[llvm] r352767 - [llvm-objcopy] Skip --localize-symbol for undefined symbols

[Jordan Rupprecht via llvm-commits]

Author: rupprecht
Date: Thu Jan 31 08:45:16 2019
New Revision: 352767

URL: http://llvm.org/viewvc/llvm-project?rev=352767&view=rev
Log:
[llvm-objcopy] Skip --localize-symbol for undefined symbols

Summary:
Include the symbol being defined in the list of requirements for using --localize-symbol.

This is used, for example, when someone is depending on two different projects that have the same (or close enough) method defined in each library, and using "-L sym" for a conflicting symbol in one of the libraries so that the definition from the other one is used. However, the library may have internal references to the symbol, which cause program crashes when those are used, i.e.:

```
$ cat foo.c
int foo() { return 5; }
$ cat bar.c
int foo();
int bar() { return 2 * foo(); }
$ cat foo2.c
int foo() { /* Safer implementation */ return 42; }
$ cat main.c
int bar();
int main() {
  __builtin_printf("bar = %d\n", bar());
  return 0;
}
$ ar rcs libfoo.a foo.o bar.o
$ ar rcs libfoo2.a foo2.o
# Picks the wrong foo() impl
$ clang main.o -lfoo -lfoo2 -L. -o main
# Picks the right foo() impl
$ objcopy -L foo libfoo.a && clang main.o -lfoo -lfoo2 -L. -o main
# Links somehow, but crashes at runtime
$ llvm-objcopy -L foo libfoo.a && clang main.o -lfoo -lfoo2 -L. -o main
```

Reviewers: jhenderson, alexshap, jakehehrlich, espindola

Subscribers: emaste, arichardson

Differential Revision: https://reviews.llvm.org/D57417

Modified:
    llvm/trunk/test/tools/llvm-objcopy/ELF/localize.test
    llvm/trunk/tools/llvm-objcopy/ELF/ELFObjcopy.cpp

Modified: llvm/trunk/test/tools/llvm-objcopy/ELF/localize.test
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/tools/llvm-objcopy/ELF/localize.test?rev=352767&r1=352766&r2=352767&view=diff
==============================================================================
--- llvm/trunk/test/tools/llvm-objcopy/ELF/localize.test (original)
+++ llvm/trunk/test/tools/llvm-objcopy/ELF/localize.test Thu Jan 31 08:45:16 2019
@@ -1,6 +1,7 @@
 # RUN: yaml2obj %s > %t
 # RUN: llvm-objcopy \
 # RUN:     --localize-symbol Global \
+# RUN:     -L GlobalUndef \
 # RUN:     -L Local \
 # RUN:     -L Weak \
 # RUN:     -L GlobalCommon \
@@ -45,6 +46,8 @@ Symbols:
       Size:     8
       Section:  .text
       Value:    0x1010
+    - Name:     GlobalUndef
+      Type:     STT_FUNC
     - Name:     GlobalCommon
       Type:     STT_OBJECT
       Index:    SHN_COMMON
@@ -89,6 +92,15 @@ Symbols:
 #CHECK-NEXT:    Section: .text
 #CHECK-NEXT:  }
 #CHECK-NEXT:  Symbol {
+#CHECK-NEXT:    Name: GlobalUndef
+#CHECK-NEXT:    Value:
+#CHECK-NEXT:    Size:
+#CHECK-NEXT:    Binding: Global
+#CHECK-NEXT:    Type: Function
+#CHECK-NEXT:    Other:
+#CHECK-NEXT:    Section: Undefined
+#CHECK-NEXT:  }
+#CHECK-NEXT:  Symbol {
 #CHECK-NEXT:    Name: GlobalCommon
 #CHECK-NEXT:    Value: 0x2006
 #CHECK-NEXT:    Size: 2

Modified: llvm/trunk/tools/llvm-objcopy/ELF/ELFObjcopy.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/tools/llvm-objcopy/ELF/ELFObjcopy.cpp?rev=352767&r1=352766&r2=352767&view=diff
==============================================================================
--- llvm/trunk/tools/llvm-objcopy/ELF/ELFObjcopy.cpp (original)
+++ llvm/trunk/tools/llvm-objcopy/ELF/ELFObjcopy.cpp Thu Jan 31 08:45:16 2019
@@ -287,7 +287,9 @@ static Error handleArgs(const CopyConfig
   // them.
   if (Obj.SymbolTable) {
     Obj.SymbolTable->updateSymbols([&](Symbol &Sym) {
-      if (!Sym.isCommon() &&
+      // Common and undefined symbols don't make sense as local symbols, and can
+      // even cause crashes if we localize those, so skip them.
+      if (!Sym.isCommon() && Sym.getShndx() != SHN_UNDEF &&
           ((Config.LocalizeHidden &&
             (Sym.Visibility == STV_HIDDEN || Sym.Visibility == STV_INTERNAL)) ||
            is_contained(Config.SymbolsToLocalize, Sym.Name)))