[ELF] Prevent crash when reporting errors if debug line cannot be parsed

LLD uses the debug info and debug line sections to determine the location of
e.g. references to undefined symbols, when producing error messages. In the
event that debug info was present, but debug line parsing failed for some
reason, then a nullptr would end up being dereferenced by the location-lookup
code.

Differential Revision: https://reviews.llvm.org/D44205

Reviewers: grimar
llvm-svn: 326899

lld/ELF/InputFiles.cpp

@@ -126,6 +126,8 @@ template <class ELFT> void ObjFile<ELFT>::initializeDwarf() {
   // CU (object file), so offset is always 0.
   const DWARFDebugLine::LineTable *LT =
       DwarfLine->getOrParseLineTable(LineData, 0, Dwarf, nullptr);
+  if (!LT)
+    return;
 
   // Return if there is no debug information about CU available.
   if (!Dwarf.getNumCompileUnits())

lld/test/ELF/Inputs/undef-bad-debug.s

@@ -0,0 +1,44 @@
+.section .text,"ax"
+sym:
+    .quad zed6
+
+.section .debug_info,"",@progbits
+    .long   .Lcu_end - .Lcu_start   # Length of Unit
+.Lcu_start:
+    .short  4                       # DWARF version number
+    .long   .Lsection_abbrev        # Offset Into Abbrev. Section
+    .byte   8                       # Address Size (in bytes)
+    .byte   1                       # Abbrev [1] 0xb:0x79 DW_TAG_compile_unit
+    .byte   2                       # Abbrev [2] 0x2a:0x15 DW_TAG_variable
+    .long   .Linfo_string           # DW_AT_name
+                                        # DW_AT_external
+    .byte   1                       # DW_AT_decl_file
+    .byte   3                       # DW_AT_decl_line
+    .byte   0                       # End Of Children Mark
+.Lcu_end:
+
+.section .debug_abbrev,"",@progbits
+.Lsection_abbrev:
+    .byte   1                       # Abbreviation Code
+    .byte   17                      # DW_TAG_compile_unit
+    .byte   1                       # DW_CHILDREN_yes
+    .byte   0                       # EOM(1)
+    .byte   0                       # EOM(2)
+    .byte   2                       # Abbreviation Code
+    .byte   52                      # DW_TAG_variable
+    .byte   0                       # DW_CHILDREN_no
+    .byte   3                       # DW_AT_name
+    .byte   14                      # DW_FORM_strp
+    .byte   63                      # DW_AT_external
+    .byte   25                      # DW_FORM_flag_present
+    .byte   58                      # DW_AT_decl_file
+    .byte   11                      # DW_FORM_data1
+    .byte   59                      # DW_AT_decl_line
+    .byte   11                      # DW_FORM_data1
+    .byte   0                       # EOM(1)
+    .byte   0                       # EOM(2)
+    .byte   0                       # EOM(3)
+
+.section .debug_str,"MS",@progbits,1
+.Linfo_string:
+    .asciz "sym"

lld/test/ELF/undef.s

@@ -2,9 +2,10 @@
 # RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
 # RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %p/Inputs/undef.s -o %t2.o
 # RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %p/Inputs/undef-debug.s -o %t3.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %p/Inputs/undef-bad-debug.s -o %t4.o
 # RUN: llvm-ar rc %t2.a %t2.o
-# RUN: not ld.lld %t.o %t2.a %t3.o -o %t.exe 2>&1 | FileCheck %s
-# RUN: not ld.lld -pie %t.o %t2.a %t3.o -o %t.exe 2>&1 | FileCheck %s
+# RUN: not ld.lld %t.o %t2.a %t3.o %t4.o -o %t.exe 2>&1 | FileCheck %s
+# RUN: not ld.lld -pie %t.o %t2.a %t3.o %t4.o -o %t.exe 2>&1 | FileCheck %s
 
 # CHECK: error: undefined symbol: foo
 # CHECK: >>> referenced by undef.s
@@ -33,6 +34,9 @@
 # CHECK: >>> referenced by undef-debug.s:11 (dir{{/|\\}}undef-debug.s:11)
 # CHECK: >>>               {{.*}}.o:(.text.2+0x0)
 
+# CHECK: error: undefined symbol: zed6
+# CHECK: >>> referenced by {{.*}}tmp4.o:(.text+0x0)
+
 # RUN: not ld.lld %t.o %t2.a -o %t.exe -no-demangle 2>&1 | \
 # RUN:   FileCheck -check-prefix=NO-DEMANGLE %s
 # NO-DEMANGLE: error: undefined symbol: _Z3fooi