[ELF] Remove empty PT_LOAD being created for program headers.

[evgeny777]

Summary

Sometimes the very first PT_LOAD segment, created by lld, can be empty. This happens when (all conditions met):

• Linker script is used
• First section in ELF image is not RO
• Not enough space for program headers.

In such case lld emits empty PT_LOAD to result ELF image. This patch fixes it

[• rafael]

LGTM with nits.

ELF/LinkerScript.cpp

659

Expand the comment a bit with the explanation you have on the description.

test/ELF/linkerscript/empty-load.s

10

Check the flags (rw/rx) too.

ELF/LinkerScript.cpp

if (HeaderSize <= MinVA && FirstPTLoad != Phdrs.end()) {
     // ELF and Program headers need to be right before the first section in
     // memory. Set their addresses accordingly.
     MinVA = alignDown(MinVA - HeaderSize, Target->PageSize);
     Out<ELFT>::ElfHeader->setVA(MinVA);
     Out<ELFT>::ProgramHeaders->setVA(Out<ELFT>::ElfHeader->getSize() + MinVA);
     FirstPTLoad->First = Out<ELFT>::ElfHeader;
     if (!FirstPTLoad->Last)
       FirstPTLoad->Last = Out<ELFT>::ProgramHeaders;
+  } else if (!FirstPTLoad->First) {
+    // Remove empty PT_LOAD and update size of ProgramHeaders.
+    Phdrs.erase(FirstPTLoad);
+    Out<ELFT>::ProgramHeaders->setSize(sizeof(typename ELFT::Phdr) *
+                                       Phdrs.size());
   }
 }
 
 // Creates program headers as instructed by PHDRS linker script command.
 template <class ELFT>
 std::vector<PhdrEntry<ELFT>> LinkerScript<ELFT>::createPhdrs() {
   std::vector<PhdrEntry<ELFT>> Ret;
 

test/ELF/linkerscript/empty-load.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
+# RUN: echo "SECTIONS { .rw : { *(.rw) } .text : { *(.text) }  }" > %t.script
+# RUN: ld.lld -o %t1 --script %t.script %t
+# RUN: llvm-objdump -private-headers %t1 | FileCheck %s
+
+## We expect 2 PT_LOAD segments
+# CHECK:       PHDR
+# CHECK-NEXT:     filesz
+# CHECK-NEXT:  LOAD
+# CHECK-NEXT:     filesz
+# CHECK-NEXT:  LOAD
+# CHECK-NEXT:     filesz
+# CHECK-NEXT:  STACK
+# CHECK-NEXT:     filesz
+
+.globl _start
+_start:
+  jmp _start
+
+.section .rw, "aw"
+ .quad 0