[LLD][ARM] Add sh_link field to .ARM.exidx sections for relocatable links

[peter.smith]

Summary

When doing a relocatable link the .ARM.exidx sections with the SHF_LINK_ORDER flag set need to set the sh_link field to the executable section they describe. We find the appropriate OutputSection by following the sh_link field of the .ARM.exidx InputSections.

Since the changes in D25232 , the getOutputSectionName() function rules make sure that when there are multiple .ARM.exidx InputSections in an OutputSection they all have the same sh_link field.

This change should be sufficient to support ARM exceptions sections in relocatable objects.

[ruiu]

LGTM

ELF/OutputSections.cpp

923 ↗

(On Diff #75293)

nit: I'd add {.

lld/trunk/ELF/OutputSections.cpp

OutputSection<ELFT>::OutputSection(StringRef Name, uint32_t Type, uintX_t Flags)
   if (Type == SHT_RELA)
     this->Header.sh_entsize = sizeof(Elf_Rela);
   else if (Type == SHT_REL)
     this->Header.sh_entsize = sizeof(Elf_Rel);
 }
 
 template <class ELFT> void OutputSection<ELFT>::finalize() {
   uint32_t Type = this->Header.sh_type;
+  if (this->Header.sh_flags & SHF_LINK_ORDER) {
+    if (!Config->Relocatable) {
       // SHF_LINK_ORDER only has meaning in relocatable objects
-  if (!Config->Relocatable)
       this->Header.sh_flags &= ~SHF_LINK_ORDER;
+    }
+    else if (!this->Sections.empty()) {
+      // When doing a relocatable link we must preserve the link order
+      // dependency of sections with the SHF_LINK_ORDER flag. The dependency
+      // is indicated by the sh_link field. We need to translate the
+      // InputSection sh_link to the OutputSection sh_link, all InputSections
+      // in the OutputSection have the same dependency.
+      if (auto *D = this->Sections.front()->getLinkOrderDep())
+        this->Header.sh_link = D->OutSec->SectionIndex;
+    }
+  }
   if (Type != SHT_RELA && Type != SHT_REL)
     return;
   this->Header.sh_link = Out<ELFT>::SymTab->SectionIndex;
   // sh_info for SHT_REL[A] sections should contain the section header index of
   // the section to which the relocation applies.
   InputSectionBase<ELFT> *S = Sections[0]->getRelocatedSection();
   this->Header.sh_info = S->OutSec->SectionIndex;
 }

lld/trunk/test/ELF/arm-exidx-relocatable.s

+// RUN: llvm-mc -filetype=obj -triple=armv7a-none-linux-gnueabi %s -o %t
+// RUN: llvm-mc -filetype=obj -triple=armv7a-none-linux-gnueabi %S/Inputs/arm-exidx-cantunwind.s -o %tcantunwind
+// Check that relocatable link maintains SHF_LINK_ORDER
+// RUN: ld.lld -r %t %tcantunwind -o %t4 2>&1
+// RUN: llvm-readobj -s %t4 | FileCheck %s
+// REQUIRES: arm
+
+// Each assembler created .ARM.exidx section has the SHF_LINK_ORDER flag set
+// with the sh_link containing the section index of the executable section
+// containing the function it describes. To maintain this property in
+// relocatable links we pass through the .ARM.exidx section, the section it
+// it has a sh_link to, and the associated relocation sections uncombined.
+
+ .syntax unified
+ .section .text, "ax",%progbits
+ .globl _start
+_start:
+ .fnstart
+ bx lr
+ .cantunwind
+ .fnend
+
+ .section .text.f1, "ax", %progbits
+ .globl f1
+f1:
+ .fnstart
+ bx lr
+ .cantunwind
+ .fnend
+
+ .section .text.f2, "ax", %progbits
+ .globl f2
+f2:
+ .fnstart
+ bx lr
+ .cantunwind
+ .fnend
+ .globl f3
+f3:
+ .fnstart
+ bx lr
+ .cantunwind
+ .fnend
+
+// CHECK:         Name: .ARM.exidx
+// CHECK-NEXT:    Type: SHT_ARM_EXIDX (0x70000001)
+// CHECK-NEXT:    Flags [ (0x82)
+// CHECK-NEXT:      SHF_ALLOC (0x2)
+// CHECK-NEXT:      SHF_LINK_ORDER (0x80)
+// CHECK:         Size: 24
+// CHECK-NEXT:    Link: 7
+// CHECK:         Name: .ARM.exidx.text.f1
+// CHECK-NEXT:    Type: SHT_ARM_EXIDX (0x70000001)
+// CHECK-NEXT:    Flags [ (0x82)
+// CHECK-NEXT:      SHF_ALLOC (0x2)
+// CHECK-NEXT:      SHF_LINK_ORDER (0x80)
+// CHECK:    Size: 8
+// CHECK-NEXT:    Link: 8
+// CHECK:         Name: .ARM.exidx.text.f2
+// CHECK-NEXT:    Type: SHT_ARM_EXIDX (0x70000001)
+// CHECK-NEXT:    Flags [ (0x82)
+// CHECK-NEXT:      SHF_ALLOC (0x2)
+// CHECK-NEXT:      SHF_LINK_ORDER (0x80)
+// CHECK:         Size: 16
+// CHECK-NEXT:    Link: 9
+// CHECK:         Name: .ARM.exidx.func1
+// CHECK-NEXT:    Type: SHT_ARM_EXIDX (0x70000001)
+// CHECK-NEXT:    Flags [ (0x82)
+// CHECK-NEXT:      SHF_ALLOC (0x2)
+// CHECK-NEXT:      SHF_LINK_ORDER (0x80)
+// CHECK:         Size: 8
+// CHECK-NEXT:    Link: 10
+// CHECK:         Name: .ARM.exidx.func2
+// CHECK-NEXT:    Type: SHT_ARM_EXIDX (0x70000001)
+// CHECK-NEXT:    Flags [ (0x82)
+// CHECK-NEXT:      SHF_ALLOC (0x2)
+// CHECK-NEXT:      SHF_LINK_ORDER (0x80)
+// CHECK:         Size: 8
+// CHECK-NEXT:    Link: 11
+// CHECK:         Name: .ARM.exidx.func3
+// CHECK-NEXT:    Type: SHT_ARM_EXIDX (0x70000001)
+// CHECK-NEXT:    Flags [ (0x82)
+// CHECK-NEXT:      SHF_ALLOC (0x2)
+// CHECK-NEXT:      SHF_LINK_ORDER (0x80)
+// CHECK:         Size: 8
+// CHECK-NEXT:    Link: 12
+// CHECK:         Index: 7
+// CHECK-NEXT:    Name: .text
+// CHECK:         Index: 8
+// CHECK-NEXT:    Name: .text.f1
+// CHECK:         Index: 9
+// CHECK-NEXT:    Name: .text.f2
+// CHECK:         Index: 10
+// CHECK-NEXT:    Name: .func1
+// CHECK:         Index: 11
+// CHECK-NEXT:    Name: .func2
+// CHECK:         Index: 12
+// CHECK-NEXT:    Name: .func3