[lld] r312727 - [ELF] Always write non-immediate bits for AArch64 branch instruction.
Peter Smith via llvm-commits
llvm-commits at lists.llvm.org
Thu Sep 7 09:29:52 PDT 2017
Author: psmith
Date: Thu Sep 7 09:29:52 2017
New Revision: 312727
URL: http://llvm.org/viewvc/llvm-project?rev=312727&view=rev
Log:
[ELF] Always write non-immediate bits for AArch64 branch instruction.
To support errata patching on AArch64 we need to be able to overwrite
an arbitrary instruction with a branch. For AArch64 it is sufficient to
always write all the bits of the branch instruction and not just the
immediate field. This is safe as the non-immediate bits of the branch
instruction are always the same.
Differential Revision: https://reviews.llvm.org/D36745
Added:
lld/trunk/test/ELF/relocation-b-aarch64.test
Modified:
lld/trunk/ELF/Arch/AArch64.cpp
Modified: lld/trunk/ELF/Arch/AArch64.cpp
URL: http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/Arch/AArch64.cpp?rev=312727&r1=312726&r2=312727&view=diff
==============================================================================
--- lld/trunk/ELF/Arch/AArch64.cpp (original)
+++ lld/trunk/ELF/Arch/AArch64.cpp Thu Sep 7 09:29:52 2017
@@ -232,8 +232,17 @@ void AArch64::relocateOne(uint8_t *Loc,
checkInt<21>(Loc, Val, Type);
write32AArch64Addr(Loc, Val);
break;
- case R_AARCH64_CALL26:
case R_AARCH64_JUMP26:
+ // Normally we would just write the bits of the immediate field, however
+ // when patching instructions for the cpu errata fix -fix-cortex-a53-843419
+ // we want to replace a non-branch instruction with a branch immediate
+ // instruction. By writing all the bits of the instruction including the
+ // opcode and the immediate (0 001 | 01 imm26) we can do this
+ // transformation by placing a R_AARCH64_JUMP26 relocation at the offset of
+ // the instruction we want to patch.
+ write32le(Loc, 0x14000000);
+ LLVM_FALLTHROUGH;
+ case R_AARCH64_CALL26:
checkInt<28>(Loc, Val, Type);
or32le(Loc, (Val & 0x0FFFFFFC) >> 2);
break;
Added: lld/trunk/test/ELF/relocation-b-aarch64.test
URL: http://llvm.org/viewvc/llvm-project/lld/trunk/test/ELF/relocation-b-aarch64.test?rev=312727&view=auto
==============================================================================
--- lld/trunk/test/ELF/relocation-b-aarch64.test (added)
+++ lld/trunk/test/ELF/relocation-b-aarch64.test Thu Sep 7 09:29:52 2017
@@ -0,0 +1,48 @@
+# REQUIRES: aarch64
+
+# RUN: yaml2obj %s -o %t.o
+# RUN: ld.lld %t.o -o %t.out
+# RUN: llvm-objdump -d -triple=aarch64-none-linux %t.out | FileCheck %s
+
+# Check that the R_AARCH64_JUMP26 writes the branch opcode as well as the
+# immediate. We use this property to overwrite instructions with a branch.
+
+# CHECK: Disassembly of section .text:
+# CHECK-NEXT: foo:
+# CHECK-NEXT: 20000: 01 00 00 14 b #4
+# CHECK: bar:
+# CHECK-NEXT: 20004: ff ff ff 17 b #-4
+
+!ELF
+FileHeader:
+ Class: ELFCLASS64
+ Data: ELFDATA2LSB
+ Type: ET_REL
+ Machine: EM_AARCH64
+Sections:
+ - Type: SHT_PROGBITS
+ Name: .text
+ Flags: [ SHF_ALLOC, SHF_EXECINSTR ]
+ Content: "0000000000000000"
+ - Type: SHT_RELA
+ Name: .rela.text
+ Link: .symtab
+ Info: .text
+ Relocations:
+ - Offset: 0
+ Symbol: bar
+ Type: R_AARCH64_JUMP26
+ - Offset: 4
+ Symbol: foo
+ Type: R_AARCH64_JUMP26
+
+Symbols:
+ Local:
+ - Type: STT_FUNC
+ Section: .text
+ Name: foo
+ Value: 0
+ - Type: STT_FUNC
+ Section: .text
+ Name: bar
+ Value: 4
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