[lld] r321578 - [ELF] Only scan executables for shlib undefined symbols

[Shoaib Meenai via llvm-commits]

Author: smeenai
Date: Sat Dec 30 00:00:44 2017
New Revision: 321578

URL: http://llvm.org/viewvc/llvm-project?rev=321578&view=rev
Log:
[ELF] Only scan executables for shlib undefined symbols

If using a version script with a `local: *` in it, symbols in shared
libraries will still get default visibility if another shared library on
the link line has an undefined reference to the symbol. This is quite
surprising. Neither bfd nor gold have this behavior when linking a
shared library, and none of LLD's tests fail without this behavior, so
it seems safe to limit scanShlibUndefined to executables.

As far as executables are concerned, gold doesn't do any automatic
default visibility marking, and bfd issues a link error about a shared
library having a reference to a hidden symbol rather than silently
giving that symbol default visibility. I think bfd's behavior here is
preferable to LLD's, but that's something to be considered in a
follow-up.

Differential Revision: https://reviews.llvm.org/D41524

Added:
    lld/trunk/test/ELF/Inputs/shlib-undefined-ref.s
    lld/trunk/test/ELF/shlib-undefined-shared.s
Modified:
    lld/trunk/ELF/Driver.cpp

Modified: lld/trunk/ELF/Driver.cpp
URL: http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/Driver.cpp?rev=321578&r1=321577&r2=321578&view=diff
==============================================================================
--- lld/trunk/ELF/Driver.cpp (original)
+++ lld/trunk/ELF/Driver.cpp Sat Dec 30 00:00:44 2017
@@ -1043,7 +1043,8 @@ template <class ELFT> void LinkerDriver:
     return;
 
   // Handle undefined symbols in DSOs.
-  Symtab->scanShlibUndefined<ELFT>();
+  if (!Config->Shared)
+    Symtab->scanShlibUndefined<ELFT>();
 
   // Handle the -exclude-libs option.
   if (Args.hasArg(OPT_exclude_libs))

Added: lld/trunk/test/ELF/Inputs/shlib-undefined-ref.s
URL: http://llvm.org/viewvc/llvm-project/lld/trunk/test/ELF/Inputs/shlib-undefined-ref.s?rev=321578&view=auto
==============================================================================
--- lld/trunk/test/ELF/Inputs/shlib-undefined-ref.s (added)
+++ lld/trunk/test/ELF/Inputs/shlib-undefined-ref.s Sat Dec 30 00:00:44 2017
@@ -0,0 +1,4 @@
+.globl f
+f:
+	call	should_not_be_exported at PLT
+	ret

Added: lld/trunk/test/ELF/shlib-undefined-shared.s
URL: http://llvm.org/viewvc/llvm-project/lld/trunk/test/ELF/shlib-undefined-shared.s?rev=321578&view=auto
==============================================================================
--- lld/trunk/test/ELF/shlib-undefined-shared.s (added)
+++ lld/trunk/test/ELF/shlib-undefined-shared.s Sat Dec 30 00:00:44 2017
@@ -0,0 +1,15 @@
+# REQUIRES: x86
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-linux-gnu -o %t1.o %S/Inputs/shlib-undefined-ref.s
+# RUN: ld.lld -shared -o %t1.so %t1.o
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-linux-gnu -o %t2.o %s
+# RUN: echo "{ local: *; };" > %t.script
+# RUN: ld.lld -shared -version-script %t.script -o %t2.so %t2.o %t1.so
+# RUN: llvm-nm -g %t2.so | FileCheck -allow-empty %s
+
+# CHECK-NOT: should_not_be_exported
+
+.globl should_not_be_exported
+should_not_be_exported:
+	ret