[llvm] r218187 - [MCJIT] Make RTDyldMemoryManager::getSymbolAddress's behaviour more consistent.

[Lang Hames]

Author: lhames
Date: Sat Sep 20 12:44:56 2014
New Revision: 218187

URL: http://llvm.org/viewvc/llvm-project?rev=218187&view=rev
Log:
[MCJIT] Make RTDyldMemoryManager::getSymbolAddress's behaviour more consistent.

This patch modifies RTDyldMemoryManager::getSymbolAddress(Name)'s behavior to
make it consistent with how clients are using it: Name should be mangled, and
getSymbolAddress should demangle it on the caller's behalf before looking the
name up in the process. This patch also fixes the one client
(MCJIT::getPointerToFunction) that had been passing unmangled names (by having
it pass mangled names instead).

Background:

RTDyldMemoryManager::getSymbolAddress(Name) has always used a re-try mechanism
when looking up symbol names in the current process. Prior to this patch
getSymbolAddress first tried to look up 'Name' exactly as the user passed it in
and then, if that failed, tried to demangle 'Name' and re-try the look up. The
implication of this behavior is that getSymbolAddress expected to be called with
unmangled names, and that handling mangled names was a fallback for convenience.
This is inconsistent with how clients (particularly the RuntimeDyldImpl
subclasses, but also MCJIT) usually use this API. Most clients pass in mangled
names, and succeed only because of the fallback case. For clients passing in
mangled names, getSymbolAddress's old behavior was actually dangerous, as it
could cause unmangled names in the process to shadow mangled names being looked
up.

For example, consider:

foo.c:

int _x = 7;
int x() { return _x; }

foo.o:

000000000000000c D __x
0000000000000000 T _x

If foo.c becomes part of the process (E.g. via dlopen("libfoo.dylib")) it will
add symbols 'x' (the function) and '_x' (the variable) to the process. However
jit clients looking for the function 'x' will be using the mangled function name
'_x' (note how function 'x' appears in foo.o). When getSymbolAddress goes
looking for '_x' it will find the variable instead, and return its address and
in place of the function, leading to JIT'd code calling the variable and
crashing (if we're lucky).

By requiring that getSymbolAddress be called with mangled names, and demangling
only when we're about to do a lookup in the process, the new behavior
implemented in this patch should eliminate any chance of names being shadowed
during lookup.

There's no good way to test this at the moment: This issue only arrises when
looking up process symbols (not JIT'd symbols). Any test case would have to
generate a platform-appropriate dylib to pass to llvm-rtdyld, and I'm not
aware of any in-tree tool for doing this in a portable way.


Modified:
    llvm/trunk/lib/ExecutionEngine/MCJIT/MCJIT.cpp
    llvm/trunk/lib/ExecutionEngine/RTDyldMemoryManager.cpp

Modified: llvm/trunk/lib/ExecutionEngine/MCJIT/MCJIT.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/ExecutionEngine/MCJIT/MCJIT.cpp?rev=218187&r1=218186&r2=218187&view=diff
==============================================================================
--- llvm/trunk/lib/ExecutionEngine/MCJIT/MCJIT.cpp (original)
+++ llvm/trunk/lib/ExecutionEngine/MCJIT/MCJIT.cpp Sat Sep 20 12:44:56 2014
@@ -351,9 +351,13 @@ uint64_t MCJIT::getFunctionAddress(const
 void *MCJIT::getPointerToFunction(Function *F) {
   MutexGuard locked(lock);
 
+  Mangler Mang(TM->getSubtargetImpl()->getDataLayout());
+  SmallString<128> Name;
+  TM->getNameWithPrefix(Name, F, Mang);
+
   if (F->isDeclaration() || F->hasAvailableExternallyLinkage()) {
     bool AbortOnFailure = !F->hasExternalWeakLinkage();
-    void *Addr = getPointerToNamedFunction(F->getName(), AbortOnFailure);
+    void *Addr = getPointerToNamedFunction(Name, AbortOnFailure);
     addGlobalMapping(F, Addr);
     return Addr;
   }
@@ -364,17 +368,18 @@ void *MCJIT::getPointerToFunction(Functi
   // Make sure the relevant module has been compiled and loaded.
   if (HasBeenAddedButNotLoaded)
     generateCodeForModule(M);
-  else if (!OwnedModules.hasModuleBeenLoaded(M))
+  else if (!OwnedModules.hasModuleBeenLoaded(M)) {
     // If this function doesn't belong to one of our modules, we're done.
+    // FIXME: Asking for the pointer to a function that hasn't been registered,
+    //        and isn't a declaration (which is handled above) should probably
+    //        be an assertion.
     return nullptr;
+  }
 
   // FIXME: Should the Dyld be retaining module information? Probably not.
   //
   // This is the accessor for the target address, so make sure to check the
   // load address of the symbol, not the local address.
-  Mangler Mang(TM->getSubtargetImpl()->getDataLayout());
-  SmallString<128> Name;
-  TM->getNameWithPrefix(Name, F, Mang);
   return (void*)Dyld.getSymbolLoadAddress(Name);
 }
 

Modified: llvm/trunk/lib/ExecutionEngine/RTDyldMemoryManager.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/ExecutionEngine/RTDyldMemoryManager.cpp?rev=218187&r1=218186&r2=218187&view=diff
==============================================================================
--- llvm/trunk/lib/ExecutionEngine/RTDyldMemoryManager.cpp (original)
+++ llvm/trunk/lib/ExecutionEngine/RTDyldMemoryManager.cpp Sat Sep 20 12:44:56 2014
@@ -253,19 +253,20 @@ uint64_t RTDyldMemoryManager::getSymbolA
   // is called before ExecutionEngine::runFunctionAsMain() is called.
   if (Name == "__main") return (uint64_t)&jit_noop;
 
+  // Try to demangle Name before looking it up in the process, otherwise symbol
+  // '_<Name>' (if present) will shadow '<Name>', and there will be no way to
+  // refer to the latter.
+
   const char *NameStr = Name.c_str();
-  void *Ptr = sys::DynamicLibrary::SearchForAddressOfSymbol(NameStr);
-  if (Ptr)
-    return (uint64_t)Ptr;
 
-  // If it wasn't found and if it starts with an underscore ('_') character,
-  // try again without the underscore.
-  if (NameStr[0] == '_') {
-    Ptr = sys::DynamicLibrary::SearchForAddressOfSymbol(NameStr+1);
-    if (Ptr)
+  if (NameStr[0] == '_')
+    if (void *Ptr = sys::DynamicLibrary::SearchForAddressOfSymbol(NameStr + 1))
       return (uint64_t)Ptr;
-  }
-  return 0;
+
+  // If we Name did not require demangling, or we failed to find the demangled
+  // name, try again without demangling.
+  if (void *Ptr = sys::DynamicLibrary::SearchForAddressOfSymbol(NameStr))
+    return (uint64_t)Ptr;
 }
 
 void *RTDyldMemoryManager::getPointerToNamedFunction(const std::string &Name,