[DWARFVerifier] Fix verification of empty line tables

[felipepiovezan]

A line table whose sole entry is an end sequence should not have the entry's file index verified, as that value corresponds to the initial value of the state machine, not to a real file index. In DWARF 5, this is particularly problematic as it uses 0-based indexing, and the state machine specifies a starting index of 1; in other words, you'd need to have two files before such index became legal "by default".

A previous attempt to fix this problem was done 1, but it was too specific in its condition, and did not capture all possible cases where this issue can happen.

[llvmbot]

@llvm/pr-subscribers-debuginfo

Author: Felipe de Azevedo Piovezan (felipepiovezan)

Changes

A line table whose sole entry is an entry sequence should not have the entry's file index verified, as that value corresponds to the initial value of the state machine, not to a real file index. In DWARF 5, this is particularly problematic as it uses 0-based indexing, and the state machine specifies a starting index of 1; in other words, you'd need to have two files before such index became legal "by default".

A previous attempt to fix this problem was done 1, but it was too specific in its condition, and did not capture all possible cases where this issue can happen.

---

Full diff: https://github.com/llvm/llvm-project/pull/81162.diff

2 Files Affected:

• (modified) llvm/lib/DebugInfo/DWARF/DWARFVerifier.cpp (+6-7)
• (added) llvm/test/tools/llvm-dwarfdump/X86/verify_empty_debug_line_sequence.yaml (+55)

diff --git a/llvm/lib/DebugInfo/DWARF/DWARFVerifier.cpp b/llvm/lib/DebugInfo/DWARF/DWARFVerifier.cpp
index 2124ff835c5727..b5235765c77b14 100644
--- a/llvm/lib/DebugInfo/DWARF/DWARFVerifier.cpp
+++ b/llvm/lib/DebugInfo/DWARF/DWARFVerifier.cpp
@@ -1025,6 +1025,11 @@ void DWARFVerifier::verifyDebugLineRows() {
       FileIndex++;
     }
 
+    // Nothing to verify in a line table with a single row containing the end
+    // sequence.
+    if (LineTable->Rows.size() == 1 && LineTable->Rows.front().EndSequence)
+      continue;
+
     // Verify rows.
     uint64_t PrevAddress = 0;
     uint32_t RowIndex = 0;
@@ -1048,13 +1053,7 @@ void DWARFVerifier::verifyDebugLineRows() {
             });
       }
 
-      // If the prologue contains no file names and the line table has only one
-      // row, do not verify the file index, this is a line table of an empty
-      // file with an end_sequence, but the DWARF standard sets the file number
-      // to 1 by default, otherwise verify file index.
-      if ((LineTable->Prologue.FileNames.size() ||
-           LineTable->Rows.size() != 1) &&
-          !LineTable->hasFileAtIndex(Row.File)) {
+      if (!LineTable->hasFileAtIndex(Row.File)) {
         ++NumDebugLineErrors;
         ErrorCategory.Report("Invalid file index in debug_line", [&]() {
           error() << ".debug_line["
diff --git a/llvm/test/tools/llvm-dwarfdump/X86/verify_empty_debug_line_sequence.yaml b/llvm/test/tools/llvm-dwarfdump/X86/verify_empty_debug_line_sequence.yaml
new file mode 100644
index 00000000000000..f1a2748115e75b
--- /dev/null
+++ b/llvm/test/tools/llvm-dwarfdump/X86/verify_empty_debug_line_sequence.yaml
@@ -0,0 +1,55 @@
+# RUN: yaml2obj %s -o %t.o
+# RUN: llvm-dwarfdump -debug-line -verify %t.o | FileCheck %s
+
+# CHECK: Verifying .debug_line...
+# CHECK: No errors
+
+# In a line table like the one below, with no rows (other than the
+# end_sequence), we should never verify the file index because the state
+# machine starts initializes the file index to 1, which is invalid in DWARF 5
+# due to its 0-based indexing.
+
+# file_names[  0]:
+#            name: "/home/umb/tests_2018/106_rnglists2"
+#       dir_index: 0
+# Address            Line   Column File   ISA Discriminator OpIndex Flags
+# ------------------ ------ ------ ------ --- ------------- ------- -------------
+# 0x0000000000000000      1      0      1   0             0       0  is_stmt end_sequence
+
+
+--- !ELF
+FileHeader:
+  Class: ELFCLASS64
+  Data:  ELFDATA2LSB
+  Type:  ET_EXEC
+DWARF:
+  debug_abbrev:
+    - ID:              0
+      Table:
+        - Code:            0x1
+          Tag:             DW_TAG_compile_unit
+          Children:        DW_CHILDREN_no
+          Attributes:
+            - Attribute:       DW_AT_stmt_list
+              Form:            DW_FORM_sec_offset
+  debug_info:
+    - Length:          0xd
+      Version:         5
+      UnitType:        DW_UT_compile
+      AbbrevTableID:   0
+      AbbrOffset:      0x0
+      AddrSize:        8
+      Entries:
+        - AbbrCode:        0x1
+          Values:
+            - Value:           0x0
+Sections:
+  - Name:            .debug_line
+    Type:            SHT_PROGBITS
+    AddressAlign:    0x1
+    Content:         300000000500080025000000010101fb0e0d00010101010000000100000101011f010000000002011f020b010000000000000101
+  - Name:            .debug_line_str
+    Type:            SHT_PROGBITS
+    Flags:           [ SHF_MERGE, SHF_STRINGS ]
+    AddressAlign:    0x1
+    Content:         2F686F6D652F756D622F74657374735F323031382F3130365F726E676C697374733200746573742E63707000

[rastogishubham]

LGTM!

[felipepiovezan]

Fixed a bad typo in the commit message: entry sequence -> end sequence