[ELF] Resolve undefined symbols from archives when linking shared objects

[evgeny777]

Summary

When linking DSO lld doesn't resolve symbols from input DSOs in case those symbols are lazy. Below is an example:

input.so - has undefined symbol 'print'
libprint.a - defines 'print'

ld.lld -shared -o output.so input.so libprint.a

Even though libprint.a has definition of 'print', output.so will have it undefined.
Later when you link application with output.so, you'll also have to provide libprint.a

This patch fixes it.

[ruiu]

How did you notice that this is an issue? I wonder if this is a real one.

ELF/SymbolTable.cpp

556

You want to set true to ExportDynamic.

ELF/SymbolTable.cpp

 // may have undefined symbols, expecting that the user program provides
 // the definitions for them. An example is BSD's __progname symbol.
 // We need to put such symbols to the main program's .dynsym so that
 // shared libraries can find them.
 // Except this, we ignore undefined symbols in DSOs.
 template <class ELFT> void SymbolTable<ELFT>::scanShlibUndefined() {
   for (SharedFile<ELFT> *File : SharedFiles)
     for (StringRef U : File->getUndefinedSymbols())
-      if (SymbolBody *Sym = find(U))
+      if (SymbolBody *Sym = find(U)) {
         if (Sym->isDefined())
           Sym->symbol()->ExportDynamic = true;
+        else if (Sym->isLazy())
+          addUndefined(U);
+      }
 }
 
 // This function processes --export-dynamic-symbol and --dynamic-list.
 template <class ELFT> void SymbolTable<ELFT>::scanDynamicList() {
   for (StringRef S : Config->DynamicList)
     if (SymbolBody *B = find(S))
       B->symbol()->ExportDynamic = true;
 }

test/ELF/Inputs/shared4.s

+.globl _shared
+_shared:
+  call _start@plt

test/ELF/shared-lazy.s

+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %p/Inputs/archive.s -o %t2.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %p/Inputs/shared4.s -o %t3.o
+# RUN: llvm-ar rcs %t2.a %t2.o
+# RUN: ld.lld -shared %t3.o -o %t3.so
+# RUN: ld.lld -shared %t.o %t2.a %t3.so -o %t.so
+# RUN: llvm-objdump -t %t.so | FileCheck --check-prefix=DEFINED %s
+
+# DEFINED: _start
+call _entry@plt