ELF: Don't crash on output sections with a mix of SHF_LINK_ORDER and non-SHF_LINK_ORDER input sections.

[pcc]

Summary

The non-SHF_LINK_ORDER sections are sorted at the end.

[grimar]

Please take a look at rLLD327006. One of the things it addressed is this crash, though
with it, LLD now reports "error: incompatible section flags for .text" for your test case,
because it restricted mixing SHF_LINK_ORDER and non-SHF_LINK_ORDER sections
(assumption behind was that since nobody faced the crash, there is no need to support that case).
So, do you want to allow mixing them or just want to fix the crash?

[• espindola]

I agree with George. We currently produce an error and it would be better if we could keep doing that.

[pcc]

I'd be fine with rejecting.

lld/ELF/Writer.cpp

 
 static bool compareByFilePosition(InputSection *A, InputSection *B) {
   // Synthetic, i. e. a sentinel section, should go last.
   if (A->kind() == InputSectionBase::Synthetic ||
       B->kind() == InputSectionBase::Synthetic)
     return A->kind() != InputSectionBase::Synthetic;
   InputSection *LA = A->getLinkOrderDep();
   InputSection *LB = B->getLinkOrderDep();
+  // Non-SHF_LINK_ORDER sections go last.
+  if (!LA || !LB)
+    return LA;
   OutputSection *AOut = LA->getParent();
   OutputSection *BOut = LB->getParent();
   if (AOut != BOut)
     return AOut->SectionIndex < BOut->SectionIndex;
   return LA->OutSecOff < LB->OutSecOff;
 }
 
 // This function is used by the --merge-exidx-entries to detect duplicate

lld/test/ELF/link-order-mixed.s

+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
+# RUN: ld.lld %t.o -o %t
+# RUN: llvm-nm %t | FileCheck %s
+
+# CHECK: 0000000000201004 N _start
+.globl _start
+_start:
+.byte 1
+
+# CHECK: 0000000000201000 n sec2
+.section .text,"ao",@progbits,_start,unique,1
+sec2:
+.byte 2