Common Sections

Why This Matters

When you run readelf -S, you see dozens of sections. Understanding the most common ones lets you navigate any ELF file and know what you're looking at.

A Simple Model

Sections are like labeled boxes in a warehouse:

.text — The code (executable instructions)
.data — Initialized global/static variables
.bss — Uninitialized globals (zero-filled at runtime)
.rodata — Read-only data (string constants, jump tables)
.symtab — Symbol table (for debugging)
.strtab — String table (names for symbols)
.dynsym — Dynamic symbol table (for linking)
.dynstr — Dynamic string table
.rela.text — Relocations for code
.debug_* — DWARF debugging info

The Actual Detail

Section	Type	Flags	Contains
.text	SHT_PROGBITS	SHF_ALLOC + SHF_EXECINSTR	Machine code
.rodata	SHT_PROGBITS	SHF_ALLOC	Read-only constants
.data	SHT_PROGBITS	SHF_ALLOC + SHF_WRITE	Initialized global variables
.bss	SHT_NOBITS	SHF_ALLOC + SHF_WRITE	Uninitialized globals (zeroed)
.symtab	SHT_SYMTAB	none	All symbols (for debugging)
.strtab	SHT_STRTAB	none	Symbol names
.dynsym	SHT_DYNSYM	SHF_ALLOC	Dynamic symbols (for linking)
.dynstr	SHT_STRTAB	SHF_ALLOC	Dynamic symbol names
.rela.text	SHT_RELA	SHF_ALLOC	Code relocations
.init	SHT_PROGBITS	SHF_ALLOC + SHF_EXECINSTR	Initialization code
.fini	SHT_PROGBITS	SHF_ALLOC + SHF_EXECINSTR	Cleanup code

Note: .bss has type SHT_NOBITS — it takes no space in the file but is zero-filled in memory.

A Real Example

$ readelf -S /bin/ls | grep -E '\.(text|data|bss|rodata|symtab|strtab)'
  [11] .text             PROGBITS   0000000000001000  00001000  001a52
  [17] .rodata           PROGBITS   0000000000003000  00003000  000ba0
  [24] .data             PROGBITS   0000000000004dc0  00003dc0  000020
  [25] .bss              NOBITS     0000000000004de0  00003de0  000040
  [26] .symtab           SYMTAB     0000000000000000  00003e28  000c90

Check Your Understanding

Why does .bss have type SHT_NOBITS?

Apply It

If .rodata and .text are in the same PT_LOAD segment, what permissions does the kernel set for .rodata?

Connect

You now know the common sections. But remember — sections and segments are different things. The next concept clarifies this distinction.

Match Code to Output

Match each section type to its correct description:

.text

.data

.bss

.rodata

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