[LLD][ELF][ARM] Do not create .ARM.exidx sections for out of range inputs

[psmith]

Summary

A linker will create .ARM.exidx sections for InputSections that don't have them. This can cause a relocation out of range error If the InputSection happens to be extremely far away from the other sections. This is often the case for the vector table on older ARM CPUs as the only two places that the table can be placed is 0 or 0xffff0000. We fix this by removing InputSections that need a linker generated .ARM.exidx section if that would cause an error.

Fixes part of pr44824 (https://bugs.llvm.org/show_bug.cgi?id=44824)

[MaskRay]

lld/ELF/SyntheticSections.cpp

3377

int64_t off = isec->getVA() - getVA();

Even better: add an explicit cast.

lld/test/ELF/arm-exidx-range.s

2

// REQUIRES: arm

6

AT(...) can be omitted.

11

The .vectors at 0xffff0000 (...) in earlier Arm CPUs.

This sentence lacks a verb.

20

Add CHECK-NOT: 0x to make sure there is the last line ("Expect only .ARM.exidx from _start and sentinel")

[psmith]

Thanks for the comments, will update diff shortly.

[psmith]

Applied previous review comments. Added missing REQUIRES, added CHECK-NOT and simplified range checking code.

[MaskRay]

LGTM.

lld/ELF/SyntheticSections.cpp

 // .ARM.exidx table entries are removed from it.
 void ARMExidxSyntheticSection::finalizeContents() {
   // The executableSections and exidxSections that we use to derive the final
   // contents of this SyntheticSection are populated before
   // processSectionCommands() and ICF. A /DISCARD/ entry in SECTIONS command or
   // ICF may remove executable InputSections and their dependent .ARM.exidx
   // section that we recorded earlier.
   auto isDiscarded = [](const InputSection *isec) { return !isec->isLive(); };
-  llvm::erase_if(executableSections, isDiscarded);
   llvm::erase_if(exidxSections, isDiscarded);
+  // We need to remove discarded InputSections and InputSections without
+  // .ARM.exidx sections that if we generated the .ARM.exidx it would be out
+  // of range.
+  auto isDiscardedOrOutOfRange = [this](InputSection *isec) {
+    if (!isec->isLive())
+      return true;
+    if (findExidxSection(isec))
+      return false;
+    int64_t off = static_cast<int64_t>(isec->getVA() - getVA());
+    return off != llvm::SignExtend64(off, 31);
+  };
+  llvm::erase_if(executableSections, isDiscardedOrOutOfRange);
 
   // Sort the executable sections that may or may not have associated
   // .ARM.exidx sections by order of ascending address. This requires the
   // relative positions of InputSections and OutputSections to be known.
   auto compareByFilePosition = [](const InputSection *a,
                                   const InputSection *b) {
     OutputSection *aOut = a->getParent();
     OutputSection *bOut = b->getParent();

lld/test/ELF/arm-exidx-range.s

+// REQUIRES: arm
+// RUN: llvm-mc --arm-add-build-attributes --triple=armv7a-linux-gnueabihf -filetype=obj %s -o %t.o
+// RUN: echo "SECTIONS { \
+// RUN:         . = 0x80000000; \
+// RUN:         .text : { *(.text) } \
+// RUN:         .vectors 0xffff0000 : { *(.vectors) } \
+// RUN: } " > %t.script
+// RUN: ld.lld --script %t.script %t.o -o %t
+// RUN: llvm-readobj -x .ARM.exidx %t | FileCheck %s
+/// Adapted from Linux kernel linker script failing due to out of range
+/// relocation. The .vectors at 0xffff0000 is a common occurrence as the vector
+/// table can only be placed at either 0 or 0xffff0000 in older ARM CPUs.
+/// In the example the .vectors won't have an exception table so if LLD creates
+/// one then we'll get a relocation out of range error. Check that we don't
+/// synthesise a table entry or place a sentinel out of range.
+
+/// Expect only .ARM.exidx from _start and sentinel
+// CHECK: Hex dump of section '.ARM.exidx':
+// CHECK-NEXT: 0x80000000 10000000 01000000 0c000000 01000000
+// CHECK-NOT:  0x80000010
+
+ .text
+ .global _start
+ .type _start, %function
+_start:
+ .fnstart
+ bx lr
+ .cantunwind
+ .fnend
+
+ .section .vectors, "ax", %progbits
+ .global vecs
+ .type vecs, %function
+vecs:
+ bx lr