[WebAssembly][lld] Fix segfault on .bss sections in mapfile

[tlively]

Summary

When memory is declared in the Wasm module, we rely on the implicit zero
initialization behavior and do not explicitly output .bss sections. The means
that they do not have associated outputSec entries, which was causing
segfaults in the mapfile support. Fix the issue by guarding against null
outputSec and falling back to using a zero offset.

[tlively]

@sbc100 Is there anything better to do here besides falling back and emitting 0 as the offset?

[sbc100]

It does seem a little unfortunate that we can't report that offset correctly here. I would expect to see my bss always following by non-bss data. Let me take a look at see if we are relying on the null outputSec ...

[sbc100]

Lets land this!

lld/test/wasm/map-file.s

 # RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-unknown %s -o %t1.o
 # RUN: wasm-ld %t1.o -o %t -M | FileCheck --match-full-lines --strict-whitespace %s
 # RUN: wasm-ld %t1.o -o %t -print-map | FileCheck --match-full-lines --strict-whitespace %s
 # RUN: wasm-ld %t1.o -o %t -Map=%t.map
 # RUN: FileCheck --match-full-lines --strict-whitespace %s < %t.map
 
 .globaltype wasm_global, i32, immutable
 wasm_global:
 
 bar:
     .functype bar () -> ()
     i32.const   somedata
+    i32.const   somezeroes
+    drop
+    drop
     end_function
 
 write_global:
     .functype write_global (i32) -> ()
     local.get 0
     global.set wasm_global
     end_function
 
     .globl _start
 _start:
     .functype _start () -> ()
     call bar
     call write_global
     end_function
 
 .section .data.somedata,"",@
 somedata:
     .int32 123
 .size somedata, 4
 
+.section .bss.somezeroes,"",@
+somezeroes:
+    .int32 0
+.size somezeroes, 4
+
 .section .debug_info,"",@
     .int32 bar
 
+
 #      CHECK:    Addr      Off     Size Out     In      Symbol
 # CHECK-NEXT:       -        8        a TYPE
 # CHECK-NEXT:       -       12        6 FUNCTION
 # CHECK-NEXT:       -       18        7 TABLE
 # CHECK-NEXT:       -       1f        5 MEMORY
 # CHECK-NEXT:       -       24        f GLOBAL
 # CHECK-NEXT:       0        0        0         __stack_pointer
 # CHECK-NEXT:       1        0        0         wasm_global
 # CHECK-NEXT:       -       33       15 EXPORT
-# CHECK-NEXT:       -       48       26 CODE
-# CHECK-NEXT:       -       49        9         {{.*}}{{/|\\}}map-file.s.tmp1.o:(bar)
-# CHECK-NEXT:       -       49        9                 bar
-# CHECK-NEXT:       -       52        b         {{.*}}{{/|\\}}map-file.s.tmp1.o:(write_global)
-# CHECK-NEXT:       -       52        b                 write_global
-# CHECK-NEXT:       -       5d        f         {{.*}}{{/|\\}}map-file.s.tmp1.o:(_start)
-# CHECK-NEXT:       -       5d        f                 _start
-# CHECK-NEXT:       -       6e        d DATA
-# CHECK-NEXT:     400       6f        4 .data
-# CHECK-NEXT:     400       75        4         {{.*}}{{/|\\}}map-file.s.tmp1.o:(.data.somedata)
-# CHECK-NEXT:     400       75        4                 somedata
-# CHECK-NEXT:       -       7b       12 CUSTOM(.debug_info)
-# CHECK-NEXT:       -       8d       50 CUSTOM(name)
+# CHECK-NEXT:       -       48       2e CODE
+# CHECK-NEXT:       -       49       11         {{.*}}{{/|\\}}map-file.s.tmp1.o:(bar)
+# CHECK-NEXT:       -       49       11                 bar
+# CHECK-NEXT:       -       5a        b         {{.*}}{{/|\\}}map-file.s.tmp1.o:(write_global)
+# CHECK-NEXT:       -       5a        b                 write_global
+# CHECK-NEXT:       -       65        f         {{.*}}{{/|\\}}map-file.s.tmp1.o:(_start)
+# CHECK-NEXT:       -       65        f                 _start
+# CHECK-NEXT:       -       76        d DATA
+# CHECK-NEXT:     400       77        4 .data
+# CHECK-NEXT:     400       7d        4         {{.*}}{{/|\\}}map-file.s.tmp1.o:(.data.somedata)
+# CHECK-NEXT:     400       7d        4                 somedata
+# CHECK-NEXT:     404       76        4 .bss
+# CHECK-NEXT:     404        0        4         {{.*}}{{/|\\}}map-file.s.tmp1.o:(.bss.somezeroes)
+# CHECK-NEXT:     404        0        4                 somezeroes
+# CHECK-NEXT:       -       83       12 CUSTOM(.debug_info)
+# CHECK-NEXT:       -       95       50 CUSTOM(name)
 
 # RUN: not wasm-ld %t1.o -o /dev/null -Map=/ 2>&1 \
 # RUN:  | FileCheck -check-prefix=FAIL %s
 # FAIL: wasm-ld: error: cannot open map file /

lld/wasm/MapFile.cpp

 static DenseMap<Symbol *, std::string>
 getSymbolStrings(ArrayRef<Symbol *> syms) {
   std::vector<std::string> str(syms.size());
   parallelForEachN(0, syms.size(), [&](size_t i) {
     raw_string_ostream os(str[i]);
     auto *chunk = syms[i]->getChunk();
     if (chunk == nullptr)
       return;
-    uint64_t fileOffset = chunk->outputSec->getOffset() + chunk->outSecOff;
+    uint64_t fileOffset = chunk->outputSec != nullptr
+                              ? chunk->outputSec->getOffset() + chunk->outSecOff
+                              : 0;
     uint64_t vma = -1;
     uint64_t size = 0;
     if (auto *DD = dyn_cast<DefinedData>(syms[i])) {
       vma = DD->getVA();
       size = DD->getSize();
       fileOffset += DD->value;
     }
     if (auto *DF = dyn_cast<DefinedFunction>(syms[i])) {
if (auto *code = dyn_cast<CodeSection>(osec)) {
           os << symStr[sym] << '\n';
       }
     } else if (auto *data = dyn_cast<DataSection>(osec)) {
       for (auto *oseg : data->segments) {
         writeHeader(os, oseg->startVA, data->getOffset() + oseg->sectionOffset,
                     oseg->size);
         os << oseg->name << '\n';
         for (auto *chunk : oseg->inputSegments) {
-          writeHeader(os, chunk->getVA(),
-                      chunk->outputSec->getOffset() + chunk->outSecOff,
-                      chunk->getSize());
+          uint64_t offset =
+              chunk->outputSec != nullptr
+                  ? chunk->outputSec->getOffset() + chunk->outSecOff
+                  : 0;
+          writeHeader(os, chunk->getVA(), offset, chunk->getSize());
           os.indent(8) << toString(chunk) << '\n';
           for (Symbol *sym : sectionSyms[chunk])
             os << symStr[sym] << '\n';
         }
       }
     } else if (auto *globals = dyn_cast<GlobalSection>(osec)) {
       for (auto *global : globals->inputGlobals) {
         writeHeader(os, global->getAssignedIndex(), 0, 0);
         os.indent(8) << global->getName() << '\n';
       }
     }
     // TODO: other section/symbol types
   }
 }