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

[jhenderson]

Summary

I noticed this whilst doing some work on the debug line parser. 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.

[grimar]

ELF/InputFiles.cpp

149 ↗

(On Diff #137373)

If there any reason why it cannot be early returned above?

const DWARFDebugLine::LineTable *LT =
    DwarfLine->getOrParseLineTable(LineData, 0, Dwarf, nullptr);
if (!LT)
  return;

[jhenderson]

ELF/InputFiles.cpp

149 ↗

(On Diff #137373)

Good point - not that I can think of.

[jhenderson]

Return early as suggested.

[grimar]

LGTM

lld/trunk/ELF/InputFiles.cpp

template <class ELFT> void ObjFile<ELFT>::initializeDwarf() {
   DWARFDataExtractor LineData(Obj, Obj.getLineSection(), Config->IsLE,
                               Config->Wordsize);
 
   // The second parameter is offset in .debug_line section
   // for compilation unit (CU) of interest. We have only one
   // 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())
     return;
 
   // Loop over variable records and insert them to VariableLoc.
   DWARFCompileUnit *CU = Dwarf.getCompileUnitAtIndex(0);
   for (const auto &Entry : CU->dies()) {

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

+.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/trunk/test/ELF/undef.s

 # REQUIRES: x86
 # 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
 # CHECK:                   {{.*}}:(.text+0x1)
 
 # CHECK: error: undefined symbol: bar
 # CHECK: >>> referenced by undef.s
 # CHECK: >>>               {{.*}}:(.text+0x6)

 # CHECK: error: undefined symbol: zed4
 # CHECK: >>> referenced by undef-debug.s:7 (dir{{/|\\}}undef-debug.s:7)
 # CHECK: >>>               {{.*}}.o:(.text.1+0x0)
 
 # CHECK: error: undefined symbol: zed5
 # 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
 
 .file "undef.s"
 
   .globl _start
 _start:
   call foo
   call bar
   call zed1
   call _Z3fooi