[ELF] Don't change binding to STB_WEAK for an undefined specified by -u

[MaskRay]

Summary

Similar to D66992.
In GNU ld, a -u specified symbol is a STB_DEFAULT undefined.
It cannot be changed to STB_WEAK by a later STB_WEAK undefined in a regular object file.

The behavior is consistent with our model because -u means "we need to fetch a lazy definition".
It should not be altered just because there is also a STB_WEAK undefined.

Note, our -u semantics are still different from GNU ld (https://github.com/ClangBuiltLinux/linux/issues/515):
we don't force the specified symbol to appear in .symtab This is a deliberate decision.

[grimar]

lld/test/ELF/weak-undef-lib.s

23

Perhaps, CHECK1 -> CHECK-UNDEF or just UNDEF.

27

Show binding?

[troyj]

This patch fixes the problem that I encountered downstream linking -u foo a.o b.a where foo is weak in a.o and provided in b.a.

[MaskRay]

Improve tests

[grimar]

LGTM

lld/ELF/Driver.cpp

template <class ELFT> void LinkerDriver::link(opt::InputArgList &args) {
 
   // Handle --trace-symbol.
   for (auto *arg : args.filtered(OPT_trace_symbol))
     symtab->insert(arg->getValue())->traced = true;
 
   // Handle -u/--undefined before input files. If both a.a and b.so define foo,
   // -u foo a.a b.so will fetch a.a.
   for (StringRef name : config->undefined)
-    addUnusedUndefined(name);
+    addUnusedUndefined(name)->referenced = true;
 
   // Add all files to the symbol table. This will add almost all
   // symbols that we need to the symbol table. This process might
   // add files to the link, via autolinking, these files are always
   // appended to the Files vector.
   {
     llvm::TimeTraceScope timeScope("Parse input files");
     for (size_t i = 0; i < files.size(); ++i)

lld/test/ELF/weak-undef-lib.s

 # CHECK:      Name: foo
 # CHECK-NEXT: Value: 0x0
 # CHECK-NEXT: Size: 0
 # CHECK-NEXT: Binding: Weak
 # CHECK-NEXT: Type: None
 # CHECK-NEXT: Other: 0
 # CHECK-NEXT: Section: Undefined
 
+## -u specifies a STB_DEFAULT undefined symbol, so the definition from %t2.o is
+## fetched.
+# RUN: ld.lld -u foo %t1.o --start-lib %t2.o -o %t1
+# RUN: llvm-readobj --syms %t1 | FileCheck %s --check-prefix=CHECK-U
+
+# CHECK-U:      Name: foo
+# CHECK-U:      Binding:
+# CHECK-U-SAME:          Global
+# CHECK-U:      Section:
+# CHECK-U-SAME:          .text
+
 .weak foo
 call foo@PLT
 
 .data
 .quad foo