[ELF] Ensure output section is also not empty in addStartEndSymbols()

[andrewng]

Summary

Fixes https://bugs.llvm.org/show_bug.cgi?id=52534.

[MaskRay]

lld/test/ELF/linkerscript/pre_init_array_empty.test

1 ↗

(On Diff #387971)

preinit-array-empty.test

6 ↗

(On Diff #387971)

7 ↗

(On Diff #387971)

Use llvm-readelf -s to check the value of __preinit_array_start.

This makes better use of a test case: not just check that the link succeeds, also catches unintended value change in the future. You can add a comment that as long as __preinit_array_start=__preinit_array_end, the exact value doesn't matter.

[andrewng]

Updated to address review comments.

However, I have also uncovered a situation where an empty .preinit_array section can be emitted and in this case, this patch would result in the value of __preinit_array_start and __preinit_array_end not matching the address of the .preinit_array section. I have created D114179 as an alternative to this patch which copes with this scenario.

[andrewng]

Abandoned in favour of D114179.

lld/ELF/Writer.cpp

template <class ELFT> void Writer<ELFT>::addStartEndSymbols() {
   //
   // In rare situations, the .text section may not exist. If that's the
   // case, use the image base address as a last resort.
   OutputSection *Default = findSection(".text");
   if (!Default)
     Default = Out::elfHeader;
 
   auto define = [=](StringRef start, StringRef end, OutputSection *os) {
-    if (os) {
+    if (os && getFirstInputSection(os)) {
       addOptionalRegular(start, os, 0);
       addOptionalRegular(end, os, -1);
     } else {
       addOptionalRegular(start, Default, 0);
       addOptionalRegular(end, Default, 0);
     }
   };
 

lld/test/ELF/linkerscript/preinit-array-empty.test

+# REQUIRES: x86
+## PR52534: https://bugs.llvm.org/show_bug.cgi?id=52534
+## Link should succeed without causing an out of range relocation error.
+
+# RUN: split-file %s %t
+# RUN: llvm-mc -filetype=obj -triple=x86_64 %t/t.s -o %t.o
+# RUN: ld.lld --script %t/t.script %t.o -o %t.out --image-base=0x80000000
+
+# RUN: llvm-readelf -s %t.out | FileCheck %s
+# CHECK:      [[#%x,ADDR:]] 0 NOTYPE  LOCAL  HIDDEN      [[#]] __preinit_array_start
+# CHECK-NEXT: [[#ADDR]]     0 NOTYPE  LOCAL  HIDDEN      [[#]] __preinit_array_end
+# CHECK-NEXT: [[#ADDR]]     0 NOTYPE  GLOBAL DEFAULT     [[#]] _start
+
+#--- t.s
+.global _start
+_start:
+ movq __preinit_array_start@GOTPCREL(%rip),%rax
+ movq __preinit_array_end@GOTPCREL(%rip),%rax
+
+#--- t.script
+SECTIONS {
+  .text : { *(.text); }
+  .preinit_array : { *(.preinit_array); }
+}