>> #13 0x0000000000946db0 lld::mingw::link(llvm::ArrayRef<char const*>,
>>> llvm::raw_ostrea
>>> m&) /ssd2/pirama/llvm-upstream/llvm/tools/lld/MinGW/Driver.cpp:255:10
>>> #14 0x00000000004d02a8 main
>>> /ssd2/pirama/llvm-upstream/llvm/tools/lld/tools/lld/lld.c
>>> pp:121:14
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>>
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>>
>
>
> --
> --
> Peter
>
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