[LLD] Fix findOrphanPos to consistently ignore "dead" OutputSection's

[andrewng]

Summary

When findOrphanPos does the reverse search to find the OutputSection
preceding the orphan's insertion point, look for a live OutputSection
and ignore "dead" OutputSection's. This matches the behaviour of the
forward search performed earlier in this function.

Added test which without the above fix fails as a result of an orphan
executable section being incorrectly placed in a non-executable segment.

[ruiu]

LGTM

[andrewng]

Committed in rL315292: [LLD] Fix findOrphanPos to consistently ignore "dead" OutputSection's.

ELF/Writer.cpp

findOrphanPos(std::vector<BaseCommand *>::iterator B,
   for (; I != E; ++I) {
     auto *CurSec = dyn_cast<OutputSection>(*I);
     if (!CurSec || !CurSec->Live)
       continue;
     if (getRankProximity(Sec, CurSec) != Proximity ||
         Sec->SortRank < CurSec->SortRank)
       break;
   }
-  auto J = std::find_if(
-      llvm::make_reverse_iterator(I), llvm::make_reverse_iterator(B),
-      [](BaseCommand *Cmd) { return isa<OutputSection>(Cmd); });
+  auto J = std::find_if(llvm::make_reverse_iterator(I),
+                        llvm::make_reverse_iterator(B), [](BaseCommand *Cmd) {
+                          auto *OS = dyn_cast<OutputSection>(Cmd);
+                          return OS && OS->Live;
+                        });
   I = J.base();
 
   // As a special case, if the orphan section is the last section, put
   // it at the very end, past any other commands.
   // This matches bfd's behavior and is convenient when the linker script fully
   // specifies the start of the file, but doesn't care about the end (the non
   // alloc sections for example).
   auto NextSec = std::find_if(

test/ELF/linkerscript/orphan-phdrs.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
+# RUN: echo "PHDRS { \
+# RUN:   exec PT_LOAD FLAGS(0x4 | 0x1); \
+# RUN:   rw   PT_LOAD FLAGS(0x4 | 0x2); \
+# RUN: } \
+# RUN: SECTIONS { \
+# RUN:  .text : { *(.text) } :exec \
+# RUN:  .empty : { *(.empty) } :rw \
+# RUN:  .rw : { *(.rw) } \
+# RUN: }" > %t.script
+# RUN: ld.lld -o %t --script %t.script %t.o
+# RUN: llvm-readobj -elf-output-style=GNU -s -l %t | FileCheck %s
+
+## Check that the orphan section is placed correctly and belongs to
+## the correct segment.
+
+# CHECK: Section Headers
+# CHECK: .text
+# CHECK-NEXT: .orphan
+# CHECK-NEXT: .rw
+
+# CHECK: Segment Sections
+# CHECK-NEXT: .text .orphan
+# CHECK-NEXT: .rw
+
+.section .text, "ax"
+ ret
+
+.section .rw, "aw"
+ .quad 0
+
+.section .orphan, "ax"
+ ret