[ELF] Fix crash when eh_frame or eh_frame_hdr is discarded in linker script

[evgeny777]

Summary

This crash was introduced by D30627

[ruiu]

LGTM

test/ELF/linkerscript/pt_gnu_eh_frame.s

3–5 ↗

(On Diff #91535)

If you can write this in one line, please do so.

lld/trunk/ELF/Writer.cpp

template <class ELFT> std::vector<PhdrEntry> Writer<ELFT>::createPhdrs() {
   PhdrEntry RelRo(PT_GNU_RELRO, PF_R);
   for (OutputSection *Sec : OutputSections)
     if (needsPtLoad<ELFT>(Sec) && isRelroSection<ELFT>(Sec))
       RelRo.add(Sec);
   if (RelRo.First)
     Ret.push_back(std::move(RelRo));
 
   // PT_GNU_EH_FRAME is a special section pointing on .eh_frame_hdr.
-  if (!In<ELFT>::EhFrame->empty() && In<ELFT>::EhFrameHdr)
+  if (!In<ELFT>::EhFrame->empty() && In<ELFT>::EhFrameHdr &&
+      In<ELFT>::EhFrame->OutSec && In<ELFT>::EhFrameHdr->OutSec)
     AddHdr(PT_GNU_EH_FRAME, In<ELFT>::EhFrameHdr->OutSec->getPhdrFlags())
         ->add(In<ELFT>::EhFrameHdr->OutSec);
 
   // PT_OPENBSD_RANDOMIZE specifies the location and size of a part of the
   // memory image of the program that must be filled with random data before any
   // code in the object is executed.
   if (OutputSection *Sec = findSection(".openbsd.randomdata"))
     AddHdr(PT_OPENBSD_RANDOMIZE, Sec->getPhdrFlags())->add(Sec);

lld/trunk/test/ELF/linkerscript/pt_gnu_eh_frame.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
+# RUN: echo "SECTIONS { /DISCARD/ : { *(.eh_frame*) *(.eh_frame_hdr*) } }" > %t.script
+# RUN: ld.lld -o %t1 --eh-frame-hdr --script %t.script %t
+
+.global _start
+_start:
+ nop
+
+.section .dah,"ax",@progbits
+.cfi_startproc
+ nop
+.cfi_endproc