[ELF] Don't emit empty PT_LOAD segment

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.

Differential revision: https://reviews.llvm.org/D25330

llvm-svn: 283760

lld/ELF/LinkerScript.cpp

@@ -655,6 +655,17 @@ void LinkerScript<ELFT>::assignAddresses(std::vector<PhdrEntry<ELFT>> &Phdrs) {
     FirstPTLoad->First = Out<ELFT>::ElfHeader;
     if (!FirstPTLoad->Last)
       FirstPTLoad->Last = Out<ELFT>::ProgramHeaders;
+  } else if (!FirstPTLoad->First) {
+    // Sometimes the very first PT_LOAD segment can be empty. 
+    // This happens if (all conditions met):
+    //  - Linker script is used
+    //  - First section in ELF image is not RO
+    //  - Not enough space for program headers.
+    // The code below removes empty PT_LOAD segment and updates
+    // program headers size.
+    Phdrs.erase(FirstPTLoad);
+    Out<ELFT>::ProgramHeaders->setSize(sizeof(typename ELFT::Phdr) *
+                                       Phdrs.size());
   }
 }
 

lld/test/ELF/linkerscript/empty-load.s

@@ -0,0 +1,22 @@
+# 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 {{0x[0-9a-f]+}} memsz {{0x[0-9a-f]+}} flags r--
+# CHECK-NEXT:  LOAD
+# CHECK-NEXT:     filesz {{0x[0-9a-f]+}} memsz {{0x[0-9a-f]+}} flags rw-
+# CHECK-NEXT:  LOAD
+# CHECK-NEXT:     filesz {{0x[0-9a-f]+}} memsz {{0x[0-9a-f]+}} flags r-x
+# CHECK-NEXT:  STACK
+# CHECK-NEXT:     filesz
+
+.globl _start
+_start:
+  jmp _start
+
+.section .rw, "aw"
+ .quad 0