[llvm] [WebAssembly] Allow absolute symbols in the linking section (symbol table) (PR #67493)

[Sam Clegg via llvm-commits]

https://github.com/sbc100 created https://github.com/llvm/llvm-project/pull/67493

Fixes a crash in `-Wl,-emit-relocs` where the linker was not able to write linker-synthetic absolute symbols to the symbol table.

This change adds a new symbol flag (`WASM_SYMBOL_ABS`), which means that the symbol's offset is absolute and not relative to a given segment. Such symbols include `__stack_low` and `__stack_low`.

Note that wasm object files never contains such symbols, only binaries linked with `-Wl,-emit-relocs`.

Fixes: #67111

>From ea09121324b22cf1897da33324209159f9c39826 Mon Sep 17 00:00:00 2001
From: Sam Clegg <sbc at chromium.org>
Date: Tue, 26 Sep 2023 14:37:15 -0700
Subject: [PATCH] [WebAssembly] Allow absolute symbols in the linking section
 (symbol table)

Fixes a crash in `-Wl,-emit-relocs` where the linker was not able to
write linker-synthetic absolute symbols to the symbol table.

This change adds a new symbol flag (`WASM_SYMBOL_ABS`), which means that
the symbol's offset is absolute and not relative to a given segment.
Such symbols include `__stack_low` and `__stack_low`.

Note that wasm object files never contains such symbols, only binaries
linked with `-Wl,-emit-relocs`.

Fixes: #67111
---
 lld/wasm/SymbolTable.cpp              |  6 ++++--
 lld/wasm/SyntheticSections.cpp        | 11 ++++++++---
 llvm/include/llvm/BinaryFormat/Wasm.h |  1 +
 llvm/lib/Object/WasmObjectFile.cpp    | 21 ++++++++++++---------
 4 files changed, 25 insertions(+), 14 deletions(-)

diff --git a/lld/wasm/SymbolTable.cpp b/lld/wasm/SymbolTable.cpp
index 9a381e95f3d9fa5..12e593577d63bd4 100644
--- a/lld/wasm/SymbolTable.cpp
+++ b/lld/wasm/SymbolTable.cpp
@@ -233,7 +233,8 @@ DefinedData *SymbolTable::addOptionalDataSymbol(StringRef name,
   else if (!s || s->isDefined())
     return nullptr;
   LLVM_DEBUG(dbgs() << "addOptionalDataSymbol: " << name << "\n");
-  auto *rtn = replaceSymbol<DefinedData>(s, name, WASM_SYMBOL_VISIBILITY_HIDDEN);
+  auto *rtn = replaceSymbol<DefinedData>(
+      s, name, WASM_SYMBOL_VISIBILITY_HIDDEN | WASM_SYMBOL_ABS);
   rtn->setVA(value);
   rtn->referenced = true;
   return rtn;
@@ -243,7 +244,8 @@ DefinedData *SymbolTable::addSyntheticDataSymbol(StringRef name,
                                                  uint32_t flags) {
   LLVM_DEBUG(dbgs() << "addSyntheticDataSymbol: " << name << "\n");
   assert(!find(name));
-  return replaceSymbol<DefinedData>(insertName(name).first, name, flags);
+  return replaceSymbol<DefinedData>(insertName(name).first, name,
+                                    flags | WASM_SYMBOL_ABS);
 }
 
 DefinedGlobal *SymbolTable::addSyntheticGlobal(StringRef name, uint32_t flags,
diff --git a/lld/wasm/SyntheticSections.cpp b/lld/wasm/SyntheticSections.cpp
index ef9547593acdcd0..3a9c147161a2d38 100644
--- a/lld/wasm/SyntheticSections.cpp
+++ b/lld/wasm/SyntheticSections.cpp
@@ -665,9 +665,14 @@ void LinkingSection::writeBody() {
       } else if (isa<DataSymbol>(sym)) {
         writeStr(sub.os, sym->getName(), "sym name");
         if (auto *dataSym = dyn_cast<DefinedData>(sym)) {
-          writeUleb128(sub.os, dataSym->getOutputSegmentIndex(), "index");
-          writeUleb128(sub.os, dataSym->getOutputSegmentOffset(),
-                       "data offset");
+          if (dataSym->segment) {
+            writeUleb128(sub.os, dataSym->getOutputSegmentIndex(), "index");
+            writeUleb128(sub.os, dataSym->getOutputSegmentOffset(),
+                         "data offset");
+          } else {
+            writeUleb128(sub.os, 0, "index");
+            writeUleb128(sub.os, dataSym->getVA(), "data offset");
+          }
           writeUleb128(sub.os, dataSym->getSize(), "data size");
         }
       } else {
diff --git a/llvm/include/llvm/BinaryFormat/Wasm.h b/llvm/include/llvm/BinaryFormat/Wasm.h
index 749d43b840d5c2c..69b286607cbd3a7 100644
--- a/llvm/include/llvm/BinaryFormat/Wasm.h
+++ b/llvm/include/llvm/BinaryFormat/Wasm.h
@@ -412,6 +412,7 @@ const unsigned WASM_SYMBOL_EXPORTED = 0x20;
 const unsigned WASM_SYMBOL_EXPLICIT_NAME = 0x40;
 const unsigned WASM_SYMBOL_NO_STRIP = 0x80;
 const unsigned WASM_SYMBOL_TLS = 0x100;
+const unsigned WASM_SYMBOL_ABS = 0x200;
 
 #define WASM_RELOC(name, value) name = value,
 
diff --git a/llvm/lib/Object/WasmObjectFile.cpp b/llvm/lib/Object/WasmObjectFile.cpp
index e9471adf66eda5d..50ef4d2648ece73 100644
--- a/llvm/lib/Object/WasmObjectFile.cpp
+++ b/llvm/lib/Object/WasmObjectFile.cpp
@@ -723,17 +723,20 @@ Error WasmObjectFile::parseLinkingSectionSymtab(ReadContext &Ctx) {
       Info.Name = readString(Ctx);
       if (IsDefined) {
         auto Index = readVaruint32(Ctx);
-        if (Index >= DataSegments.size())
-          return make_error<GenericBinaryError>("invalid data segment index",
-                                                object_error::parse_failed);
         auto Offset = readVaruint64(Ctx);
         auto Size = readVaruint64(Ctx);
-        size_t SegmentSize = DataSegments[Index].Data.Content.size();
-        if (Offset > SegmentSize)
-          return make_error<GenericBinaryError>(
-              "invalid data symbol offset: `" + Info.Name + "` (offset: " +
-                  Twine(Offset) + " segment size: " + Twine(SegmentSize) + ")",
-              object_error::parse_failed);
+        if (!(Info.Flags & wasm::WASM_SYMBOL_ABS)) {
+          if (static_cast<size_t>(Index) >= DataSegments.size())
+            return make_error<GenericBinaryError>(
+                "invalid data segment index: " + Twine(Index),
+                object_error::parse_failed);
+          size_t SegmentSize = DataSegments[Index].Data.Content.size();
+          if (Offset > SegmentSize)
+            return make_error<GenericBinaryError>(
+                "invalid data symbol offset: `" + Info.Name + "` (offset: " +
+                    Twine(Offset) + " segment size: " + Twine(SegmentSize) + ")",
+                object_error::parse_failed);
+        }
         Info.DataRef = wasm::WasmDataReference{Index, Offset, Size};
       }
       break;