[PATCH] D43658: Make undefined symbol in DSO to pull out object files from archive files.

[Rui Ueyama via Phabricator via llvm-commits]

ruiu created this revision.
ruiu added a reviewer: rafael.
Herald added subscribers: arichardson, emaste.

We have an internal program that does't link without this patch. I don't
know of any open-source program that needs this, but there might be.
Since this patch improves compatibility with GNU linkers with a few lines
of code, I think it's worth to be committed.

The problem is about undefined symbols in DSOs. Some programs depend on
the GNU linkers' behavior that they pull out object files from archive
files to resolve undefined symbols in DSOs. We already allow that kind of
"reverse" dependency (from DSOs to the main executable) for regular
symbols, in particular, for "__progname" symbol, but that didn't work
if the symbol is in an archive file. This patch is to make it work.


https://reviews.llvm.org/D43658

Files:
  lld/ELF/SymbolTable.cpp
  lld/test/ELF/shlib-undefined-archive.s


Index: lld/test/ELF/shlib-undefined-archive.s
===================================================================
--- /dev/null
+++ lld/test/ELF/shlib-undefined-archive.s
@@ -0,0 +1,18 @@
+# REQUIRES: x86
+
+# Undefined symbols in a DSO should pull out object files from archives
+# to resolve them.
+
+# RUN: echo '.globl foo' | llvm-mc -filetype=obj -triple=x86_64-linux-gnu -o %t1.o -
+# RUN: ld.lld -shared -o %t.so %t1.o
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-linux-gnu -o %t2.o %s
+# RUN: llvm-ar cru %t.a %t2.o
+# RUN: ld.lld -o %t.exe %t.a %t.so
+# RUN: llvm-nm -D %t.exe | FileCheck %s
+
+# CHECK: T foo
+
+.globl foo
+foo:
+  ret
Index: lld/ELF/SymbolTable.cpp
===================================================================
--- lld/ELF/SymbolTable.cpp
+++ lld/ELF/SymbolTable.cpp
@@ -599,9 +599,14 @@
   for (InputFile *F : SharedFiles) {
     for (StringRef U : cast<SharedFile<ELFT>>(F)->getUndefinedSymbols()) {
       Symbol *Sym = find(U);
-      if (!Sym || !Sym->isDefined())
+      if (!Sym)
         continue;
-      Sym->ExportDynamic = true;
+      if (auto *L = dyn_cast<Lazy>(Sym))
+        if (InputFile *File = L->fetch())
+          addFile<ELFT>(File);
+
+      if (Sym->isDefined())
+        Sym->ExportDynamic = true;
     }
   }
 }


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