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Fix crash after looking up dwo_id=0 in CU index.
In the current state, if getFromHash(0) is called and there's no CU with dwo_id=0, the lookup will stop at an empty slot, then the check `Rows[H].getSignature() != S` won't cause the lookup to fail and return a nullptr (as it should), because the empty slot has a 0 in the signature field, and a pointer to the empty slot will be incorrectly returned. This patch fixes this by using the index field in the hash entry to check for empty slots: signature = 0 can match a valid hash but according to the spec the index for an occupied slot will always be non-zero. Differential Revision: https://reviews.llvm.org/D91670 (cherry picked from commit 314a0d7)
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‎llvm/lib/DebugInfo/DWARF/DWARFUnitIndex.cpp

Lines changed: 6 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -286,10 +286,14 @@ const DWARFUnitIndex::Entry *DWARFUnitIndex::getFromHash(uint64_t S) const {
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auto H = S & Mask;
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auto HP = ((S >> 32) & Mask) | 1;
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while (Rows[H].getSignature() != S && Rows[H].getSignature() != 0)
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// The spec says "while 0 is a valid hash value, the row index in a used slot
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// will always be non-zero". Loop until we find a match or an empty slot.
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while (Rows[H].getSignature() != S && Rows[H].Index != nullptr)
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H = (H + HP) & Mask;
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if (Rows[H].getSignature() != S)
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// If the slot is empty, we don't care whether the signature matches (it could
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// be zero and still match the zeros in the empty slot).
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if (Rows[H].Index == nullptr)
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return nullptr;
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return &Rows[H];

‎llvm/test/tools/llvm-symbolizer/split-dwarf-zero-signature-not-found.s

Lines changed: 118 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,118 @@
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## This test checks that looking up a zero hash in the .debug_cu_index hash
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## table works correctly when there's no CU with signature = 0.
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##
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## LLVM used to check just the signature bits to decide if the hash lookup ended
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## at a match or at an empty slot. This is wrong when signature = 0 because
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## empty slots have all zeros in the signature field too, and LLVM would return
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## the empty slot as a valid result.
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# REQUIRES: x86-registered-target
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# RUN: llvm-mc --filetype=obj --triple x86_64 %s -o %t --defsym MAIN=0
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# RUN: llvm-mc --filetype=obj --triple x86_64 %s -o %t.dwp
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# RUN: llvm-symbolizer --obj=%t --dwp=%t.dwp 0x0 | FileCheck %s
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## This expected output is very uninteresting, but it's better than a crash.
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# CHECK: ??:0:0
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.section .debug_abbrev,"",@progbits
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.byte 1 # Abbreviation Code
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.byte 17 # DW_TAG_compile_unit
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.byte 0 # DW_CHILDREN_no
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.ascii "\260B" # DW_AT_GNU_dwo_name
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.byte 8 # DW_FORM_string
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.ascii "\261B" # DW_AT_GNU_dwo_id
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.byte 7 # DW_FORM_data8
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.ascii "\263B" # DW_AT_GNU_addr_base
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.byte 23 # DW_FORM_sec_offset
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.byte 85 # DW_AT_ranges
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.byte 23 # DW_FORM_sec_offset
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.byte 0 # EOM(1)
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.byte 0 # EOM(2)
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.byte 0 # EOM(3)
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## Create two CUs, with dwo_ids 0 and 1 respectively.
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.ifdef MAIN
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.irpc I,01
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.data
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A\I:
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.long \I
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.text
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F\I:
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nop
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.section .debug_info,"",@progbits
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.Lcu_begin\I:
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.long .Ldebug_info_end\I-.Ldebug_info_start\I # Length of Unit
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.Ldebug_info_start\I:
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.short 4 # DWARF version number
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.long .debug_abbrev # Offset Into Abbrev. Section
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.byte 8 # Address Size (in bytes)
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.byte 1 # Abbrev [1] 0xb:0x25 DW_TAG_compile_unit
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.asciz "A.dwo" # DW_AT_GNU_dwo_name
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.quad \I # DW_AT_GNU_dwo_id
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.long .debug_addr # DW_AT_GNU_addr_base
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.long .Lranges\I # DW_AT_ranges
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.Ldebug_info_end\I:
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.section .debug_addr,"",@progbits
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.quad A\I
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.quad F\I
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.section .debug_ranges,"",@progbits
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.Lranges\I:
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.quad F\I
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.quad F\I+1
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.quad 0
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.quad 0
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.endr
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.else
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## Deliberately omit compile unit 0 in the DWP. We want to check the case where
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## a signature = 0 matches an empty hash slot in .debug_cu_index and the index
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## in the parallel table has to be checked.
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.section .debug_abbrev.dwo,"e",@progbits
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.Labbrev1:
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.byte 1 # Abbreviation Code
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.byte 17 # DW_TAG_compile_unit
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.byte 0 # DW_CHILDREN_no
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.byte 37 # DW_AT_producer
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.byte 8 # DW_FORM_string
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.byte 3 # DW_AT_name
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.byte 8 # DW_FORM_string
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.byte 0 # EOM(1)
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.byte 0 # EOM(2)
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.byte 0 # EOM(3)
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.Labbrev_end1:
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.section .debug_info.dwo,"e",@progbits
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.Lcu_begin1:
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.long .Ldebug_info_end1-.Ldebug_info_start1 # Length of Unit
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.Ldebug_info_start1:
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.short 4 # DWARF version number
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.long 0 # Offset Into Abbrev. Section
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.byte 8 # Address Size (in bytes)
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.byte 1 # Abbrev DW_TAG_compile_unit
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.asciz "Hand-written DWARF" # DW_AT_producer
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.byte '1', '.', 'c', 0 # DW_AT_name
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.Ldebug_info_end1:
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.section .debug_cu_index,"",@progbits
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.long 2 # DWARF version number
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.long 2 # Section count
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.long 1 # Unit count
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.long 8 # Slot count
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.quad 1, 0, 0, 0, 0, 0, 0, 0 # Hash table
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.long 1, 0, 0, 0, 0, 0, 0, 0 # Index table
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.long 1 # DW_SECT_INFO
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.long 3 # DW_SECT_ABBREV
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.long .Lcu_begin1-.debug_info.dwo
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.long .Labbrev1-.debug_abbrev.dwo
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.long .Ldebug_info_end1-.Lcu_begin1
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.long .Labbrev_end1-.Labbrev1
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.endif

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