[llvm] r362849 - [COFF] Fix /export:foo=bar when bar is a weak alias

[Reid Kleckner via llvm-commits]

Author: rnk
Date: Fri Jun  7 15:05:12 2019
New Revision: 362849

URL: http://llvm.org/viewvc/llvm-project?rev=362849&view=rev
Log:
[COFF] Fix /export:foo=bar when bar is a weak alias

Summary:
When handling exports from the command line or from .def files, the
linker does a "fuzzy" string lookup to allow finding mangled symbols.
However, when the symbol is re-exported under a new name, the linker has
to transfer the decorations from the exported symbol over to the new
name. This is implemented by taking the mangled symbol that was found in
the object and replacing the original symbol name with the export name.

Before this patch, LLD implemented the fuzzy search by adding an
undefined symbol with the unmangled name, and then during symbol
resolution, checking if similar mangled symbols had been added after the
last round of symbol resolution. If so, LLD makes the original symbol a
weak alias of the mangled symbol. Later, to get the original symbol
name, LLD would look through the weak alias and forward it on to the
import library writer, which copies the symbol decorations. This
approach doesn't work when bar is itself a weak alias, as is the case in
asan. It's especially bad when the aliasee of bar contains the string
"bar", consider "bar_default". In this case, we would end up exporting
the symbol "foo_default" when we should've exported just "foo".

To fix this, don't look through weak aliases to find the mangled name.
Save the mangled name earlier during fuzzy symbol lookup.

Fixes PR42074

Reviewers: mstorsjo, ruiu

Subscribers: thakis, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62984

Modified:
    llvm/trunk/include/llvm/Object/COFFImportFile.h

Modified: llvm/trunk/include/llvm/Object/COFFImportFile.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Object/COFFImportFile.h?rev=362849&r1=362848&r2=362849&view=diff
==============================================================================
--- llvm/trunk/include/llvm/Object/COFFImportFile.h (original)
+++ llvm/trunk/include/llvm/Object/COFFImportFile.h Fri Jun  7 15:05:12 2019
@@ -69,9 +69,21 @@ private:
 };
 
 struct COFFShortExport {
+  /// The name of the export as specified in the .def file or on the command
+  /// line, i.e. "foo" in "/EXPORT:foo", and "bar" in "/EXPORT:foo=bar". This
+  /// may lack mangling, such as underscore prefixing and stdcall suffixing.
   std::string Name;
+
+  /// The external, exported name. Only non-empty when export renaming is in
+  /// effect, i.e. "foo" in "/EXPORT:foo=bar".
   std::string ExtName;
+
+  /// The real, mangled symbol name from the object file. Given
+  /// "/export:foo=bar", this could be "_bar at 8" if bar is stdcall.
   std::string SymbolName;
+
+  /// Creates a weak alias. This is the name of the weak aliasee. In a .def
+  /// file, this is "baz" in "EXPORTS\nfoo = bar == baz".
   std::string AliasTarget;
 
   uint16_t Ordinal = 0;