[lld] 1c1fbf5 - [lld][WebAssembly] Fix TLS-relative relocations when linking without shared memory (#116136)

[via llvm-commits]

Author: Sam Clegg
Date: 2024-11-19T14:00:37-08:00
New Revision: 1c1fbf51b5ec9657e5da7fa94ee892273255544a

URL: https://github.com/llvm/llvm-project/commit/1c1fbf51b5ec9657e5da7fa94ee892273255544a
DIFF: https://github.com/llvm/llvm-project/commit/1c1fbf51b5ec9657e5da7fa94ee892273255544a.diff

LOG: [lld][WebAssembly] Fix TLS-relative relocations when linking without shared memory (#116136)

TLS-relative relocations always need to be relative the TLS section
since they get added to `__tls_base` at runtime.

Without this change the tls base address was effectively being added to
the final value twice in this case.

This only effects code the is built with `-pthread` but linked without
shared memory (i.e. without threads).

Fixes: https://github.com/emscripten-core/emscripten/issues/22880

Added: 
    

Modified: 
    lld/test/wasm/tls-non-shared-memory.s
    lld/wasm/Symbols.cpp
    lld/wasm/Symbols.h
    lld/wasm/SyntheticSections.cpp

Removed: 
    


################################################################################
diff  --git a/lld/test/wasm/tls-non-shared-memory.s b/lld/test/wasm/tls-non-shared-memory.s
index 1754fd6254bb80..04fbb62228a7e2 100644
--- a/lld/test/wasm/tls-non-shared-memory.s
+++ b/lld/test/wasm/tls-non-shared-memory.s
@@ -44,6 +44,7 @@ tls1:
 
 # RUN: wasm-ld --no-gc-sections --no-entry -o %t.wasm %t.o
 # RUN: obj2yaml %t.wasm | FileCheck %s
+# RUN: llvm-objdump --disassemble-symbols=get_tls1 --no-show-raw-insn --no-leading-addr %t.wasm | FileCheck %s --check-prefixes DIS
 
 # RUN: wasm-ld --experimental-pic -shared -o %t.so %t.o
 # RUN: obj2yaml %t.so | FileCheck %s --check-prefixes=SHARED,PIC
@@ -97,6 +98,14 @@ tls1:
 # CHECK-NEXT:        Content:         2A000000
 # CHECK-NEXT:  - Type:            CUSTOM
 
+# The constant value here which we add to `__tls_base` should not be absolute
+# but relative to `__tls_base`, in this case zero rather than 1024.
+# DIS:      <get_tls1>:
+# DIS-EMPTY:
+# DIS-NEXT:  global.get 1
+# DIS-NEXT:  i32.const 0
+# DIS-NEXT:  i32.add
+# DIS-NEXT:  end
 
 # In PIC mode we expect TLS data and non-TLS data to be merged into
 # a single segment which is initialized via the  __memory_base import

diff  --git a/lld/wasm/Symbols.cpp b/lld/wasm/Symbols.cpp
index b2bbd11c53ef23..e62e7bec609f54 100644
--- a/lld/wasm/Symbols.cpp
+++ b/lld/wasm/Symbols.cpp
@@ -310,12 +310,11 @@ uint32_t DefinedFunction::getExportedFunctionIndex() const {
   return function->getFunctionIndex();
 }
 
-uint64_t DefinedData::getVA() const {
+uint64_t DefinedData::getVA(bool absolute) const {
   LLVM_DEBUG(dbgs() << "getVA: " << getName() << "\n");
-  // In the shared memory case, TLS symbols are relative to the start of the TLS
-  // output segment (__tls_base).  When building without shared memory, TLS
-  // symbols absolute, just like non-TLS.
-  if (isTLS() && config->sharedMemory)
+  // TLS symbols (by default) are relative to the start of the TLS output
+  // segment (__tls_base).
+  if (isTLS() && !absolute)
     return getOutputSegmentOffset();
   if (segment)
     return segment->getVA(value);

diff  --git a/lld/wasm/Symbols.h b/lld/wasm/Symbols.h
index 5ce3ecbc4ab194..80b658773bd20b 100644
--- a/lld/wasm/Symbols.h
+++ b/lld/wasm/Symbols.h
@@ -315,7 +315,9 @@ class DefinedData : public DataSymbol {
   static bool classof(const Symbol *s) { return s->kind() == DefinedDataKind; }
 
   // Returns the output virtual address of a defined data symbol.
-  uint64_t getVA() const;
+  // For TLS symbols, by default (unless absolute is set), this returns an
+  // address relative the `__tls_base`.
+  uint64_t getVA(bool absolute = false) const;
   void setVA(uint64_t va);
 
   // Returns the offset of a defined data symbol within its OutputSegment.

diff  --git a/lld/wasm/SyntheticSections.cpp b/lld/wasm/SyntheticSections.cpp
index a3bc90cfe759ca..1454c3324af989 100644
--- a/lld/wasm/SyntheticSections.cpp
+++ b/lld/wasm/SyntheticSections.cpp
@@ -514,7 +514,10 @@ void GlobalSection::writeBody() {
     } else {
       WasmInitExpr initExpr;
       if (auto *d = dyn_cast<DefinedData>(sym))
-        initExpr = intConst(d->getVA(), is64);
+        // In the sharedMemory case TLS globals are set during
+        // `__wasm_apply_global_tls_relocs`, but in the non-shared case
+        // we know the absolute value at link time.
+        initExpr = intConst(d->getVA(/*absolute=*/!config->sharedMemory), is64);
       else if (auto *f = dyn_cast<FunctionSymbol>(sym))
         initExpr = intConst(f->isStub ? 0 : f->getTableIndex(), is64);
       else {