[lld] 15dc93e - [lld-macho] Ignore LLVM segments to prevent duplicate syms

[Vincent Lee via llvm-commits]

Author: Vincent Lee
Date: 2021-08-16T12:41:03-07:00
New Revision: 15dc93e61c21abe1966484cd3f091d63616f29ab

URL: https://github.com/llvm/llvm-project/commit/15dc93e61c21abe1966484cd3f091d63616f29ab
DIFF: https://github.com/llvm/llvm-project/commit/15dc93e61c21abe1966484cd3f091d63616f29ab.diff

LOG: [lld-macho] Ignore LLVM segments to prevent duplicate syms

There was an instance of a third-party archive containing multiple
_llvm symbols from different files that clashed with each other
producing duplicate symbols. Symbols under the LLVM segment
don't seem to be producing any meaningful value, so just ignore them.

Reviewed By: #lld-macho, int3

Differential Revision: https://reviews.llvm.org/D108016

Added: 
    lld/test/MachO/discard-llvm-sections.s

Modified: 
    lld/MachO/InputFiles.cpp

Removed: 
    


################################################################################
diff  --git a/lld/MachO/InputFiles.cpp b/lld/MachO/InputFiles.cpp
index 04cdd981929b9..32279da1cf627 100644
--- a/lld/MachO/InputFiles.cpp
+++ b/lld/MachO/InputFiles.cpp
@@ -282,16 +282,24 @@ void ObjFile::parseSections(ArrayRef<Section> sections) {
     } else {
       auto *isec =
           make<ConcatInputSection>(segname, name, this, data, align, flags);
-      if (!(isDebugSection(isec->getFlags()) &&
-            isec->getSegName() == segment_names::dwarf)) {
-        subsections.push_back({{0, isec}});
-      } else {
+      if (isDebugSection(isec->getFlags()) &&
+          isec->getSegName() == segment_names::dwarf) {
         // Instead of emitting DWARF sections, we emit STABS symbols to the
         // object files that contain them. We filter them out early to avoid
         // parsing their relocations unnecessarily. But we must still push an
         // empty map to ensure the indices line up for the remaining sections.
         subsections.push_back({});
         debugSections.push_back(isec);
+      } else if (isec->getSegName() == segment_names::llvm) {
+        // ld64 does not appear to emit contents from sections within the __LLVM
+        // segment. Symbols within those sections point to bitcode metadata
+        // instead of actual symbols. Global symbols within those sections could
+        // have the same name without causing duplicate symbol errors. Push an
+        // empty map to ensure indices line up for the remaining sections.
+        // TODO: Evaluate whether the bitcode metadata is needed.
+        subsections.push_back({});
+      } else {
+        subsections.push_back({{0, isec}});
       }
     }
   }

diff  --git a/lld/test/MachO/discard-llvm-sections.s b/lld/test/MachO/discard-llvm-sections.s
new file mode 100644
index 0000000000000..571fdbb0b2da0
--- /dev/null
+++ b/lld/test/MachO/discard-llvm-sections.s
@@ -0,0 +1,46 @@
+# REQUIRES: x86
+# RUN: rm -rf %t; split-file %s %t
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/foo.s -o %t/foo.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/bar.s -o %t/bar.o
+
+## "_llvm." symbols are not special. LLD would produce duplicate symbol errors
+## if they were not within the LLVM segment.
+
+## 1/ Test that LLD does not produce duplicate symbols errors when linking global symbols
+##    with the same name under the LLVM segment.
+# RUN: %lld -dylib %t/foo.o %t/bar.o -o %t/libDuplicate.dylib
+
+## 2/ Test that all sections within an LLVM segment are dropped.
+# RUN: llvm-objdump --section-headers %t/libDuplicate.dylib | FileCheck %s
+
+# CHECK-LABEL: Sections:
+# CHECK-NEXT:  Idx  Name    Size      VMA            Type
+# CHECK-NEXT:  0    __text  00000000  {{[0-9a-f]+}}  TEXT
+
+## 3/ Test that linking global symbol that is not under the LLVM segment produces duplicate
+##    symbols
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin --defsym TEXT=0 %t/foo.s -o %t/foo.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin --defsym TEXT=0 %t/bar.s -o %t/bar.o
+# RUN: not %lld -dylib %t/foo.o %t/bar.o -o %t/libDuplicate.dylib 2>&1 | FileCheck %s --check-prefix=DUP
+
+# DUP: ld64.lld: error: duplicate symbol: _llvm.foo
+
+#--- foo.s
+.globl _llvm.foo
+.ifdef TEXT
+  .section __TEXT,__cstring
+.else
+  .section __LLVM,__bitcode
+.endif
+  _llvm.foo:
+    .asciz "test"
+
+#--- bar.s
+.globl _llvm.foo
+.ifdef TEXT
+  .section __TEXT,__cstring
+.else
+  .section __LLVM,__bitcode
+.endif
+  _llvm.foo:
+    .asciz "test"