Where is the “Section to segment mapping” stored i

问题:

As part of trying to write a compiler completely from scratch, I'm currently working on the part the handles ELF files.

After skimming through several articles and specifications about them, I still don't quite understand where section to segment mappings are stored. When observing small executables generated by NASM+ld, I can see that the .text section is somehow mapped onto a LOAD-type program header, but how?

A small piece of readelf's output when given a small (working) executable as input:

Program Headers:
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flags  Align
  LOAD           0x0000000000000000 0x0000000000400000 0x0000000000400000
                 0x0000000000000084 0x0000000000000084  R E    200000

 Section to Segment mapping:
  Segment Sections...
   00     .text 

Is this mapping even required to have a working executable? Or can they be omitted completely and you would still have a valid executable?

回答1:

I still don't quite understand where section to segment mappings are stored.

They are not stored anywhere.

Rather, readelf computes the mapping by looking at file offset and size of sections and segments.

回答2:

I did a test according to the @Employed Russian.

readelf -l ./libandroid_servers.so

Elf file type is DYN (Shared object file)
Entry point 0x0
There are 6 program headers, starting at offset 52

Program Headers:
  Type           Offset   VirtAddr   PhysAddr   FileSiz MemSiz  Flg Align
  PHDR           0x000034 0x00000034 0x00000034 0x000c0 0x000c0 R   0x4
  LOAD           0x000000 0x00000000 0x00000000 0x0f830 0x0f830 R E 0x1000
  LOAD           0x010000 0x00010000 0x00010000 0x00cf4 0x011ac RW  0x1000
  DYNAMIC        0x010540 0x00010540 0x00010540 0x00130 0x00130 RW  0x4
  GNU_STACK      0x000000 0x00000000 0x00000000 0x00000 0x00000 RW  0
  EXIDX          0x00f2e8 0x0000f2e8 0x0000f2e8 0x00548 0x00548 R   0x4

 Section to Segment mapping:
  Segment Sections...
   00     
   01     .hash .dynsym .dynstr .rel.plt .rel.dyn .plt .text .rodata .ARM.extab .ARM.exidx 
   02     .init_array .fini_array .data.rel.ro .dynamic .got .data .bss 
   03     .dynamic 
   04     
   05     .ARM.exidx 

01 LOAD offset： 0x000000 fileSize 0x0f830
.ARM.exidx section end addr： hex(0xF2E8 + 1352) = 0xf830

02 LOAD offset: 0x010000 fileSize: 0x00cf4
.init_array section begin addr： 10000h
.bss section end addr： hex(0x10cf4 + 0 ) = 0x10cf4

You see the readelf surely print the sections in a segments by computes. They match well.