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

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Merged
merged 1 commit into from
Nov 19, 2024
Merged

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

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9 changes: 9 additions & 0 deletions lld/test/wasm/tls-non-shared-memory.s
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Expand Up @@ -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
Expand Down Expand Up @@ -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
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9 changes: 4 additions & 5 deletions lld/wasm/Symbols.cpp
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Expand Up @@ -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);
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4 changes: 3 additions & 1 deletion lld/wasm/Symbols.h
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Expand Up @@ -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.
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5 changes: 4 additions & 1 deletion lld/wasm/SyntheticSections.cpp
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Expand Up @@ -514,7 +514,10 @@ void GlobalSection::writeBody() {
} else {
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does the extended-const case also need to be updated?

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No since in that case we do the addition with __tls_base as part of the const expression. i.e. its don't for us at instantiation time.

So I guess there are 3 ways this addition can happen:

  1. At instantiation time via extended const
  2. As part of __wasm_apply_global_tls_relocs at runtime
  3. At static link time via getVA(absolute=true)

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 {
Expand Down
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