[ELF] do not allow .bss to occupy the file space when producing relocatable output.

[grimar]

Summary

When generating relocatable output SHT_NOBITS sections
were still occupy the file space.

Patch fixed that.

[grimar]

• Corrected comment in testcase.

[ruiu]

ELF/Writer.cpp

1325

Do you have to set file offsets to bss sections?

[grimar]

ELF/Writer.cpp

1325

We just do the same in assignAddresses():

if (Sec->getType() != SHT_NOBITS)
  FileOff = alignTo(FileOff, Align);
Sec->setFileOffset(FileOff);
if (Sec->getType() != SHT_NOBITS)
  FileOff += Sec->getSize();

[grimar]

ELF/Writer.cpp

1325

And yes, I think we need to set the offset.
Below is relocatable output from gold for the testcase.
.bss has the Offset set.
I am just not sure why we might not to do that ?

Section Headers:

[Nr] Name              Type             Address           Offset
     Size              EntSize          Flags  Link  Info  Align
[ 0]                   NULL             0000000000000000  00000000
     0000000000000000  0000000000000000           0     0     0
[ 1] .text             PROGBITS         0000000000000000  00000040
     0000000000000001  0000000000000000  AX       0     0     4
[ 2] .bss              NOBITS           0000000000000000  00000041
     0000000000500000  0000000000000000  WA       0     0     1
[ 3] .symtab           SYMTAB           0000000000000000  00000048
     0000000000000030  0000000000000018           4     1     8

[ruiu]

ELF/Writer.cpp

1325

That gold sets the file offset does not mean that we have to do the same. I believe that behavior is not implemented to gold intentionally, but just that it happened to be that behavior. For us,

for (OutputSectionBase<ELFT> *Sec : OutputSections) {
  if (Sec->getType() == SHT_NOBITS)
    continue;
  FileOff = alignTo(FileOff, Sec->getAlign());
  Sec->setFileOffset(FileOff);
  FileOff += Sec->getSize();
}

is more straightforward, no?

[grimar]

r262650, got LGTM from Rui in llvm-mails.

ELF/Writer.cpp

 
 // Used for relocatable output (-r). In this case we create only ELF file
 // header, do not create program headers. Also assign of section addresses
 // is very straightforward: we just put all sections sequentually to the file.
 template <class ELFT> void Writer<ELFT>::assignAddressesRelocatable() {
   Out<ELFT>::ElfHeader->setSize(sizeof(Elf_Ehdr));
   uintX_t FileOff = 0;
   for (OutputSectionBase<ELFT> *Sec : OutputSections) {
+    if (Sec->getType() != SHT_NOBITS)
       FileOff = alignTo(FileOff, Sec->getAlign());
     Sec->setFileOffset(FileOff);
+    if (Sec->getType() != SHT_NOBITS)
       FileOff += Sec->getSize();
   }
   SectionHeaderOff = alignTo(FileOff, sizeof(uintX_t));
   FileSize = SectionHeaderOff + getNumSections() * sizeof(Elf_Shdr);
 }
 
 // Visits all headers in PhdrTable and assigns the adresses to
 // the output sections. Also creates common and special headers.
 template <class ELFT> void Writer<ELFT>::assignAddresses() {

test/ELF/relocatable-bss.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t1.o
+# RUN: ld.lld -r %t1.o -o %t
+# RUN: llvm-readobj -file-headers -sections -program-headers -symbols -r %t | FileCheck %s
+
+## We check here that .bss does not occupy the space in file.
+## If it would, the SectionHeaderOffset would have offset about 5 megabytes.
+# CHECK:       ElfHeader {
+# CHECK-NEXT:  Ident {
+# CHECK-NEXT:    Magic: (7F 45 4C 46)
+# CHECK-NEXT:    Class: 64-bit
+# CHECK-NEXT:    DataEncoding: LittleEndian
+# CHECK-NEXT:    FileVersion: 1
+# CHECK-NEXT:    OS/ABI: SystemV
+# CHECK-NEXT:    ABIVersion: 0
+# CHECK-NEXT:    Unused: (00 00 00 00 00 00 00)
+# CHECK-NEXT:  }
+# CHECK-NEXT:  Type: Relocatable
+# CHECK-NEXT:  Machine: EM_X86_64
+# CHECK-NEXT:  Version:
+# CHECK-NEXT:  Entry:
+# CHECK-NEXT:   ProgramHeaderOffset:
+# CHECK-NEXT:   SectionHeaderOffset: 0xA8
+# CHECK-NEXT:  Flags [
+# CHECK-NEXT:  ]
+# CHECK-NEXT:  HeaderSize:
+# CHECK-NEXT:  ProgramHeaderEntrySize:
+# CHECK-NEXT:  ProgramHeaderCount:
+# CHECK-NEXT:  SectionHeaderEntrySize:
+# CHECK-NEXT:  SectionHeaderCount:
+# CHECK-NEXT:  StringTableSectionIndex:
+# CHECK-NEXT:  }
+
+.text
+.globl _start;
+_start:
+ nop
+
+.bss
+ .space 5242880

test/ELF/relocatable.s

 # CHECK-NEXT:    ABIVersion: 0
 # CHECK-NEXT:    Unused: (00 00 00 00 00 00 00)
 # CHECK-NEXT:  }
 # CHECK-NEXT:  Type: Relocatable
 # CHECK-NEXT:  Machine: EM_X86_64
 # CHECK-NEXT:  Version: 1
 # CHECK-NEXT:  Entry: 0x0
 # CHECK-NEXT:  ProgramHeaderOffset: 0x0
-# CHECK-NEXT:  SectionHeaderOffset: 0x2D8
+# CHECK-NEXT:  SectionHeaderOffset: 0x2C0
 # CHECK-NEXT:  Flags [
 # CHECK-NEXT:  ]
 # CHECK-NEXT:  HeaderSize: 64
 # CHECK-NEXT:  ProgramHeaderEntrySize: 0
 # CHECK-NEXT:  ProgramHeaderCount: 0
 # CHECK-NEXT:  SectionHeaderEntrySize: 64
 # CHECK-NEXT:  SectionHeaderCount: 7
 # CHECK-NEXT:  StringTableSectionIndex: 5