[LLD][ARM] Accept first SHT_ARM_ATTRIBUTES section

[peter.smith]

Summary

The eglibc C library, as used by Ubuntu 14.04, and similar Debian bases systems, requires the presence of an SHT_ARM_ATTRIBUTES section in for the purposes of checking hard/soft float compatibility when dlopen() is used. Unfortunately when the section is not present dlopen() fails with a generic could not find file message.

This change makes lld keep the first .ARM.attributes section that it encounters and propagates it to the output. This is not a complete SHT_ARM_ATTRIBUTES implementation, that would involve reading the contents of the section and joining each individual attribute. It should suffice for a homogenous build all libraries and executables on the same system with a compatible set of command line options.

My understanding is that later versions of glibc, which eglibc was merged into a couple of years ago do not have this check. However Trusty (14.04) and Debian is common on the consumer devices that people do ARM development on such as Raspberry Pis and Chromebooks so I think it is worth making this change to get dlopen() to work.

[ruiu]

LGTM

ELF/InputFiles.cpp

344

So you are using In<ELFT>::ARMAttributes as a boolean flag rather than an InputSection object in the sense that the variable is used only to check if we have already created a section instance for ARMAttribute. It doesn't feel like the right approach to me, but honestly I cannot think of a better way. So, LGTM, but do you think any other ways?

[peter.smith]

Thanks for the review. My first idea was to put a flag in Config, but that didn't seem right either. If we were wanting to merge the build attributes sections we'd probably want to do so via a SyntheticSection so I thought it would be easier to start that way.

I'll commit tomorrow as I need to leave the office in a few minutes.

ELF/InputFiles.cpp

 //===- InputFiles.cpp -----------------------------------------------------===//
 //
 //                             The LLVM Linker
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 
 #include "InputFiles.h"
 #include "Driver.h"
 #include "Error.h"
 #include "InputSection.h"
 #include "LinkerScript.h"
 #include "SymbolTable.h"
 #include "Symbols.h"
+#include "SyntheticSections.h"
 #include "lld/Support/Memory.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/Bitcode/BitcodeReader.h"
 #include "llvm/CodeGen/Analysis.h"
 #include "llvm/DebugInfo/DWARF/DWARFContext.h"
 #include "llvm/IR/LLVMContext.h"
 #include "llvm/IR/Module.h"
 #include "llvm/LTO/LTO.h"

 InputSectionBase<ELFT> *
 elf::ObjectFile<ELFT>::createInputSection(const Elf_Shdr &Sec,
                                           StringRef SectionStringTable) {
   StringRef Name =
       check(this->getObj().getSectionName(&Sec, SectionStringTable));
 
   switch (Sec.sh_type) {
   case SHT_ARM_ATTRIBUTES:
-    // FIXME: ARM meta-data section. At present attributes are ignored,
-    // they can be used to reason about object compatibility.
+    // FIXME: ARM meta-data section. Retain the first attribute section
+    // we see. The eglibc ARM dynamic loaders require the presence of an
+    // attribute section for dlopen to work.
+    // In a full implementation we would merge all attribute sections.
+    if (In<ELFT>::ARMAttributes == nullptr) {
+      In<ELFT>::ARMAttributes = make<InputSection<ELFT>>(this, &Sec, Name);
+      return In<ELFT>::ARMAttributes;
+    }
     return &InputSection<ELFT>::Discarded;
   case SHT_RELA:
   case SHT_REL: {
     // This section contains relocation information.
     // If -r is given, we do not interpret or apply relocation
     // but just copy relocation sections to output.
     if (Config->Relocatable)
       return make<InputSection<ELFT>>(this, &Sec, Name);

ELF/SyntheticSections.h

 };
 
 template <class ELFT> InputSection<ELFT> *createCommonSection();
 template <class ELFT> InputSection<ELFT> *createInterpSection();
 template <class ELFT> MergeInputSection<ELFT> *createCommentSection();
 
 // Linker generated sections which can be used as inputs.
 template <class ELFT> struct In {
+  static InputSection<ELFT> *ARMAttributes;
   static BuildIdSection<ELFT> *BuildId;
   static InputSection<ELFT> *Common;
   static DynamicSection<ELFT> *Dynamic;
   static StringTableSection<ELFT> *DynStrTab;
   static SymbolTableSection<ELFT> *DynSymTab;
   static EhFrameHeader<ELFT> *EhFrameHdr;
   static GnuHashTableSection<ELFT> *GnuHashTab;
   static GdbIndexSection<ELFT> *GdbIndex;
template <class ELFT> struct In {
   static StringTableSection<ELFT> *ShStrTab;
   static StringTableSection<ELFT> *StrTab;
   static SymbolTableSection<ELFT> *SymTab;
   static VersionDefinitionSection<ELFT> *VerDef;
   static VersionTableSection<ELFT> *VerSym;
   static VersionNeedSection<ELFT> *VerNeed;
 };
 
+template <class ELFT> InputSection<ELFT> *In<ELFT>::ARMAttributes;
 template <class ELFT> BuildIdSection<ELFT> *In<ELFT>::BuildId;
 template <class ELFT> InputSection<ELFT> *In<ELFT>::Common;
 template <class ELFT> DynamicSection<ELFT> *In<ELFT>::Dynamic;
 template <class ELFT> StringTableSection<ELFT> *In<ELFT>::DynStrTab;
 template <class ELFT> SymbolTableSection<ELFT> *In<ELFT>::DynSymTab;
 template <class ELFT> EhFrameHeader<ELFT> *In<ELFT>::EhFrameHdr;
 template <class ELFT> GdbIndexSection<ELFT> *In<ELFT>::GdbIndex;
 template <class ELFT> GnuHashTableSection<ELFT> *In<ELFT>::GnuHashTab;

test/ELF/Inputs/arm-attributes1.s

-// RUN: llvm-mc -filetype=obj -triple=armv7a-none-linux-gnueabi %s -o %t.o
-// RUN: ld.lld %t.o -o %t
-// RUN: llvm-readobj -s %t | FileCheck %s
-// RUN: ld.lld %t.o -shared -o %t2
-// RUN: llvm-readobj -s %t2 | FileCheck %s
-// RUN: ld.lld %t.o -r -o %t3
-// RUN: llvm-readobj -s %t3 | FileCheck %s
-
-// The .ARM.attributes section should be removed from executables and
-// shared objects.
-
-// At present we remove it from the -r object output as well which isn't ideal.
-// Unfortunately combining per-object attributes cannot be safely done by just
-// concatentation of input sections.
-
-// CHECK-NOT: Name: .ARM.attributes
-// REQUIRES: arm
+// Input that generates an object with a populated SHT_ARM_ATTRIBUTES section
  .text
  .syntax unified
  .eabi_attribute        67, "2.09"      @ Tag_conformance
  .cpu    cortex-a8
  .eabi_attribute 6, 10   @ Tag_CPU_arch
  .eabi_attribute 7, 65   @ Tag_CPU_arch_profile
  .eabi_attribute 8, 1    @ Tag_ARM_ISA_use
  .eabi_attribute 9, 2    @ Tag_THUMB_ISA_use
  .fpu    neon
  .eabi_attribute 15, 1   @ Tag_ABI_PCS_RW_data
  .eabi_attribute 16, 1   @ Tag_ABI_PCS_RO_data
  .eabi_attribute 17, 2   @ Tag_ABI_PCS_GOT_use
  .eabi_attribute 20, 1   @ Tag_ABI_FP_denormal
  .eabi_attribute 21, 1   @ Tag_ABI_FP_exceptions
  .eabi_attribute 23, 3   @ Tag_ABI_FP_number_model
  .eabi_attribute 34, 1   @ Tag_CPU_unaligned_access
  .eabi_attribute 24, 1   @ Tag_ABI_align_needed
  .eabi_attribute 25, 1   @ Tag_ABI_align_preserved
  .eabi_attribute 38, 1   @ Tag_ABI_FP_16bit_format
  .eabi_attribute 18, 4   @ Tag_ABI_PCS_wchar_t
  .eabi_attribute 26, 2   @ Tag_ABI_enum_size
  .eabi_attribute 14, 0   @ Tag_ABI_PCS_R9_use
  .eabi_attribute 68, 1   @ Tag_Virtualization_use
- .globl  _start
+ .globl  func
  .p2align        2
- .type   _start,%function
-_start:
+ .type   func,%function
+func:
  bx lr

test/ELF/arm-attributes.s

+// RUN: llvm-mc -filetype=obj -triple=armv7a-none-linux-gnueabi %S/Inputs/arm-attributes1.s -o %t1.o
+// RUN: llvm-mc -filetype=obj -triple=armv7a-none-linux-gnueabi %s -o %t2.o
+
+// RUN: ld.lld %t1.o %t2.o -o %t
+// RUN: llvm-readobj -arm-attributes %t | FileCheck %s
+// RUN: ld.lld %t1.o %t2.o -shared -o %t2
+// RUN: llvm-readobj -arm-attributes %t2 | FileCheck %s
+// RUN: ld.lld %t1.o %t2.o -r -o %t3
+// RUN: llvm-readobj -arm-attributes %t3 | FileCheck %s
+// REQUIRES: arm
+
+// Check that we retain only 1 SHT_ARM_ATTRIBUTES section. At present we do not
+// try and merge or use the contents of SHT_ARM_ATTRIBUTES sections. We just
+// pass the first one through.
+ .text
+ .syntax unified
+ .eabi_attribute        67, "2.09"      @ Tag_conformance
+ .cpu    cortex-a8
+ .eabi_attribute 6, 10   @ Tag_CPU_arch
+ .eabi_attribute 7, 65   @ Tag_CPU_arch_profile
+ .eabi_attribute 8, 1    @ Tag_ARM_ISA_use
+ .eabi_attribute 9, 2    @ Tag_THUMB_ISA_use
+ .fpu    neon
+ .eabi_attribute 15, 1   @ Tag_ABI_PCS_RW_data
+ .eabi_attribute 16, 1   @ Tag_ABI_PCS_RO_data
+ .eabi_attribute 17, 2   @ Tag_ABI_PCS_GOT_use
+ .eabi_attribute 20, 1   @ Tag_ABI_FP_denormal
+ .eabi_attribute 21, 1   @ Tag_ABI_FP_exceptions
+ .eabi_attribute 23, 3   @ Tag_ABI_FP_number_model
+ .eabi_attribute 34, 1   @ Tag_CPU_unaligned_access
+ .eabi_attribute 24, 1   @ Tag_ABI_align_needed
+ .eabi_attribute 25, 1   @ Tag_ABI_align_preserved
+ .eabi_attribute 38, 1   @ Tag_ABI_FP_16bit_format
+ .eabi_attribute 18, 4   @ Tag_ABI_PCS_wchar_t
+ .eabi_attribute 26, 2   @ Tag_ABI_enum_size
+ .eabi_attribute 14, 0   @ Tag_ABI_PCS_R9_use
+ .eabi_attribute 68, 1   @ Tag_Virtualization_use
+ .globl  _start
+ .p2align        2
+ .type   _start,%function
+_start:
+ .globl func
+ bl func
+ bx lr
+
+// CHECK: BuildAttributes {
+// CHECK-NEXT:   FormatVersion: 0x41
+// CHECK-NEXT:   Section 1 {
+// CHECK-NEXT:     SectionLength: 72
+// CHECK-NEXT:     Vendor: aeabi
+// CHECK-NEXT:     Tag: Tag_File (0x1)
+// CHECK-NEXT:     Size: 62
+// CHECK-NEXT:     FileAttributes {
+// CHECK-NEXT:       Attribute {
+// CHECK-NEXT:         Tag: 67
+// CHECK-NEXT:         TagName: conformance
+// CHECK-NEXT:         Value: 2.09
+// CHECK-NEXT:       }
+// CHECK-NEXT:       Attribute {
+// CHECK-NEXT:         Tag: 5
+// CHECK-NEXT:         TagName: CPU_name
+// CHECK-NEXT:         Value: cortex-a8
+// CHECK-NEXT:       }
+// CHECK-NEXT:       Attribute {
+// CHECK-NEXT:         Tag: 6
+// CHECK-NEXT:         Value: 10
+// CHECK-NEXT:         TagName: CPU_arch
+// CHECK-NEXT:         Description: ARM v7
+// CHECK-NEXT:       }
+// CHECK-NEXT:       Attribute {
+// CHECK-NEXT:         Tag: 7
+// CHECK-NEXT:         Value: 65
+// CHECK-NEXT:         TagName: CPU_arch_profile
+// CHECK-NEXT:         Description: Application
+// CHECK-NEXT:       }
+// CHECK-NEXT:       Attribute {
+// CHECK-NEXT:         Tag: 8
+// CHECK-NEXT:         Value: 1
+// CHECK-NEXT:         TagName: ARM_ISA_use
+// CHECK-NEXT:         Description: Permitted
+// CHECK-NEXT:       }
+// CHECK-NEXT:       Attribute {
+// CHECK-NEXT:         Tag: 9
+// CHECK-NEXT:         Value: 2
+// CHECK-NEXT:         TagName: THUMB_ISA_use
+// CHECK-NEXT:         Description: Thumb-2
+// CHECK-NEXT:       }
+// CHECK-NEXT:       Attribute {
+// CHECK-NEXT:         Tag: 10
+// CHECK-NEXT:         Value: 3
+// CHECK-NEXT:         TagName: FP_arch
+// CHECK-NEXT:         Description: VFPv3
+// CHECK-NEXT:       }
+// CHECK-NEXT:       Attribute {
+// CHECK-NEXT:         Tag: 12
+// CHECK-NEXT:         Value: 1
+// CHECK-NEXT:         TagName: Advanced_SIMD_arch
+// CHECK-NEXT:         Description: NEONv1
+// CHECK-NEXT:       }
+// CHECK-NEXT:       Attribute {
+// CHECK-NEXT:         Tag: 14
+// CHECK-NEXT:         Value: 0
+// CHECK-NEXT:         TagName: ABI_PCS_R9_use
+// CHECK-NEXT:         Description: v6
+// CHECK-NEXT:       }
+// CHECK-NEXT:       Attribute {
+// CHECK-NEXT:         Tag: 15
+// CHECK-NEXT:         Value: 1
+// CHECK-NEXT:         TagName: ABI_PCS_RW_data
+// CHECK-NEXT:         Description: PC-relative
+// CHECK-NEXT:       }
+// CHECK-NEXT:       Attribute {
+// CHECK-NEXT:         Tag: 16
+// CHECK-NEXT:         Value: 1
+// CHECK-NEXT:         TagName: ABI_PCS_RO_data
+// CHECK-NEXT:         Description: PC-relative
+// CHECK-NEXT:       }
+// CHECK-NEXT:       Attribute {
+// CHECK-NEXT:         Tag: 17
+// CHECK-NEXT:         Value: 2
+// CHECK-NEXT:         TagName: ABI_PCS_GOT_use
+// CHECK-NEXT:         Description: GOT-Indirect
+// CHECK-NEXT:       }
+// CHECK-NEXT:       Attribute {
+// CHECK-NEXT:         Tag: 18
+// CHECK-NEXT:         Value: 4
+// CHECK-NEXT:         TagName: ABI_PCS_wchar_t
+// CHECK-NEXT:         Description: 4-byte
+// CHECK-NEXT:       }
+// CHECK-NEXT:       Attribute {
+// CHECK-NEXT:         Tag: 20
+// CHECK-NEXT:         Value: 1
+// CHECK-NEXT:         TagName: ABI_FP_denormal
+// CHECK-NEXT:         Description: IEEE-754
+// CHECK-NEXT:       }
+// CHECK-NEXT:       Attribute {
+// CHECK-NEXT:         Tag: 21
+// CHECK-NEXT:         Value: 1
+// CHECK-NEXT:         TagName: ABI_FP_exceptions
+// CHECK-NEXT:         Description: IEEE-754
+// CHECK-NEXT:       }
+// CHECK-NEXT:       Attribute {
+// CHECK-NEXT:         Tag: 23
+// CHECK-NEXT:         Value: 3
+// CHECK-NEXT:         TagName: ABI_FP_number_model
+// CHECK-NEXT:         Description: IEEE-754
+// CHECK-NEXT:       }
+// CHECK-NEXT:       Attribute {
+// CHECK-NEXT:         Tag: 24
+// CHECK-NEXT:         Value: 1
+// CHECK-NEXT:         TagName: ABI_align_needed
+// CHECK-NEXT:         Description: 8-byte alignment
+// CHECK-NEXT:       }
+// CHECK-NEXT:       Attribute {
+// CHECK-NEXT:         Tag: 25
+// CHECK-NEXT:         Value: 1
+// CHECK-NEXT:         TagName: ABI_align_preserved
+// CHECK-NEXT:         Description: 8-byte data alignment
+// CHECK-NEXT:       }
+// CHECK-NEXT:       Attribute {
+// CHECK-NEXT:         Tag: 26
+// CHECK-NEXT:         Value: 2
+// CHECK-NEXT:         TagName: ABI_enum_size
+// CHECK-NEXT:         Description: Int32
+// CHECK-NEXT:       }
+// CHECK-NEXT:       Attribute {
+// CHECK-NEXT:         Tag: 34
+// CHECK-NEXT:         Value: 1
+// CHECK-NEXT:         TagName: CPU_unaligned_access
+// CHECK-NEXT:         Description: v6-style
+// CHECK-NEXT:       }
+// CHECK-NEXT:       Attribute {
+// CHECK-NEXT:         Tag: 38
+// CHECK-NEXT:         Value: 1
+// CHECK-NEXT:         TagName: ABI_FP_16bit_format
+// CHECK-NEXT:         Description: IEEE-754
+// CHECK-NEXT:       }
+// CHECK-NEXT:       Attribute {
+// CHECK-NEXT:         Tag: 68
+// CHECK-NEXT:         Value: 1
+// CHECK-NEXT:         TagName: Virtualization_use
+// CHECK-NEXT:         Description: TrustZone
+// CHECK-NEXT:       }