Index: lld/trunk/ELF/InputFiles.cpp
===================================================================
--- lld/trunk/ELF/InputFiles.cpp
+++ lld/trunk/ELF/InputFiles.cpp
@@ -999,25 +999,25 @@
   for (size_t I = 0; I < Syms.size(); ++I) {
     const Elf_Sym &Sym = Syms[I];
 
-    StringRef Name = CHECK(Sym.getName(this->StringTable), this);
-    if (Sym.isUndefined()) {
-      Symbol *S = Symtab->addUndefined<ELFT>(Name, Sym.getBinding(),
-                                             Sym.st_other, Sym.getType(),
-                                             /*CanOmitFromDynSym=*/false, this);
-      S->ExportDynamic = true;
-      continue;
-    }
-
     // ELF spec requires that all local symbols precede weak or global
     // symbols in each symbol table, and the index of first non-local symbol
     // is stored to sh_info. If a local symbol appears after some non-local
     // symbol, that's a violation of the spec.
+    StringRef Name = CHECK(Sym.getName(this->StringTable), this);
     if (Sym.getBinding() == STB_LOCAL) {
       warn("found local symbol '" + Name +
            "' in global part of symbol table in file " + toString(this));
       continue;
     }
 
+    if (Sym.isUndefined()) {
+      Symbol *S = Symtab->addUndefined<ELFT>(Name, Sym.getBinding(),
+                                             Sym.st_other, Sym.getType(),
+                                             /*CanOmitFromDynSym=*/false, this);
+      S->ExportDynamic = true;
+      continue;
+    }
+
     // MIPS BFD linker puts _gp_disp symbol into DSO files and incorrectly
     // assigns VER_NDX_LOCAL to this section global symbol. Here is a
     // workaround for this bug.
Index: lld/trunk/test/ELF/invalid/undefined-local-symbol-in-dso.test
===================================================================
--- lld/trunk/test/ELF/invalid/undefined-local-symbol-in-dso.test
+++ lld/trunk/test/ELF/invalid/undefined-local-symbol-in-dso.test
@@ -0,0 +1,66 @@
+# REQUIRES: x86
+
+# LLD used to crash when linking against a DSO with an undefined STB_LOCAL
+# symbol in the global part of the dynamic symbol table (i.e. an STB_LOCAL
+# symbol with an index >= the sh_info of the dynamic symbol table section). Such
+# a DSO is very broken, because local symbols should precede all global symbols
+# in the symbol table, and because having a symbol that's both undefined and
+# STB_LOCAL is a nonsensical combination. Nevertheless, we should warn on such
+# input files instead of crashing.
+
+# We've found actual broken DSOs of this sort in the wild, but for this test, we
+# created a reduced broken input file. There are no tools capable of producing a
+# broken DSO of this nature, so instead we created a valid DSO with an undefiend
+# global symbol in the dynamic symbol table and then manually edited the binary
+# to make that symbol local. The valid DSO was created as follows:
+
+```
+% cat undef.s
+.hidden bar
+bar:
+	movq	foo@GOT, %rax
+
+% llvm-mc -triple=x86_64-linux-gnu -filetype=obj -o undef.o undef.s
+% ld.lld --no-rosegment -shared -o undefined-local-symbol-in-dso.so undef.o
+% strip undef.so
+```
+
+# (--no-rosegment and stripping are unnecessary; they just produce a smaller
+# binary)
+
+# This DSO should only have a single dynamic symbol table entry for foo, and
+# then we can use a small C program to modify that symbol table entry to be
+# STB_LOCAL instead of STB_GLOBAL.
+
+```
+#include <elf.h>
+#include <stdio.h>
+
+int main(int argc, char *argv[]) {
+  FILE *F = fopen(argv[1], "rb+");
+
+  Elf64_Ehdr Ehdr;
+  fread(&Ehdr, sizeof(Ehdr), 1, F);
+  fseek(F, Ehdr.e_shoff, SEEK_SET);
+
+  Elf64_Shdr Shdr;
+  do {
+    fread(&Shdr, sizeof(Shdr), 1, F);
+  } while (Shdr.sh_type != SHT_DYNSYM);
+
+  Elf64_Sym Sym;
+  fseek(F, Shdr.sh_offset + sizeof(Elf64_Sym), SEEK_SET);
+  fread(&Sym, sizeof(Sym), 1, F);
+  Sym.st_info = STB_LOCAL << 4 | ELF64_ST_TYPE(Sym.st_info);
+  fseek(F, Shdr.sh_offset + sizeof(Elf64_Sym), SEEK_SET);
+  fwrite(&Sym, sizeof(Sym), 1, F);
+  fclose(F);
+}
+```
+
+# (the C program just takes its input DSO and modifies the binding of the first
+# dynamic symbol table entry to be STB_LOCAL instead of STB_GLOBAL)
+
+# RUN: ld.lld %p/Inputs/undefined-local-symbol-in-dso.so -o %t 2>&1 | \
+# RUN:   FileCheck -check-prefix=WARN %s
+# WARN: found local symbol 'foo' in global part of symbol table in file {{.*}}undefined-local-symbol-in-dso.so