[ELF][MIPS] Calculate combined addend for R_MIPS_GOT16 against local symbol

[atanasyan]

Summary

R_MIPS_GOT16 relocation against local symbol requires index of a local GOT entry which contains page address corresponds to sum of the symbol address and addend. The addend in that case is calculated using addends from the R_MIPS_GOT16 and paired R_MIPS_LO16 relocations.

[ruiu]

This seems to be yet another oddity of the MIPS ABI, but the code LGTM.

ELF/InputSection.cpp

256

You can remove 0xffff, no? In SignExtend64<16>(X), only the least significant 16 bits in X matters, I think.

ELF/InputSection.cpp

 //===- InputSection.cpp ---------------------------------------------------===//
 //
 //                             The LLVM Linker
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 
 #include "InputSection.h"
 #include "Config.h"
 #include "Error.h"
 #include "InputFiles.h"
 #include "OutputSections.h"
 #include "Target.h"
 
+#include "llvm/Support/Endian.h"
+
 using namespace llvm;
 using namespace llvm::ELF;
 using namespace llvm::object;
+using namespace llvm::support::endian;
 
 using namespace lld;
 using namespace lld::elf2;
 
 template <class ELFT>
 InputSectionBase<ELFT>::InputSectionBase(ObjectFile<ELFT> *File,
                                          const Elf_Shdr *Header,
                                          Kind SectionKind)
if (SymIndex < SymTab->sh_info)
       fatal("Relocation against local symbols is not supported yet");
 
     SymbolBody *Body = this->File->getSymbolBody(SymIndex)->repl();
     P->r_offset = RelocatedSection->getOffset(Rel.r_offset);
     P->setSymbolAndType(Body->DynsymIndex, Type, Config->Mips64EL);
   }
 }
 
+static uint32_t getMipsPairedRelocType(uint32_t Type) {
+  if (Config->EMachine != EM_MIPS)
+    return R_MIPS_NONE;
+  switch (Type) {
+  case R_MIPS_HI16:
+    return R_MIPS_LO16;
+  case R_MIPS_PCHI16:
+    return R_MIPS_PCLO16;
+  case R_MICROMIPS_HI16:
+    return R_MICROMIPS_LO16;
+  default:
+    return R_MIPS_NONE;
+  }
+}
+
 template <class ELFT>
 template <bool isRela>
 uint8_t *
 InputSectionBase<ELFT>::findMipsPairedReloc(uint8_t *Buf, uint32_t SymIndex,
                                             uint32_t Type,
                                             RelIteratorRange<isRela> Rels) {
   // Some MIPS relocations use addend calculated from addend of the relocation
   // itself and addend of paired relocation. ABI requires to compute such
   // combined addend in case of REL relocation record format only.
   // See p. 4-17 at ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf
-  if (isRela || Config->EMachine != EM_MIPS)
-    return nullptr;
-  if (Type == R_MIPS_HI16)
-    Type = R_MIPS_LO16;
-  else if (Type == R_MIPS_PCHI16)
-    Type = R_MIPS_PCLO16;
-  else if (Type == R_MICROMIPS_HI16)
-    Type = R_MICROMIPS_LO16;
-  else
+  if (isRela || Type == R_MIPS_NONE)
     return nullptr;
   for (const auto &RI : Rels) {
     if (RI.getType(Config->Mips64EL) != Type)
       continue;
     if (RI.getSymbol(Config->Mips64EL) != SymIndex)
       continue;
     uintX_t Offset = getOffset(RI.r_offset);
     if (Offset == (uintX_t)-1)
if (Target->canRelaxTls(Type, Body)) {
       continue;
     }
 
     // Handle relocations for local symbols -- they never get
     // resolved so we don't allocate a SymbolBody.
     uintX_t A = getAddend<ELFT>(RI);
     if (!Body) {
       uintX_t SymVA = getLocalRelTarget(*File, RI, A);
+      uint8_t *PairedLoc = nullptr;
       if (Config->EMachine == EM_MIPS) {
         if (Type == R_MIPS_GPREL16 || Type == R_MIPS_GPREL32)
           // We need to adjust SymVA value in case of R_MIPS_GPREL16/32
           // relocations because they use the following expression to calculate
           // the relocation's result for local symbol: S + A + GP0 - G.
           SymVA += File->getMipsGp0();
-        else if (Type == R_MIPS_GOT16)
+        else if (Type == R_MIPS_GOT16) {
           // R_MIPS_GOT16 relocation against local symbol requires index of
           // a local GOT entry which contains page address corresponds
-          // to the symbol address.
-          SymVA = Out<ELFT>::Got->getMipsLocalPageAddr(SymVA);
+          // to sum of the symbol address and addend. The addend in that case
+          // is calculated using addends from R_MIPS_GOT16 and paired
+          // R_MIPS_LO16 relocations.
+          const endianness E = ELFT::TargetEndianness;
+          uint8_t *LowLoc =
+              findMipsPairedReloc(Buf, SymIndex, R_MIPS_LO16, NextRelocs);
+          uint64_t AHL = (read32<E>(BufLoc) & 0xffff) << 16;
+          if (LowLoc)
+            AHL += SignExtend64<16>(read32<E>(LowLoc) & 0xffff);
+          SymVA = Out<ELFT>::Got->getMipsLocalPageAddr(SymVA + AHL);
+        } else
+          PairedLoc = findMipsPairedReloc(
+              Buf, SymIndex, getMipsPairedRelocType(Type), NextRelocs);
       }
-      Target->relocateOne(BufLoc, BufEnd, Type, AddrLoc, SymVA, 0,
-                          findMipsPairedReloc(Buf, SymIndex, Type, NextRelocs));
+      Target->relocateOne(BufLoc, BufEnd, Type, AddrLoc, SymVA, 0, PairedLoc);
       continue;
     }
 
     if (Target->isTlsGlobalDynamicRel(Type) &&
         !Target->canRelaxTls(Type, Body)) {
       Target->relocateOne(BufLoc, BufEnd, Type, AddrLoc,
                           Out<ELFT>::Got->getGlobalDynAddr(*Body) +
                               getAddend<ELFT>(RI));
if (Target->needsPlt<ELFT>(Type, *Body)) {
         SymVA = getMipsGpAddr<ELFT>() - AddrLoc;
       else if (Type == R_MIPS_LO16 && Body == Config->MipsGpDisp)
         SymVA = getMipsGpAddr<ELFT>() - AddrLoc + 4;
       else if (Body == Config->MipsLocalGp)
         SymVA = getMipsGpAddr<ELFT>();
     }
     uintX_t Size = Body->getSize<ELFT>();
     Target->relocateOne(BufLoc, BufEnd, Type, AddrLoc, SymVA + A, Size + A,
-                        findMipsPairedReloc(Buf, SymIndex, Type, NextRelocs));
+                        findMipsPairedReloc(Buf, SymIndex,
+                                            getMipsPairedRelocType(Type),
+                                            NextRelocs));
   }
 }
 
 template <class ELFT> void InputSection<ELFT>::writeTo(uint8_t *Buf) {
   if (this->Header->sh_type == SHT_NOBITS)
     return;
   // Copy section contents from source object file to output file.
   ArrayRef<uint8_t> Data = this->getSectionData();

test/ELF/mips-got16.s

 # Check R_MIPS_GOT16 relocation calculation.
 
 # RUN: llvm-mc -filetype=obj -triple=mips-unknown-linux %s -o %t.o
 # RUN: ld.lld %t.o -shared -o %t.so
 # RUN: llvm-objdump -d -t %t.so | FileCheck %s
 # RUN: llvm-readobj -r -mips-plt-got %t.so | FileCheck -check-prefix=GOT %s
 
 # REQUIRES: mips
 
 # CHECK:      Disassembly of section .text:
 # CHECK-NEXT: __start:
 # CHECK-NEXT:    10000:       8f 88 80 18     lw      $8, -32744($gp)
-# CHECK-NEXT:    10004:       21 08 00 1c     addi    $8, $8, 28
+# CHECK-NEXT:    10004:       21 08 00 2c     addi    $8, $8, 44
 # CHECK-NEXT:    10008:       8f 88 80 1c     lw      $8, -32740($gp)
-# CHECK-NEXT:    1000c:       21 08 00 00     addi    $8, $8, 0
+# CHECK-NEXT:    1000c:       21 08 90 00     addi    $8, $8, -28672
 # CHECK-NEXT:    10010:       8f 88 80 20     lw      $8, -32736($gp)
-# CHECK-NEXT:    10014:       21 08 00 04     addi    $8, $8, 4
-# CHECK-NEXT:    10018:       8f 88 80 24     lw      $8, -32732($gp)
+# CHECK-NEXT:    10014:       21 08 90 04     addi    $8, $8, -28668
+# CHECK-NEXT:    10018:       8f 88 80 20     lw      $8, -32736($gp)
+# CHECK-NEXT:    1001c:       21 08 10 04     addi    $8, $8, 4100
+# CHECK-NEXT:    10020:       8f 88 80 24     lw      $8, -32732($gp)
+# CHECK-NEXT:    10024:       21 08 10 08     addi    $8, $8, 4104
+# CHECK-NEXT:    10028:       8f 88 80 2c     lw      $8, -32724($gp)
 #
 # CHECK: SYMBOL TABLE:
-# CHECK: 0001001c         .text           00000000 $LC0
-# CHECK: 00030000         .data           00000000 $LC1
-# CHECK: 00030004         .data           00000000 .hidden bar
+# CHECK: 0001002c         .text           00000000 $LC0
+# CHECK: 00039000         .data           00000000 $LC1
+# CHECK: 00051008         .data           00000000 .hidden bar
 # CHECK: 00000000         *UND*           00000000 foo
 
 # GOT:      Relocations [
 # GOT-NEXT: ]
 
 # GOT:      Primary GOT {
 # GOT-NEXT:   Canonical gp value: 0x27FF0
 # GOT-NEXT:   Reserved entries [

 # GOT-NEXT:       Purpose: Module pointer (GNU extension)
 # GOT-NEXT:     }
 # GOT-NEXT:   ]
 # GOT-NEXT:   Local entries [
 # GOT-NEXT:     Entry {
 # GOT-NEXT:       Address: 0x20008
 # GOT-NEXT:       Access: -32744
 # GOT-NEXT:       Initial: 0x10000
+#                          ^-- (0x1002c + 0x8000) & ~0xffff
 # GOT-NEXT:     }
 # GOT-NEXT:     Entry {
 # GOT-NEXT:       Address: 0x2000C
 # GOT-NEXT:       Access: -32740
-# GOT-NEXT:       Initial: 0x30000
+# GOT-NEXT:       Initial: 0x40000
+#                          ^-- (0x39000 + 0x8000) & ~0xffff
 # GOT-NEXT:     }
 # GOT-NEXT:     Entry {
 # GOT-NEXT:       Address: 0x20010
 # GOT-NEXT:       Access: -32736
-# GOT-NEXT:       Initial: 0x30004
+# GOT-NEXT:       Initial: 0x50000
+#                          ^-- (0x39000 + 0x10004 + 0x8000) & ~0xffff
+#                          ^-- (0x39000 + 0x18004 + 0x8000) & ~0xffff
 # GOT-NEXT:     }
-# GOT-NEXT:   ]
-# GOT-NEXT:   Global entries [
 # GOT-NEXT:     Entry {
 # GOT-NEXT:       Address: 0x20014
 # GOT-NEXT:       Access: -32732
+# GOT-NEXT:       Initial: 0x51008
+#                          ^-- 'bar' address
+# GOT-NEXT:     }
+# GOT-NEXT:     Entry {
+# GOT-NEXT:       Address: 0x20018
+# GOT-NEXT:       Access: -32728
+# GOT-NEXT:       Initial: 0x0
+#                          ^-- redundant unused entry
+# GOT-NEXT:     }
+# GOT-NEXT:   ]
+# GOT-NEXT:   Global entries [
+# GOT-NEXT:     Entry {
+# GOT-NEXT:       Address: 0x2001C
+# GOT-NEXT:       Access: -32724
 # GOT-NEXT:       Initial: 0x0
 # GOT-NEXT:       Value: 0x0
 # GOT-NEXT:       Type: None
 # GOT-NEXT:       Section: Undefined
 # GOT-NEXT:       Name: foo@
 # GOT-NEXT:     }
 # GOT-NEXT:   ]
 # GOT-NEXT:   Number of TLS and multi-GOT entries: 0
 # GOT-NEXT: }
 
   .text
   .globl  __start
 __start:
   lw      $t0,%got($LC0)($gp)
   addi    $t0,$t0,%lo($LC0)
   lw      $t0,%got($LC1)($gp)
   addi    $t0,$t0,%lo($LC1)
+  lw      $t0,%got($LC1+0x10004)($gp)
+  addi    $t0,$t0,%lo($LC1+0x10004)
+  lw      $t0,%got($LC1+0x18004)($gp)
+  addi    $t0,$t0,%lo($LC1+0x18004)
   lw      $t0,%got(bar)($gp)
   addi    $t0,$t0,%lo(bar)
   lw      $t0,%got(foo)($gp)
 $LC0:
   nop
 
   .data
+  .space 0x9000
 $LC1:
   .word 0
+  .space 0x18000
+  .word 0
 .global bar
 .hidden bar
 bar:
   .word 0