[ELF2][MIPS] Support R_MIPS_CALL16 relocation

[atanasyan]

Summary

R_MIPS_CALL16 relocation provides the same result as R_MIPS_GOT16 relocation but does not need to check the result on overflow and should reject local symbol as a target.

The patch adds new argument to the relocateOne method. It is a pointer to SymbolBody. In case of local symbol it is nullptr.

[ruiu]

Did you pass a SymbolBody just for checking if its a global symbol? Do we need that? (We don't have to find all ABI violations -- if input is garbage, output may be garbage.)

[atanasyan]

In D14950#296022, @ruiu wrote:

Did you pass a SymbolBody just for checking if its a global symbol? Do we need that? (We don't have to find all ABI violations -- if input is garbage, output may be garbage.)

In this patch I pass a SymbolBody just for the checking. But later I will anyway need to recognize _gp_disp symbol and handle relocations against this symbol in a special way.

[ruiu]

Could you remove that error checking and the parameters from this patch and move that to the other patch in which you use that?

[atanasyan]

Removed checking for R_MIPS_CALL16 target symbol (local/global). Now we just calculate and apply the relocation.

[ruiu]

LGTM with this fix.

ELF/Target.cpp

116

Please remove this function from this patch.

[atanasyan]

ELF/Target.cpp

116

Hmm, if I remove this function I will always get R_MIPS_GOT16 relocation type into the relocateOne routine and so I will not be able to process R_MIPS_GOT16 and R_MIPS_CALL16 differently. In case of R_MIPS_CALL16 I should not check the result on overflow.

[ruiu]

LGTM

ELF/Target.cpp

116

I'm sorry, I overlooked "override" and thought that this is a dead function.

[atanasyan]

Closed by commit rL254092.

ELF/Target.cpp

public:
   bool relocNeedsPlt(uint32_t Type, const SymbolBody &S) const override;
   void relocateOne(uint8_t *Loc, uint8_t *BufEnd, uint32_t Type, uint64_t P,
                    uint64_t SA) const override;
 };
 
 template <class ELFT> class MipsTargetInfo final : public TargetInfo {
 public:
   MipsTargetInfo();
+  unsigned getGotRefReloc(unsigned Type) const override;
   void writeGotHeaderEntries(uint8_t *Buf) const override;
   void writeGotPltEntry(uint8_t *Buf, uint64_t Plt) const override;
   void writePltZeroEntry(uint8_t *Buf, uint64_t GotEntryAddr,
                          uint64_t PltEntryAddr) const override;
   void writePltEntry(uint8_t *Buf, uint64_t GotEntryAddr,
                      uint64_t PltEntryAddr, int32_t Index) const override;
   bool relocNeedsGot(uint32_t Type, const SymbolBody &S) const override;
   bool relocNeedsPlt(uint32_t Type, const SymbolBody &S) const override;

 
 template <class ELFT> MipsTargetInfo<ELFT>::MipsTargetInfo() {
   PageSize = 65536;
   GotRefReloc = R_MIPS_GOT16;
   GotHeaderEntriesNum = 2;
 }
 
 template <class ELFT>
+unsigned MipsTargetInfo<ELFT>::getGotRefReloc(unsigned Type) const {
+  return Type;
+}
+
+template <class ELFT>
 void MipsTargetInfo<ELFT>::writeGotHeaderEntries(uint8_t *Buf) const {
   typedef typename llvm::object::ELFFile<ELFT>::Elf_Off Elf_Off;
   auto *P = reinterpret_cast<Elf_Off *>(Buf);
   // Module pointer
   P[1] = ELFT::Is64Bits ? 0x8000000000000000 : 0x80000000;
 }
 
 template <class ELFT>
 void MipsTargetInfo<ELFT>::writeGotPltEntry(uint8_t *Buf, uint64_t Plt) const {}
 template <class ELFT>
 void MipsTargetInfo<ELFT>::writePltZeroEntry(uint8_t *Buf, uint64_t GotEntryAddr,
                                        uint64_t PltEntryAddr) const {}
 template <class ELFT>
 void MipsTargetInfo<ELFT>::writePltEntry(uint8_t *Buf, uint64_t GotEntryAddr,
                                          uint64_t PltEntryAddr, int32_t Index) const {}
 
 template <class ELFT>
 bool MipsTargetInfo<ELFT>::relocNeedsGot(uint32_t Type,
                                          const SymbolBody &S) const {
-  return Type == R_MIPS_GOT16;
+  return Type == R_MIPS_GOT16 || Type == R_MIPS_CALL16;
 }
 
 template <class ELFT>
 bool MipsTargetInfo<ELFT>::relocNeedsPlt(uint32_t Type,
                                          const SymbolBody &S) const {
   return false;
 }
 
 template <class ELFT>
 void MipsTargetInfo<ELFT>::relocateOne(uint8_t *Loc, uint8_t *BufEnd,
                                        uint32_t Type, uint64_t P,
                                        uint64_t SA) const {
   const endianness E = ELFT::TargetEndianness;
   switch (Type) {
   case R_MIPS_32:
     add32<E>(Loc, SA);
     break;
+  case R_MIPS_CALL16:
   case R_MIPS_GOT16: {
     int64_t V = SA - getMipsGpAddr<ELFT>();
-    if (!isInt<16>(V))
+    if (Type == R_MIPS_GOT16 && !isInt<16>(V))
       error("Relocation R_MIPS_GOT16 out of range");
     write32<E>(Loc, (read32<E>(Loc) & 0xffff0000) | (V & 0xffff));
     break;
   }
   default:
     error("unrecognized reloc " + Twine(Type));
   }
 }

test/ELF/mips-call16.s

+# Check R_MIPS_CALL16 relocation calculation.
+
+# RUN: llvm-mc -filetype=obj -triple=mips-unknown-linux %s -o %t.o
+# RUN: ld.lld %t.o -shared -o %t.exe
+# RUN: llvm-objdump -d %t.exe | FileCheck %s
+# RUN: llvm-readobj -mips-plt-got -symbols %t.exe \
+# RUN:   | FileCheck -check-prefix=GOT %s
+
+# REQUIRES: mips
+
+  .text
+  .globl  __start
+__start:
+  lw      $t0,%call16(g1)($gp)
+
+  .globl g1
+  .type  g1,@function
+g1:
+  nop
+
+# CHECK:      Disassembly of section .text:
+# CHECK-NEXT: __start:
+# CHECK-NEXT:      10000:   8f 88 80 18   lw   $8, -32744
+
+# GOT:      Name: g1
+# GOT-NEXT: Value: 0x[[ADDR:[0-9A-F]+]]
+
+# GOT:      Local entries [
+# GOT-NEXT: ]
+# GOT-NEXT: Global entries [
+# GOT-NEXT:   Entry {
+# GOT-NEXT:     Address:
+# GOT-NEXT:     Access: -32744
+# GOT-NEXT:     Initial: 0x[[ADDR]]
+# GOT-NEXT:     Value: 0x[[ADDR]]
+# GOT-NEXT:     Type: Function
+# GOT-NEXT:     Section: .text
+# GOT-NEXT:     Name: g1
+# GOT-NEXT:   }
+# GOT-NEXT: ]