[ELF] Linkerscript: allow .eh_frame_hdr to go before .eh_frame

[evgeny777]

Summary

Currently .eh_frame_hdr must be written after .eh_frame, because EhFrameHeader is filled in EhOutputSection::writeTo(). This patch fixes it in a cost of one extra disk seek.

[ruiu]

LGTM

ELF/Writer.cpp

1312 ↗

(On Diff #69718)

Add a comment saying that EhFrameHdr depends on EhFrame contents therefore it needs to be written after EhFrame is writtern.

lld/trunk/ELF/Writer.cpp

template <class ELFT> void Writer<ELFT>::writeSections() {
   // before processing relocations in code-containing sections.
   Out<ELFT>::Opd = findSection(".opd");
   if (Out<ELFT>::Opd) {
     Out<ELFT>::OpdBuf = Buf + Out<ELFT>::Opd->getFileOff();
     Out<ELFT>::Opd->writeTo(Buf + Out<ELFT>::Opd->getFileOff());
   }
 
   for (OutputSectionBase<ELFT> *Sec : OutputSections)
-    if (Sec != Out<ELFT>::Opd)
+    if (Sec != Out<ELFT>::Opd && Sec != Out<ELFT>::EhFrameHdr)
       Sec->writeTo(Buf + Sec->getFileOff());
+
+  // The .eh_frame_hdr depends on .eh_frame section contents, therefore
+  // it should be written after .eh_frame is written.
+  if (!Out<ELFT>::EhFrame->empty() && Out<ELFT>::EhFrameHdr)
+    Out<ELFT>::EhFrameHdr->writeTo(Buf + Out<ELFT>::EhFrameHdr->getFileOff());
 }
 
 template <class ELFT> void Writer<ELFT>::writeBuildId() {
   if (!Out<ELFT>::BuildId)
     return;
 
   // Compute a hash of all sections except .debug_* sections.
   // We skip debug sections because they tend to be very large

lld/trunk/test/ELF/linkerscript/linkerscript-eh-frame-hdr.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
+# RUN: echo "SECTIONS { \
+# RUN:          .eh_frame_hdr : {} \
+# RUN:          .eh_frame : {} \
+# RUN:       }" > %t.script
+# RUN: ld.lld -o %t1 --eh-frame-hdr --script %t.script %t
+# RUN: llvm-objdump -s -section=".eh_frame_hdr" %t1 | FileCheck %s
+
+# CHECK:      0158 011b033b 14000000 01000000 49000000
+# CHECK-NEXT: 0168 30000000
+
+.global _start
+_start:
+ nop
+
+.section .dah,"ax",@progbits
+.cfi_startproc
+ nop
+.cfi_endproc