[llvm] [DWARFVerifier] Fix verification of empty line tables (PR #81162)

[Felipe de Azevedo Piovezan via llvm-commits]

https://github.com/felipepiovezan updated https://github.com/llvm/llvm-project/pull/81162

>From a78820d5c848d20cc1f7503eb5f967c07548d78e Mon Sep 17 00:00:00 2001
From: Felipe de Azevedo Piovezan <fpiovezan at apple.com>
Date: Thu, 8 Feb 2024 09:17:39 -0800
Subject: [PATCH] [DWARFVerifier] Fix verification of empty line tables

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.

[1]: https://github.com/llvm/llvm-project/pull/77004
---
 llvm/lib/DebugInfo/DWARF/DWARFVerifier.cpp    | 13 ++---
 .../X86/verify_empty_debug_line_sequence.yaml | 55 +++++++++++++++++++
 2 files changed, 61 insertions(+), 7 deletions(-)
 create mode 100644 llvm/test/tools/llvm-dwarfdump/X86/verify_empty_debug_line_sequence.yaml

diff --git a/llvm/lib/DebugInfo/DWARF/DWARFVerifier.cpp b/llvm/lib/DebugInfo/DWARF/DWARFVerifier.cpp
index 2124ff835c572..b5235765c77b1 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 0000000000000..f1a2748115e75
--- /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