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

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. Differential Revision: https://reviews.llvm.org/D102951
llvm · Jul 8, 2021 · 0fd5e7b · 0fd5e7b
1 parent f8c5a4c
commit 0fd5e7b
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Showing 2 changed files with 33 additions and 17 deletions.
diff --git a/lld/test/wasm/map-file.s b/lld/test/wasm/map-file.s
@@ -10,6 +10,9 @@ wasm_global:
 bar:
     .functype bar () -> ()
     i32.const   somedata
+    i32.const   somezeroes
+    drop
+    drop
     end_function
 
 write_global:
@@ -30,9 +33,15 @@ 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
@@ -42,19 +51,22 @@ somedata:
 # 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

diff --git a/lld/wasm/MapFile.cpp b/lld/wasm/MapFile.cpp
@@ -80,7 +80,9 @@ getSymbolStrings(ArrayRef<Symbol *> syms) {
     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])) {
@@ -138,9 +140,11 @@ void lld::wasm::writeMapFile(ArrayRef<OutputSection *> outputSections) {
                     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';