[LLD][COFF] Add support for IMPORT_NAME_EXPORTAS import library names. (#83211)

This allows handling importlibs produced by llvm-dlltool in #78772.
ARM64EC import libraries use it by default, but it's supported by MSVC
link.exe on other platforms too.

This also avoids assuming null-terminated input, like in #78769.

Author: cjacek

lld/COFF/InputFiles.cpp

@@ -944,18 +944,20 @@ ImportFile::ImportFile(COFFLinkerContext &ctx, MemoryBufferRef m)
     : InputFile(ctx, ImportKind, m), live(!ctx.config.doGC), thunkLive(live) {}
 
 void ImportFile::parse() {
-  const char *buf = mb.getBufferStart();
-  const auto *hdr = reinterpret_cast<const coff_import_header *>(buf);
+  const auto *hdr =
+      reinterpret_cast<const coff_import_header *>(mb.getBufferStart());
 
   // Check if the total size is valid.
-  if (mb.getBufferSize() != sizeof(*hdr) + hdr->SizeOfData)
+  if (mb.getBufferSize() < sizeof(*hdr) ||
+      mb.getBufferSize() != sizeof(*hdr) + hdr->SizeOfData)
     fatal("broken import library");
 
   // Read names and create an __imp_ symbol.
-  StringRef name = saver().save(StringRef(buf + sizeof(*hdr)));
+  StringRef buf = mb.getBuffer().substr(sizeof(*hdr));
+  StringRef name = saver().save(buf.split('\0').first);
   StringRef impName = saver().save("__imp_" + name);
-  const char *nameStart = buf + sizeof(coff_import_header) + name.size() + 1;
-  dllName = std::string(StringRef(nameStart));
+  buf = buf.substr(name.size() + 1);
+  dllName = buf.split('\0').first;
   StringRef extName;
   switch (hdr->getNameType()) {
   case IMPORT_ORDINAL:
@@ -971,6 +973,9 @@ void ImportFile::parse() {
     extName = ltrim1(name, "?@_");
     extName = extName.substr(0, extName.find('@'));
     break;
+  case IMPORT_NAME_EXPORTAS:
+    extName = buf.substr(dllName.size() + 1).split('\0').first;
+    break;
   }
 
   this->hdr = hdr;

lld/test/COFF/exportas.test

@@ -0,0 +1,19 @@
+REQUIRES: x86
+RUN: split-file %s %t.dir && cd %t.dir
+
+Link to an import library containing EXPORTAS and verify that we use proper name for the import.
+
+RUN: llvm-mc -filetype=obj -triple=x86_64-windows test.s -o test.obj
+RUN: llvm-lib -machine:amd64 -out:test.lib -def:test.def
+RUN: lld-link -out:out1.dll -dll -noentry test.obj test.lib
+RUN: llvm-readobj --coff-imports out1.dll | FileCheck --check-prefix=IMPORT %s
+IMPORT: Symbol: expfunc
+
+#--- test.s
+    .section ".test", "rd"
+    .rva __imp_func
+
+#--- test.def
+LIBRARY test.dll
+EXPORTS
+    func EXPORTAS expfunc