segments within a executable C program

Question

I was reading about sections and segments. Seems you could list the mapping between sections and segments as below.

$ readelf -l test

Elf file type is EXEC (Executable file)
Entry point 0x8048330
There are 9 program headers, starting at offset 52

Program Headers:
  Type           Offset   VirtAddr   PhysAddr   FileSiz MemSiz  Flg Align
  PHDR           0x000034 0x08048034 0x08048034 0x00120 0x00120 R E 0x4
  INTERP         0x000154 0x08048154 0x08048154 0x00013 0x00013 R   0x1
      [Requesting program interpreter: /lib/ld-linux.so.2]
  LOAD           0x000000 0x08048000 0x08048000 0x0065c 0x0065c R E 0x1000
  LOAD           0x000f14 0x08049f14 0x08049f14 0x00104 0x00110 RW  0x1000
  DYNAMIC        0x000f28 0x08049f28 0x08049f28 0x000c8 0x000c8 RW  0x4
  NOTE           0x000168 0x08048168 0x08048168 0x00044 0x00044 R   0x4
  GNU_EH_FRAME   0x000564 0x08048564 0x08048564 0x00034 0x00034 R   0x4
  GNU_STACK      0x000000 0x00000000 0x00000000 0x00000 0x00000 RW  0x4
  GNU_RELRO      0x000f14 0x08049f14 0x08049f14 0x000ec 0x000ec R   0x1

 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 .rel.dyn .rel.plt .init .plt .text .fini .rodata .eh_frame_hdr .eh_frame 
   03     .ctors .dtors .jcr .dynamic .got .got.plt .data .bss 
   04     .dynamic 
   05     .note.ABI-tag .note.gnu.build-id 
   06     .eh_frame_hdr 
   07     
   08     .ctors .dtors .jcr .dynamic .got

My questions,

1. I couldn't understand what the program headers mean? How are they related to segments?
2. Section to segment mapping is clear. But could someone name it? I see only numbers. I identified the code seg (03), data seg (02) and stack (07).

Answer 1

Program headers in an ELF binary describe how the binary should be run. The interesting parts are the LOAD headers which load part of the binary into different places in memory. There could be almost arbitrary number of LOAD headers in a binary, but usually the linker puts everything read-only and executable into one and everything read/write into another. There are operating systems which will have read-only data LOAD header, read-write data and read-only executable code for slightly increased security.

Segments here just mean parts of the binary loaded in different places in memory. So basically the different LOAD headers.

Sections is how the data was organized during linking. For various reasons you want to have better granularity organizing things than just data/code. Some data is read-only, it's put in ".rodata" in your example. The code is in ".text", initialized data is in ".data" while data in variables that are zeroed on program start are in ".bss".

The "section to segment mapping" tells you which sections are in which segments (different LOAD headers). So ".text" and ".rodata" are in the first LOAD header (the third program header) and ".data" is in the second LOAD header (fourth program header).

The stack is something that the operating system gives you on execution and it's not described by an ELF binary.

Answer 2

To understand the output of readelf it will help for you to understand the format of an ELF file. Please reference this document.

As far as understanding how to interpret the output of readelf this link may be of help.

As to your question 2, this link describes the segments. In that document search for "Various sections hold program and control information:" to find the area where the segment names are described.

That document describes the segments as follows:

Various sections hold program and control information:

   .bss      This section holds uninitialized data that contributes to the
             program's memory image.  By definition, the system initializes the
             data with zeros when the program begins to run.  This section is of
             type SHT_NOBITS.  The attribute types are SHF_ALLOC and SHF_WRITE.

   .comment  This section holds version control information.  This section is of
             type SHT_PROGBITS.  No attribute types are used.

   .ctors    This section holds initialized pointers to the C++ constructor
             functions.  This section is of type SHT_PROGBITS.  The attribute
             types are SHF_ALLOC and SHF_WRITE.

   .data     This section holds initialized data that contribute to the program's
             memory image.  This section is of type SHT_PROGBITS.  The attribute
             types are SHF_ALLOC and SHF_WRITE.

   .data1    This section holds initialized data that contribute to the program's
             memory image.  This section is of type SHT_PROGBITS.  The attribute
             types are SHF_ALLOC and SHF_WRITE.

   .debug    This section holds information for symbolic debugging.  The contents
             are unspecified.  This section is of type SHT_PROGBITS.  No
             attribute types are used.

   .dtors    This section holds initialized pointers to the C++ destructor
             functions.  This section is of type SHT_PROGBITS.  The attribute
             types are SHF_ALLOC and SHF_WRITE.

   .dynamic  This section holds dynamic linking information.  The section's
             attributes will include the SHF_ALLOC bit.  Whether the SHF_WRITE
             bit is set is processor-specific.  This section is of type
             SHT_DYNAMIC.  See the attributes above.

   .dynstr   This section holds strings needed for dynamic linking, most commonly
             the strings that represent the names associated with symbol table
             entries.  This section is of type SHT_STRTAB.  The attribute type
             used is SHF_ALLOC.

   .dynsym   This section holds the dynamic linking symbol table.  This section
             is of type SHT_DYNSYM.  The attribute used is SHF_ALLOC.

   .fini     This section holds executable instructions that contribute to the
             process termination code.  When a program exits normally the system
             arranges to execute the code in this section.  This section is of
             type SHT_PROGBITS.  The attributes used are SHF_ALLOC and
             SHF_EXECINSTR.

   .gnu.version
             This section holds the version symbol table, an array of ElfN_Half
             elements.  This section is of type SHT_GNU_versym.  The attribute
             type used is SHF_ALLOC.

   .gnu.version_d
             This section holds the version symbol definitions, a table of
             ElfN_Verdef structures.  This section is of type SHT_GNU_verdef.
             The attribute type used is SHF_ALLOC.

   .gnu.version_r
             This section holds the version symbol needed elements, a table of
             ElfN_Verneed structures.  This section is of type SHT_GNU_versym.
             The attribute type used is SHF_ALLOC.

   .got      This section holds the global offset table.  This section is of type
             SHT_PROGBITS.  The attributes are processor specific.

   .hash     This section holds a symbol hash table.  This section is of type
             SHT_HASH.  The attribute used is SHF_ALLOC.

   .init     This section holds executable instructions that contribute to the
             process initialization code.  When a program starts to run the
             system arranges to execute the code in this section before calling
             the main program entry point.  This section is of type SHT_PROGBITS.
             The attributes used are SHF_ALLOC and SHF_EXECINSTR.

   .interp   This section holds the pathname of a program interpreter.  If the
             file has a loadable segment that includes the section, the section's
             attributes will include the SHF_ALLOC bit.  Otherwise, that bit will
             be off.  This section is of type SHT_PROGBITS.

   .line     This section holds line number information for symbolic debugging,
             which describes the correspondence between the program source and
             the machine code.  The contents are unspecified.  This section is of
             type SHT_PROGBITS.  No attribute types are used.

   .note     This section holds information in the "Note Section" format.  This
             section is of type SHT_NOTE.  No attribute types are used.  OpenBSD
             native executables usually contain a .note.openbsd.ident section to
             identify themselves, for the kernel to bypass any compatibility ELF
             binary emulation tests when loading the file.

   .note.GNU-stack
             This section is used in Linux object files for declaring stack
             attributes.  This section is of type SHT_PROGBITS.  The only
             attribute used is SHF_EXECINSTR.  This indicates to the GNU linker
             that the object file requires an executable stack.

   .plt      This section holds the procedure linkage table.  This section is of
             type SHT_PROGBITS.  The attributes are processor specific.

   .relNAME  This section holds relocation information as described below.  If
             the file has a loadable segment that includes relocation, the
             section's attributes will include the SHF_ALLOC bit.  Otherwise the
             bit will be off.  By convention, "NAME" is supplied by the section
             to which the relocations apply.  Thus a relocation section for .text
             normally would have the name .rel.text.  This section is of type
             SHT_REL.

   .relaNAME This section holds relocation information as described below.  If
             the file has a loadable segment that includes relocation, the
             section's attributes will include the SHF_ALLOC bit.  Otherwise the
             bit will be off.  By convention, "NAME" is supplied by the section
             to which the relocations apply.  Thus a relocation section for .text
             normally would have the name .rela.text.  This section is of type
             SHT_RELA.

   .rodata   This section holds read-only data that typically contributes to a
             nonwritable segment in the process image.  This section is of type
             SHT_PROGBITS.  The attribute used is SHF_ALLOC.

   .rodata1  This section holds read-only data that typically contributes to a
             nonwritable segment in the process image.  This section is of type
             SHT_PROGBITS.  The attribute used is SHF_ALLOC.

   .shstrtab This section holds section names.  This section is of type
             SHT_STRTAB.  No attribute types are used.

   .strtab   This section holds strings, most commonly the strings that represent
             the names associated with symbol table entries.  If the file has a
             loadable segment that includes the symbol string table, the
             section's attributes will include the SHF_ALLOC bit.  Otherwise the
             bit will be off.  This section is of type SHT_STRTAB.

   .symtab   This section holds a symbol table.  If the file has a loadable
             segment that includes the symbol table, the section's attributes
             will include the SHF_ALLOC bit.  Otherwise the bit will be off.
             This section is of type SHT_SYMTAB.

   .text     This section holds the "text", or executable instructions, of a
             program.  This section is of type SHT_PROGBITS.  The attributes used
             are SHF_ALLOC and SHF_EXECINSTR.