[LLD][COFF] Don't resolve weak aliases when performing local import

Fixes crashes reported in #151255.

The alias may have already been stored for later resolution, which can lead
to treating a resolved alias as if it were still undefined. Instead, use the
alias target directly for the import.

Also extended the test to make reproducing the problem more likely, and added
an assert that catches the issue.

Author: cjacek

lld/COFF/Driver.cpp

@@ -2714,8 +2714,10 @@ void LinkerDriver::linkerMain(ArrayRef<const char *> argsArr) {
   }
 
   ctx.symtab.initializeSameAddressThunks();
-  for (auto alias : aliases)
+  for (auto alias : aliases) {
+    assert(alias->kind() == Symbol::UndefinedKind);
     alias->resolveWeakAlias();
+  }
 
   if (config->mingw) {
     // Make sure the crtend.o object is the last object file. This object

lld/COFF/SymbolTable.cpp

@@ -478,17 +478,11 @@ void SymbolTable::resolveRemainingUndefines(std::vector<Undefined *> &aliases) {
     if (name.starts_with("__imp_")) {
       auto findLocalSym = [&](StringRef n) {
         Symbol *sym = find(n);
-        if (auto undef = dyn_cast_or_null<Undefined>(sym)) {
-          // The unprefixed symbol might come later in symMap, so handle it now
-          // if needed.
-          if (!undef->resolveWeakAlias())
-            sym = nullptr;
-        }
-        return sym;
+        return sym ? sym->getDefined() : nullptr;
       };
 
       StringRef impName = name.substr(strlen("__imp_"));
-      Symbol *imp = findLocalSym(impName);
+      Defined *imp = findLocalSym(impName);
       if (!imp && isEC()) {
         // Try to use the mangled symbol on ARM64EC.
         std::optional<std::string> mangledName =
@@ -502,11 +496,10 @@ void SymbolTable::resolveRemainingUndefines(std::vector<Undefined *> &aliases) {
             imp = findLocalSym(*mangledName);
         }
       }
-      if (imp && isa<Defined>(imp)) {
-        auto *d = cast<Defined>(imp);
-        replaceSymbol<DefinedLocalImport>(sym, ctx, name, d);
+      if (imp) {
+        replaceSymbol<DefinedLocalImport>(sym, ctx, name, imp);
         localImportChunks.push_back(cast<DefinedLocalImport>(sym)->getChunk());
-        localImports[sym] = d;
+        localImports[sym] = imp;
         continue;
       }
     }

lld/test/COFF/import_weak_alias.test

@@ -4,17 +4,24 @@
 # RUN: llvm-mc --filetype=obj -triple=x86_64-windows-msvc %t.dir/foo.s -o %t.foo.obj
 # RUN: llvm-mc --filetype=obj -triple=x86_64-windows-msvc %t.dir/qux.s -o %t.qux.obj
 # RUN: lld-link %t.qux.obj %t.foo.obj -out:%t.dll -dll
+# RUN: lld-link %t.foo.obj %t.qux.obj -out:%t.dll -dll
 #
 #--- foo.s
 .text
 bar:
   ret
 
-.weak foo
-.set foo, bar
+.weak a
+.weak b
+.weak c
+.set a, bar
+.set b, bar
+.set c, bar
 #--- qux.s
 .text
 .global _DllMainCRTStartup
 _DllMainCRTStartup:
-  call *__imp_foo(%rip)
+  call *__imp_a(%rip)
+  call *__imp_b(%rip)
+  call *__imp_c(%rip)
   ret