commit 0fbf28f7aae0ceb70071cac56de345e3ff04439c
author Fangrui Song <maskray@google.com> Thu Jan 23 11:52:03 2020 -0800
committer Fangrui Song <maskray@google.com> Thu Jan 23 12:25:15 2020 -0800
tree 96923d7586645e3b2bd9d0e85a9440d81de2225c
parent e5caa156b446150adc2343760f68ba3e1555c277

[ELF] --no-dynamic-linker: don't emit undefined weak symbols to .dynsym

I felt really sad to push this commit for my selfish purpose to make
glibc -static-pie build with lld. Some code constructs in glibc require
R_X86_64_GOTPCREL/R_X86_64_REX_GOTPCRELX referencing undefined weak to
be resolved to a GOT entry not relocated by R_X86_64_GLOB_DAT (GNU ld
behavior), e.g.

csu/libc-start.c
  if (__pthread_initialize_minimal != NULL)
    __pthread_initialize_minimal ();

elf/dl-object.c
  void
  _dl_add_to_namespace_list (struct link_map *new, Lmid_t nsid)
  {
    /* We modify the list of loaded objects.  */
    __rtld_lock_lock_recursive (GL(dl_load_write_lock));

Emitting a GLOB_DAT will make the address equal &__ehdr_start (true
value) and cause elf/ldconfig to segfault. glibc really should move away
from weak references, which do not have defined semantics.

Temporarily special case --no-dynamic-linker.

lld/ELF/Config.h

diff --git a/lld/ELF/Config.h b/lld/ELF/Config.h
index 06ba88a..ef1edbc 100644
--- a/lld/ELF/Config.h
+++ b/lld/ELF/Config.h
@@ -169,6 +169,7 @@
   bool mipsN32Abi = false;
   bool mmapOutputFile;
   bool nmagic;
+  bool noDynamicLinker = false;
   bool noinhibitExec;
   bool nostdlib;
   bool oFormatBinary;

lld/ELF/Driver.cpp

diff --git a/lld/ELF/Driver.cpp b/lld/ELF/Driver.cpp
index 61d4247..e85183e 100644
--- a/lld/ELF/Driver.cpp
+++ b/lld/ELF/Driver.cpp
@@ -602,8 +602,13 @@
 
 static StringRef getDynamicLinker(opt::InputArgList &args) {
   auto *arg = args.getLastArg(OPT_dynamic_linker, OPT_no_dynamic_linker);
-  if (!arg || arg->getOption().getID() == OPT_no_dynamic_linker)
+  if (!arg)
     return "";
+  if (arg->getOption().getID() == OPT_no_dynamic_linker) {
+    // --no-dynamic-linker suppresses undefined weak symbols in .dynsym
+    config->noDynamicLinker = true;
+    return "";
+  }
   return arg->getValue();
 }
 

lld/ELF/Symbols.cpp

diff --git a/lld/ELF/Symbols.cpp b/lld/ELF/Symbols.cpp
index f0f6121..0dcf347 100644
--- a/lld/ELF/Symbols.cpp
+++ b/lld/ELF/Symbols.cpp
@@ -278,7 +278,11 @@
   if (computeBinding() == STB_LOCAL)
     return false;
   if (!isDefined() && !isCommon())
-    return true;
+    // This should unconditionally return true, unfortunately glibc -static-pie
+    // expects undefined weak symbols not to exist in .dynsym, e.g.
+    // __pthread_mutex_lock reference in _dl_add_to_namespace_list,
+    // __pthread_initialize_minimal reference in csu/libc-start.c.
+    return !(config->noDynamicLinker && isUndefWeak());
 
   return exportDynamic || inDynamicList;
 }