Reading the program header contents of an ELF file

Question

How is it possible to extract loadable program headers individually from ELF files? By examining a binary using readelf one can get output similar to:

$ readelf -l helloworld

Elf file type is EXEC (Executable file)
Entry point 0x400440
There are 9 program headers, starting at offset 64

Program Headers:
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flags  Align
  PHDR           0x0000000000000040 0x0000000000400040 0x0000000000400040
                 0x00000000000001f8 0x00000000000001f8  R E    8
  INTERP         0x0000000000000238 0x0000000000400238 0x0000000000400238
                 0x000000000000001c 0x000000000000001c  R      1
      [Requesting program interpreter: /lib64/ld-linux-x86-64.so.2]
  LOAD           0x0000000000000000 0x0000000000400000 0x0000000000400000
                 0x000000000000070c 0x000000000000070c  R E    200000
  LOAD           0x0000000000000e10 0x0000000000600e10 0x0000000000600e10
                 0x0000000000000230 0x0000000000000238  RW     200000
  DYNAMIC        0x0000000000000e28 0x0000000000600e28 0x0000000000600e28
                 0x00000000000001d0 0x00000000000001d0  RW     8
  NOTE           0x0000000000000254 0x0000000000400254 0x0000000000400254
                 0x0000000000000044 0x0000000000000044  R      4
  GNU_EH_FRAME   0x00000000000005e4 0x00000000004005e4 0x00000000004005e4
                 0x0000000000000034 0x0000000000000034  R      4
  GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
                 0x0000000000000000 0x0000000000000000  RW     10
  GNU_RELRO      0x0000000000000e10 0x0000000000600e10 0x0000000000600e10
                 0x00000000000001f0 0x00000000000001f0  R      1

 Section to Segment mapping:
  Segment Sections...
   00     
   01     .interp 
   02     .interp .note.ABI-tag .note.gnu.build-id .gnu.hash .dynsym .dynstr .gnu.version .gnu.version_r .rela.dyn .rela.plt .init .plt .text .fini .rodata .eh_frame_hdr .eh_frame 
   03     .init_array .fini_array .jcr .dynamic .got .got.plt .data .bss 
   04     .dynamic 
   05     .note.ABI-tag .note.gnu.build-id 
   06     .eh_frame_hdr 
   07     
   08     .init_array .fini_array .jcr .dynamic .got 

This question answers how loadable headers are being mapped to memory(and where) but does not specify from where(from which offset and size) are the sections read within the given binary.
Is it determined by the current program header's fields p_offset and p_filesz?

Answer 1

Quoting man elf (but you can find the same info from the ELF spec):

p_offset: This member holds the offset from the beginning of the file at which the first byte of the segment resides.

p_filesz: This member holds the number of bytes in the file image of the segment. It may be zero.

p_memsz: This member holds the number of bytes in the memory image of the segment. It may be zero.

To summarize:

• p_offset is the offset of the segment in the ELF file;

• p_memsz is the size of segment in memory;

• p_filesz is the size of the segment in the file.

The size in the file may be smaller than than the size in memory p_memsz: in this case the remaining part of the segment is filled with zersos (on POSIX-ish systems this part is mmap-ped using MAP_ANONYMOUS).

References from GNU ld.so:

• where the mmap are done;

• where some of the data is computed for the program header.

You might be interested in the code of elfcat (a tool I wrote) which dumps a given segment to stdout.