[DebugInfo] Read CIE pointer as a relocatable value.

[ikudrin]

Summary

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 difference. However, in partially linked object files there are multiple CIE records and the relocations should be applied to recover the right offset value.

[jhenderson]

llvm/test/DebugInfo/debug-frame-cieid-reloc.test

1 ↗

(On Diff #244655)

As this is testing the CIE_pointer field of FDEs, it's porbably best to rename the test to debug-frame-cie-pointer-reloc.test.

7 ↗

(On Diff #244655)

Add a comma after "reference"

18–21 ↗

(On Diff #244655)

I'm guessing you used obj2yaml to generate this file? It generates excessive whitespace between keys and values, which I find can make it harder to read, and would prefer be removed to make something like this:

FileHeader:
  Class:   ELFCLASS64
  Data:    ELFDATA2LSB
  Type:    ET_REL
  Machine: EM_X86_64

Same goes further below.

25 ↗

(On Diff #244655)

No need for the AddressAlign field. Delete it.

29–31 ↗

(On Diff #244655)

No need for these three fields.

40 ↗

(On Diff #244655)

Does Addend allow a hex number? If so, I feel like 0x28 would be clearer, as it matches the value in the output.

[ikudrin]

Addressed all the suggestions above. Thanks, @jhenderson!

[jhenderson]

LGTM.

llvm/lib/DebugInfo/DWARF/DWARFDebugFrame.cpp

while (Data.isValidOffset(Offset)) {
     // At this point, Offset points to the next field after Length.
     // Length is the structure size excluding itself. Compute an offset one
     // past the end of the structure (needed to know how many instructions to
     // read).
     uint64_t StartStructureOffset = Offset;
     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));
 
     if (IsCIE) {
       uint8_t Version = Data.getU8(&Offset);
       const char *Augmentation = Data.getCStr(&Offset);
       StringRef AugmentationString(Augmentation ? Augmentation : "");
       uint8_t AddressSize = Version < 4 ? Data.getAddressSize() :

llvm/test/DebugInfo/debug-frame-cie-pointer-reloc.test

+# 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
+    Content: 0C000000FFFFFFFF02000178100000001400000000000000000000000000000001000000000000000C000000FFFFFFFF0200017810000000140000000000000010000000000000000100000000000000
+  - Name:    .rela.debug_frame
+    Type:    SHT_RELA
+    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: 0x28
+Symbols:
+  - Name:    .debug_frame
+    Type:    STT_SECTION
+    Section: .debug_frame
+...