[lld] [lld][WebAssembly] Fix for shared library symbols WRT replacing lazy symbols (PR #124619)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-lld-wasm

Author: Sam Clegg (sbc100)

<details>
<summary>Changes</summary>

The rule here, which I'm copying from the ELF linker, is that shared library symbols should take presence, unless the symbol has already be extracted from the archive.  e.g:

```
$ wasm-ld foo.a foo.so ref.o  // .so wins
$ wasm-ld foo.a ref.o foo.so  // .a wins
```

In the first case the shared library takes precedence because the lazy symbol is replaced by the .so symbol before it is extracted from the archive.  In the second example the ref.o file causes the archive to be exracted before the .so file is processed, so in that case the archive file wins.

Fixes: https://github.com/emscripten-core/emscripten/issues/23501

---
Full diff: https://github.com/llvm/llvm-project/pull/124619.diff


2 Files Affected:

- (added) lld/test/wasm/shared-lazy.s (+79) 
- (modified) lld/wasm/SymbolTable.cpp (+10-2) 


``````````diff
diff --git a/lld/test/wasm/shared-lazy.s b/lld/test/wasm/shared-lazy.s
new file mode 100644
index 00000000000000..f1044547203a2a
--- /dev/null
+++ b/lld/test/wasm/shared-lazy.s
@@ -0,0 +1,79 @@
+##  Based on lld/test/ELF/shared-lazy.s
+
+# RUN: rm -rf %t && split-file %s %t && cd %t
+# RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-emscripten a.s -o a.o
+# RUN: wasm-ld a.o --experimental-pic -shared -o a.so
+# RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-emscripten b.s -o b.o
+# RUN: wasm-ld b.o --experimental-pic -shared -o b.so
+# RUN: llvm-ar rc a.a a.o
+# RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-emscripten ref.s -o ref.o
+# RUN: wasm-ld a.a b.so ref.o --experimental-pic -shared -o 1.so
+# RUN: obj2yaml 1.so | FileCheck %s
+# RUN: wasm-ld a.so a.a ref.o --experimental-pic -shared -o 1.so
+# RUN: obj2yaml 1.so | FileCheck %s
+
+## The definitions from a.so are used and we don't extract a member from the
+## archive.
+
+# CHECK:   - Type:            IMPORT
+# CHECK:            - Module:          GOT.mem
+# CHECK-NEXT:         Field:           x1
+# CHECK-NEXT:         Kind:            GLOBAL
+# CHECK-NEXT:         GlobalType:      I32
+# CHECK-NEXT:         GlobalMutable:   true
+# CHECK-NEXT:       - Module:          GOT.mem
+# CHECK-NEXT:         Field:           x2
+# CHECK-NEXT:         Kind:            GLOBAL
+# CHECK-NEXT:         GlobalType:      I32
+# CHECK-NEXT:         GlobalMutable:   true
+
+## The extracted x1 is defined as STB_GLOBAL.
+# RUN: wasm-ld ref.o a.a b.so -o 2.so --experimental-pic -shared
+# RUN: obj2yaml 2.so | FileCheck %s --check-prefix=CHECK2
+# RUN: wasm-ld a.a ref.o b.so -o 2.so --experimental-pic -shared
+# RUN: obj2yaml 2.so | FileCheck %s --check-prefix=CHECK2
+
+# CHECK2:       - Type:            EXPORT
+# CHECK2-NEXT:    Exports:
+# CHECK2-NEXT:      - Name:            __wasm_call_ctors
+# CHECK2-NEXT:        Kind:            FUNCTION
+# CHECK2-NEXT:        Index:
+# CHECK2-NEXT:      - Name:            x1
+# CHECK2-NEXT:        Kind:            GLOBAL
+# CHECK2-NEXT:        Index:
+# CHECK2-NEXT:      - Name:            x2
+# CHECK2-NEXT:        Kind:            GLOBAL
+
+#--- a.s
+.section .data.x1,"",@
+.global x1
+x1:
+  .byte 0
+.size x1, 1
+
+.section .data.x2,"",@
+.weak x2
+x2:
+  .byte 0
+.size x2, 1
+#--- b.s
+.section .data.x1,"",@
+.globl x1
+x1:
+  .byte 0
+.size x1, 1
+
+.section .data.x2,"",@
+.globl x2
+x2:
+  .byte 0
+.size x2, 1
+#--- ref.s
+.globl x1
+.globl x2
+.globl d
+.section .data.d,"",@
+d:
+  .int x1
+  .int x2
+.size d, 8
diff --git a/lld/wasm/SymbolTable.cpp b/lld/wasm/SymbolTable.cpp
index 6309cf35fe5523..7e8b4aa632a321 100644
--- a/lld/wasm/SymbolTable.cpp
+++ b/lld/wasm/SymbolTable.cpp
@@ -363,7 +363,7 @@ Symbol *SymbolTable::addSharedFunction(StringRef name, uint32_t flags,
     replaceSymbol<SharedFunctionSymbol>(sym, name, flags, file, sig);
   };
 
-  if (wasInserted) {
+  if (wasInserted || s->isLazy()) {
     replaceSym(s);
     return s;
   }
@@ -408,10 +408,18 @@ Symbol *SymbolTable::addSharedData(StringRef name, uint32_t flags,
   bool wasInserted;
   std::tie(s, wasInserted) = insert(name, file);
 
-  if (wasInserted || s->isUndefined()) {
+  if (wasInserted || s->isLazy()) {
     replaceSymbol<SharedData>(s, name, flags, file);
+    return s;
+  }
+
+  // Shared symbols should never replace locally-defined ones
+  if (s->isDefined()) {
+    return s;
   }
 
+  checkDataType(s, file);
+  replaceSymbol<SharedData>(s, name, flags, file);
   return s;
 }
 

``````````

</details>


https://github.com/llvm/llvm-project/pull/124619