[PATCH] D74612: [DebugInfo] Read CIE pointer as a relocatable value.

[Igor Kudrin via Phabricator via llvm-commits]

ikudrin created this revision.
ikudrin added reviewers: probinson, dblaikie, jhenderson, aprantl.
ikudrin added projects: LLVM, debug-info.
Herald added a subscriber: hiraditya.

The CIE pointer field of an FDE record contains an offset to a corresponding CIE record. In object files, this value comes with relocation because the value has to be fixed when a linker combines the final section from multiple sources. In most object files there is only one CIE record at offset 0 of the .debug_frame section, so reading a relocated or a raw value makes no differences. However, in partially linked object files there are multiple CIE records and the relocations should be applied to recover the right offset value.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D74612

Files:
  llvm/lib/DebugInfo/DWARF/DWARFDebugFrame.cpp
  llvm/test/DebugInfo/debug-frame-cieid-reloc.test


Index: llvm/test/DebugInfo/debug-frame-cieid-reloc.test
===================================================================
--- /dev/null
+++ llvm/test/DebugInfo/debug-frame-cieid-reloc.test
@@ -0,0 +1,45 @@
+# RUN: yaml2obj %s -o - | \
+# RUN:   llvm-dwarfdump -debug-frame - | \
+# RUN:   FileCheck %s
+
+## This checks that we resolve relocations when reading CIE pointers.
+## The second FDE references the second CIE. The value for the CIE pointer
+## field in the raw section data is 0, thus, to recover the real reference
+## it is required to read the addend from the corresponding RELA relocation.
+
+# CHECK: .debug_frame contents:
+# CHECK: 00000000 0000000c ffffffff CIE
+# CHECK: 00000010 00000014 00000000 FDE cie=00000000 pc=00000000...00000001
+# CHECK: 00000028 0000000c ffffffff CIE
+# CHECK: 00000038 00000014 00000028 FDE cie=00000028 pc=00000010...00000011
+
+--- !ELF
+FileHeader:
+  Class:           ELFCLASS64
+  Data:            ELFDATA2LSB
+  Type:            ET_REL
+  Machine:         EM_X86_64
+Sections:
+  - Name:            .debug_frame
+    Type:            SHT_PROGBITS
+    AddressAlign:    0x0000000000000001
+    Content:         0C000000FFFFFFFF02000178100000001400000000000000000000000000000001000000000000000C000000FFFFFFFF0200017810000000140000000000000010000000000000000100000000000000
+  - Name:            .rela.debug_frame
+    Type:            SHT_RELA
+    Link:            .symtab
+    AddressAlign:    0x0000000000000008
+    EntSize:         0x0000000000000018
+    Info:            .debug_frame
+    Relocations:
+      - Offset:          0x0000000000000014
+        Symbol:          .debug_frame
+        Type:            R_X86_64_32
+      - Offset:          0x000000000000003C
+        Symbol:          .debug_frame
+        Type:            R_X86_64_32
+        Addend:          40
+Symbols:
+  - Name:            .debug_frame
+    Type:            STT_SECTION
+    Section:         .debug_frame
+...
Index: llvm/lib/DebugInfo/DWARF/DWARFDebugFrame.cpp
===================================================================
--- llvm/lib/DebugInfo/DWARF/DWARFDebugFrame.cpp
+++ llvm/lib/DebugInfo/DWARF/DWARFDebugFrame.cpp
@@ -380,7 +380,7 @@
     uint64_t EndStructureOffset = Offset + Length;
 
     // The Id field's size depends on the DWARF format
-    Id = Data.getUnsigned(&Offset, (IsDWARF64 && !IsEH) ? 8 : 4);
+    Id = Data.getRelocatedValue((IsDWARF64 && !IsEH) ? 8 : 4, &Offset);
     bool IsCIE =
         ((IsDWARF64 && Id == DW64_CIE_ID) || Id == DW_CIE_ID || (IsEH && !Id));
 


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