[llvm-readobj/elf] - Fix the PREL31 relocation computation used for dumping arm32 unwind info (-u).

[grimar]

Summary

This is a part of https://bugs.llvm.org/show_bug.cgi?id=47581.

We have the following computation:

(1) uint64_t Location = Address & 0x7fffffff;
(2) if (Location & 0x04000000)
(3)   Location |= (uint64_t) ~0x7fffffff;
(4) return Location + Place;

At line 2 there is a mistype. The constant should be 0x40000000,
not 0x04000000, because the intention here is to sign extend the Location,
which is a 31 bit signed value.

[psmith]

LGTM. Agree that it should be bit31 that is sign extended. Apologies for not catching this last time.

[grimar]

In D88407#2297935, @psmith wrote:

LGTM. Agree that it should be bit31 that is sign extended. Apologies for not catching this last time.

No worries. This and few other issues are/were mentioned in comments on the bug page. They are about 6 years old,
I guess nobody tried to heavily use "llvm-readobj -u" before. I am going to try to fix all of them. Thanks for quick review!

[MaskRay]

LGTM

In D88407#2298019, @grimar wrote:

In D88407#2297935, @psmith wrote:

LGTM. Agree that it should be bit31 that is sign extended. Apologies for not catching this last time.

No worries. This and few other issues are/were mentioned in comments on the bug page. They are about 6 years old,
I guess nobody tried to heavily use "llvm-readobj -u" before. I am going to try to fix all of them. Thanks for quick review!

Thanks for fixing this! I tend to agree with nobody tried to heavily use "llvm-readobj -u" before (not nobody but definitely not many (Android folks noticed a crash last year))..

[grimar]

In D88407#2298859, @MaskRay wrote:

I tend to agree with nobody tried to heavily use "llvm-readobj -u" before (not nobody but definitely not many (Android folks noticed a crash last year))..

There is one more issue that exist in this area: currently llvm-readelf produce the same LLVM-style output as llvm-readobj does.
We probably want to implement the proper GNU style output.

llvm/test/tools/llvm-readobj/ELF/ARM/unwind-non-relocatable.test

 # UNWIND-NEXT:           0xB0      ; finish
 # UNWIND-NEXT:           0xB0      ; finish
 # UNWIND-NEXT:         ]
 # UNWIND-NEXT:       }
 # UNWIND-NEXT:       Entry {
 # UNWIND-NEXT:         FunctionAddress: 0x24C
 # UNWIND-NEXT:         Model: CantUnwind
 # UNWIND-NEXT:       }
+# UNWIND-NEXT:       Entry {
+# UNWIND-NEXT:         FunctionAddress: 0x4000026B
+# UNWIND-NEXT:         FunctionName: func4
+# UNWIND-NEXT:         Model: Compact (Inline)
+# UNWIND-NEXT:         PersonalityIndex: 0
+# UNWIND-NEXT:         Opcodes [
+# UNWIND-NEXT:           0xB0      ; finish
+# UNWIND-NEXT:           0xB0      ; finish
+# UNWIND-NEXT:           0xB0      ; finish
+# UNWIND-NEXT:         ]
+# UNWIND-NEXT:       }
 # UNWIND-NEXT:     ]
 # UNWIND-NEXT:   }
 # UNWIND-NEXT: }
 
 --- !ELF
 FileHeader:
   Class:   ELFCLASS32
   Data:    ELFDATA2LSB
## Address of .ARM.exidx (0x24C) + entry offset (8) + 0x7fffffe0 (31 bit) == 0x234 (func2).
       - Offset: 0x7FFFFFE0
         Value:  0x809B8480 ## arbitrary opcodes.
 ## Address of .ARM.exidx (0x24C) + entry offset (16) + 0x7fffffec (31 bit) == 0x248 (func2).
       - Offset: 0x7FFFFFEC
         Value:  0x80B0B0B0 ## arbitrary opcodes.
 ## Address of .ARM.exidx (0x24C) + entry offset (24) + 0x7fffffe8 (31 bit) == 0x24C.
       - Offset: 0x7FFFFFE8
         Value:  EXIDX_CANTUNWIND
+## Address of .ARM.exidx (0x24C) + entry offset (32) + 0x3FFFFFFF (31 bit) == 0x4000026b.
+      - Offset: 0x3FFFFFFF
+        Value:  0x80B0B0B0 ## arbitrary opcodes.
 Symbols:
   - Name:    func1
     Type:    STT_FUNC
     Section: .text
     Value:   0x230
   - Name:    func2
     Type:    STT_FUNC
     Section: .text
     Value:   0x234
   - Name:    func3
     Type:    STT_FUNC
     Section: .text
     Value:   0x248
+  - Name:    func4
+    Type:    STT_FUNC
+    Section: .text
+    Value:   0x4000026b

llvm/tools/llvm-readobj/ARMEHABIPrinter.h

class PrinterContext {
   StringRef FileName;
   const Elf_Shdr *Symtab;
   ArrayRef<Elf_Word> ShndxTable;
 
   static const size_t IndexTableEntrySize;
 
   static uint64_t PREL31(uint32_t Address, uint32_t Place) {
     uint64_t Location = Address & 0x7fffffff;
-    if (Location & 0x04000000)
+    if (Location & 0x40000000)
       Location |= (uint64_t) ~0x7fffffff;
     return Location + Place;
   }
 
   ErrorOr<StringRef> FunctionAtAddress(uint64_t Address,
                                        Optional<unsigned> SectionIndex) const;
   const Elf_Shdr *FindExceptionTable(unsigned IndexTableIndex,
                                      off_t IndexTableOffset) const;