From 9509f4e6578b78b6380a07c0b1c846b9c0d1682a Mon Sep 17 00:00:00 2001
From: Alvin Wong <alvin@alvinhc.com>
Date: Tue, 20 Sep 2022 10:44:45 +0300
Subject: [PATCH] [LLD][COFF] Improve symbol table info for import thunk

Import thunks themselves contain a jump or branch, which is code by
nature. Therefore the import thunk symbol should be marked as function
type in the symbol table to help with debugging.

The `__imp_` import symbol associated to the import thunk is also useful
for debugging. However, when the import symbol isn't directly referenced
outside of the import thunk, it doesn't normally get added to the symbol
table. This change teaches LLD to add the import symbol explicitly.

Reviewed By: mstorsjo

Differential Revision: https://reviews.llvm.org/D134169
---
 lld/COFF/Writer.cpp       | 12 ++++++++++++
 lld/test/COFF/symtab.test | 18 ++++++++++++++++++
 2 files changed, 30 insertions(+)

diff --git a/lld/COFF/Writer.cpp b/lld/COFF/Writer.cpp
index 0883841..b3761ba 100644
--- a/lld/COFF/Writer.cpp
+++ b/lld/COFF/Writer.cpp
@@ -1179,6 +1179,10 @@ Optional<coff_symbol16> Writer::createSymbol(Defined *def) {
     COFFSymbolRef ref = d->getCOFFSymbol();
     sym.Type = ref.getType();
     sym.StorageClass = ref.getStorageClass();
+  } else if (def->kind() == Symbol::DefinedImportThunkKind) {
+    sym.Type = (IMAGE_SYM_DTYPE_FUNCTION << SCT_COMPLEX_TYPE_SHIFT) |
+               IMAGE_SYM_TYPE_NULL;
+    sym.StorageClass = IMAGE_SYM_CLASS_EXTERNAL;
   } else {
     sym.Type = IMAGE_SYM_TYPE_NULL;
     sym.StorageClass = IMAGE_SYM_CLASS_EXTERNAL;
@@ -1225,6 +1229,14 @@ void Writer::createSymbolAndStringTable() {
 
         if (Optional<coff_symbol16> sym = createSymbol(d))
           outputSymtab.push_back(*sym);
+
+        if (auto *dthunk = dyn_cast<DefinedImportThunk>(d)) {
+          if (!dthunk->wrappedSym->writtenToSymtab) {
+            dthunk->wrappedSym->writtenToSymtab = true;
+            if (Optional<coff_symbol16> sym = createSymbol(dthunk->wrappedSym))
+              outputSymtab.push_back(*sym);
+          }
+        }
       }
     }
   }
diff --git a/lld/test/COFF/symtab.test b/lld/test/COFF/symtab.test
index 949e960..ccf26fd 100644
--- a/lld/test/COFF/symtab.test
+++ b/lld/test/COFF/symtab.test
@@ -15,6 +15,15 @@
 # CHECK-NEXT:     Value: 80
 # CHECK-NEXT:     Section: .text (1)
 # CHECK-NEXT:     BaseType: Null (0x0)
+# CHECK-NEXT:     ComplexType: Function (0x2)
+# CHECK-NEXT:     StorageClass: External (0x2)
+# CHECK-NEXT:     AuxSymbolCount: 0
+# CHECK-NEXT:   }
+# CHECK-NEXT:   Symbol {
+# CHECK-NEXT:     Name: __imp_MessageBoxA
+# CHECK-NEXT:     Value:
+# CHECK-NEXT:     Section: .rdata (2)
+# CHECK-NEXT:     BaseType: Null (0x0)
 # CHECK-NEXT:     ComplexType: Null (0x0)
 # CHECK-NEXT:     StorageClass: External (0x2)
 # CHECK-NEXT:     AuxSymbolCount: 0
@@ -24,6 +33,15 @@
 # CHECK-NEXT:     Value: 64
 # CHECK-NEXT:     Section: .text (1)
 # CHECK-NEXT:     BaseType: Null (0x0)
+# CHECK-NEXT:     ComplexType: Function (0x2)
+# CHECK-NEXT:     StorageClass: External (0x2)
+# CHECK-NEXT:     AuxSymbolCount: 0
+# CHECK-NEXT:   }
+# CHECK-NEXT:   Symbol {
+# CHECK-NEXT:     Name: __imp_ExitProcess
+# CHECK-NEXT:     Value:
+# CHECK-NEXT:     Section: .rdata (2)
+# CHECK-NEXT:     BaseType: Null (0x0)
 # CHECK-NEXT:     ComplexType: Null (0x0)
 # CHECK-NEXT:     StorageClass: External (0x2)
 # CHECK-NEXT:     AuxSymbolCount: 0
-- 
2.7.4