[ELF] Linkerscript: do not fail if there is no space for program headers and there is PHDRS directive

[evgeny777]

Summary

After http://llvm.org/viewvc/llvm-project?view=revision&revision=281660
linking no works for me, because if first section VA is less than size of headers lld emits fatal error.

However in case we have PHDRS directive it doesn't really have to do so, because with PHDRS we can tell lld not to emit program headers or assign separate segment for them.
This patch does two things:

a) Do not emit fatal error in case hasPhdrsCommands() is true
b) Set elf headers VA to the end of the section, so there are more chances it is valid.

[emaste]

I've encountered this in FreeBSD as well after rL281660. The patch does not apply to HEAD, at rL281836 though.

[ruiu]

ELF/LinkerScript.cpp

472–479

I thought about this new code probably more than 15 minutes but am still not sure if this is correct. What is the problem you are trying to fix with this change?

test/ELF/linkerscript/phdrs.s

23–28

Is this linker script expected to create a valid executable?

[evgeny777]

This works for me, because I have separate load segment for ELF and file headers, but now I think that in general it shouldn't

ELF/LinkerScript.cpp

template <class ELFT> void LinkerScript<ELFT>::assignAddresses() {
   for (OutputSectionBase<ELFT> *Sec : *OutputSections) {
     StringRef Name = Sec->getName();
     if (getSectionIndex(Name) == INT_MAX)
       Opt.Commands.push_back(llvm::make_unique<OutputSectionCommand>(Name));
   }
 
   // Assign addresses as instructed by linker script SECTIONS sub-commands.
   Dot = getHeaderSize();
-  uintX_t MinVA = std::numeric_limits<uintX_t>::max();
+  uintX_t HeaderVA = std::numeric_limits<uintX_t>::max();
   uintX_t ThreadBssOffset = 0;
 
   for (const std::unique_ptr<BaseCommand> &Base : Opt.Commands) {
     if (auto *Cmd = dyn_cast<SymbolAssignment>(Base.get())) {
       if (Cmd->Name == ".") {
         Dot = Cmd->Expression(Dot);
       } else if (Cmd->Sym) {
         cast<DefinedRegular<ELFT>>(Cmd->Sym)->Value = Cmd->Expression(Dot);
for (OutputSectionBase<ELFT> *Sec :
       if (!(Sec->getFlags() & SHF_ALLOC)) {
         assignOffsets(Cmd, Sec);
         continue;
       }
 
       Dot = alignTo(Dot, Sec->getAlignment());
       Sec->setVA(Dot);
       assignOffsets(Cmd, Sec);
-      MinVA = std::min(MinVA, Dot);
+      HeaderVA = std::min(HeaderVA, Dot);
       Dot += Sec->getSize();
     }
   }
 
   uintX_t HeaderSize =
       Out<ELFT>::ElfHeader->getSize() + Out<ELFT>::ProgramHeaders->getSize();
-  if (HeaderSize > MinVA)
+  if (HeaderSize > HeaderVA && !hasPhdrsCommands())
     fatal("Not enough space for ELF and program headers");
 
-  // 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);
+  HeaderVA = HeaderSize > HeaderVA
+                 ? alignTo(Dot, Target->PageSize)
+                 : alignDown(HeaderVA - HeaderSize, Target->PageSize);
+  Out<ELFT>::ElfHeader->setVA(HeaderVA);
+  Out<ELFT>::ProgramHeaders->setVA(Out<ELFT>::ElfHeader->getSize() + HeaderVA);
 }
 
 // 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;
 
   // Process PHDRS and FILEHDR keywords because they are not

test/ELF/linkerscript/phdrs.s

 # RUN:       SECTIONS { \
 # RUN:           . = 0x10000200; \
 # RUN:           .text : {*(.text*)} :all \
 # RUN:           .foo : {*(.foo.*)} :all \
 # RUN:           .data : {*(.data.*)} :all}" > %t.script
 # RUN: ld.lld -o %t1 --script %t.script %t
 # RUN: llvm-readobj -program-headers %t1 | FileCheck --check-prefix=AT %s
 
+## Check PHDR VA.
+# RUN: echo "PHDRS { header PT_PHDR PHDRS; other PT_LOAD; } \
+# RUN:       SECTIONS { \
+# RUN:           . = 0; \
+# RUN:           .text : {*(.text*)} : other \
+# RUN:           .foo : {*(.foo.*)} : other \
+# RUN:           .data : {*(.data.*)} : other }" > %t.script
+# RUN: ld.lld -o %t1 --script %t.script %t
+# RUN: llvm-readobj -program-headers %t1 | FileCheck --check-prefix=PHDR %s
+
 # CHECK:     ProgramHeaders [
 # CHECK-NEXT:  ProgramHeader {
 # CHECK-NEXT:    Type: PT_LOAD (0x1)
 # CHECK-NEXT:    Offset: 0x0
 # CHECK-NEXT:    VirtualAddress: 0x10000000
 # CHECK-NEXT:    PhysicalAddress: 0x10000000
 # CHECK-NEXT:    FileSize: 521
 # CHECK-NEXT:    MemSize: 521

 # AT-NEXT:      FileSize: 521
 # AT-NEXT:      MemSize: 521
 # AT-NEXT:      Flags [ (0x7)
 # AT-NEXT:        PF_R (0x4)
 # AT-NEXT:        PF_W (0x2)
 # AT-NEXT:        PF_X (0x1)
 # AT-NEXT:      ]
 
+# PHDR:     ProgramHeaders [
+# PHDR-NEXT:  ProgramHeader {
+# PHDR-NEXT:    Type: PT_PHDR (0x6)
+# PHDR-NEXT:    Offset: 0x40
+# PHDR-NEXT:    VirtualAddress: 0x1040
+# PHDR-NEXT:    PhysicalAddress: 0x1040
+# PHDR-NEXT:    FileSize: 112
+# PHDR-NEXT:    MemSize: 112
+# PHDR-NEXT:    Flags [ (0x4)
+# PHDR-NEXT:      PF_R (0x4)
+# PHDR-NEXT:    ]
+# PHDR-NEXT:    Alignment: 8
+# PHDR-NEXT:  }
+# PHDR-NEXT:  ProgramHeader {
+# PHDR-NEXT:    Type: PT_LOAD (0x1)
+# PHDR-NEXT:    Offset: 0x1000
+# PHDR-NEXT:    VirtualAddress: 0x0
+# PHDR-NEXT:    PhysicalAddress: 0x0
+# PHDR-NEXT:    FileSize: 9
+# PHDR-NEXT:    MemSize: 9
+# PHDR-NEXT:    Flags [ (0x7)
+# PHDR-NEXT:      PF_R (0x4)
+# PHDR-NEXT:      PF_W (0x2)
+# PHDR-NEXT:      PF_X (0x1)
+# PHDR-NEXT:    ]
+# PHDR-NEXT:    Alignment: 4096
+# PHDR-NEXT:  }
+# PHDR-NEXT: ]
+
 .global _start
 _start:
  nop
 
 .section .foo.1,"a"
 foo1:
  .long 0
 
 .section .foo.2,"aw"
 foo2:
  .long 0