Index: llvm/test/tools/llvm-dwp/X86/unknown-section-id.s
===================================================================
--- /dev/null
+++ llvm/test/tools/llvm-dwp/X86/unknown-section-id.s
@@ -0,0 +1,117 @@
+## The test checks that llvm-dwp avoids an out of bound access when there is
+## an unknown section identifier in an index section. Without the fix, the test
+## failed when LLVM is built with UBSan.
+## Note that additional sections (.debug_abbrev.dwo, .debug_info.dwo, and
+## .debug_types.dwo) are required to reach the test points in the code.
+
+# RUN: llvm-mc -triple x86_64-unknown-linux-gnu %s -filetype=obj -o %t.dwp
+# RUN: llvm-dwp %t.dwp -o -
+
+.section .debug_abbrev.dwo, "e", @progbits
+.LCUAbbrevBegin:
+    .uleb128 1                          # Abbreviation Code
+    .uleb128 0x11                       # DW_TAG_compile_unit
+    .byte 1                             # DW_CHILDREN_no
+    .uleb128 0x2131                     # DW_AT_GNU_dwo_id
+    .uleb128 7                          # DW_FORM_data8
+    .byte 0                             # EOM(1)
+    .byte 0                             # EOM(2)
+    .byte 0                             # EOM(3)
+.LCUAbbrevEnd:
+
+.LTUAbbrevBegin:
+    .uleb128 1                          # Abbreviation Code
+    .uleb128 0x41                       # DW_TAG_type_unit
+    .byte 1                             # DW_CHILDREN_yes
+    .byte 0                             # EOM(1)
+    .byte 0                             # EOM(2)
+    .uleb128 2                          # Abbreviation Code
+    .uleb128 0x13                       # DW_TAG_structure_type
+    .byte 0                             # DW_CHILDREN_no
+    .byte 0                             # EOM(1)
+    .byte 0                             # EOM(2)
+    .byte 0                             # EOM(3)
+.LTUAbbrevEnd:
+
+    .section .debug_info.dwo, "e", @progbits
+.LCUBegin:
+    .long .LCUEnd-.LCUVersion           # Length of Unit
+.LCUVersion:
+    .short 4                            # Version
+    .long 0                             # Abbrev offset
+    .byte 8                             # Address size
+    .uleb128 1                          # Abbrev [1] DW_TAG_compile_unit
+    .quad 0x1100002222222222            # DW_AT_GNU_dwo_id
+.LCUEnd:
+
+    .section .debug_types.dwo, "e", @progbits
+.LTUBegin:
+    .long .LTUEnd-.LTUVersion           # Length of Unit
+.LTUVersion:
+    .short 4                            # Version
+    .long 0                             # Abbrev offset
+    .byte 8                             # Address size
+    .quad 0x1100003333333333            # Type signature
+    .long .LTUType-.LTUBegin            # Type offset
+    .uleb128 1                          # Abbrev [1] DW_TAG_type_unit
+.LTUType:
+    .uleb128 2                          # Abbrev [2] DW_TAG_structure_type
+.LTUEnd:
+
+    .section .debug_cu_index, "", @progbits
+## Header:
+    .long 2                             # Version
+    .long 4                             # Section count
+    .long 1                             # Unit count
+    .long 2                             # Slot count
+## Hash Table of Signatures:
+    .quad 0x1100002222222222
+    .quad 0
+## Parallel Table of Indexes:
+    .long 1
+    .long 0
+## Table of Section Offsets:
+## Row 0:
+    .long 1                             # DW_SECT_INFO
+    .long 3                             # DW_SECT_ABBREV
+    .long 0                             # Invalid ID, less than DW_SECT_INFO
+    .long 9                             # Invalid ID, greater than DW_SECT_MACRO
+## Row 1:
+    .long 0                             # Offset in .debug_info.dwo
+    .long 0                             # Offset in .debug_abbrev.dwo
+    .long 0
+    .long 0
+## Table of Section Sizes:
+    .long .LCUEnd-.LCUBegin             # Size in .debug_info.dwo
+    .long .LCUAbbrevEnd-.LCUAbbrevBegin # Size in .debug_abbrev.dwo
+    .long 1
+    .long 1
+
+    .section .debug_tu_index, "", @progbits
+## Header:
+    .long 2                             # Version
+    .long 4                             # Section count
+    .long 1                             # Unit count
+    .long 2                             # Slot count
+## Hash Table of Signatures:
+    .quad 0x1100003333333333
+    .quad 0
+## Parallel Table of Indexes:
+    .long 1
+    .long 0
+## Table of Section Offsets:
+## Row 0:
+    .long 2                             # DW_SECT_TYPES
+    .long 3                             # DW_SECT_ABBREV
+    .long 0                             # Invalid ID, less than DW_SECT_INFO
+    .long 9                             # Invalid ID, greater than DW_SECT_MACRO
+## Row 1:
+    .long 0                             # Offset in .debug_types.dwo
+    .long 0                             # Offset in .debug_abbrev.dwo
+    .long 0
+    .long 0
+## Table of Section Sizes:
+    .long .LTUEnd-.LTUBegin             # Size in .debug_types.dwo
+    .long .LTUAbbrevEnd-.LTUAbbrevBegin # Size in .debug_abbrev.dwo
+    .long 1
+    .long 1
Index: llvm/tools/llvm-dwp/llvm-dwp.cpp
===================================================================
--- llvm/tools/llvm-dwp/llvm-dwp.cpp
+++ llvm/tools/llvm-dwp/llvm-dwp.cpp
@@ -216,6 +216,10 @@
   StringRef DWPName;
 };
 
+static bool isSupportedSectionKind(DWARFSectionKind Kind) {
+  return Kind >= DW_SECT_INFO && Kind <= DW_SECT_MACRO;
+}
+
 static StringRef getSubsection(StringRef Section,
                                const DWARFUnitIndex::Entry &Entry,
                                DWARFSectionKind Kind) {
@@ -241,6 +245,8 @@
     // Zero out the debug_info contribution
     Entry.Contributions[0] = {};
     for (auto Kind : TUIndex.getColumnKinds()) {
+      if (!isSupportedSectionKind(Kind))
+        continue;
       auto &C = Entry.Contributions[Kind - DW_SECT_INFO];
       C.Offset += I->Offset;
       C.Length = I->Length;
@@ -618,6 +624,8 @@
       NewEntry.DWOName = ID.DWOName;
       NewEntry.DWPName = Input;
       for (auto Kind : CUIndex.getColumnKinds()) {
+        if (!isSupportedSectionKind(Kind))
+          continue;
         auto &C = NewEntry.Contributions[Kind - DW_SECT_INFO];
         C.Offset += I->Offset;
         C.Length = I->Length;