How to Use the 'readelf' Command (with examples)

How to Use the 'readelf' Command (with examples)

The readelf command is a powerful utility in Unix-based systems designed to display information about ELF (Executable and Linkable Format) files. ELF is a common standard file format for executables, object code, shared libraries, and core dumps in Unix-like operating systems. By using readelf, developers and system administrators can dissect and analyze these ELF files, gaining insights into their structure and contents. This command is particularly useful for debugging, development, and system auditing tasks.

Display All Information About the ELF File

Code:

readelf -all path/to/binary

Motivation:

The command readelf -all path/to/binary is used when you need a comprehensive overview of an ELF file. If you’re dealing with debugging processes or conducting an in-depth analysis, being aware of every aspect of the ELF file can help in understanding how the file is structured, what components it comprises, and how it’s supposed to function within the system.

Explanation:

  • readelf: This is the command used to read and interpret ELF files.
  • -all: This option tells readelf to display all available information about the ELF file. It is a comprehensive switch that encompasses all the various sections and elements of the ELF file.

Example Output:

ELF Header:
  Magic:   7f 45 4c 46 ...
  Class:                             ELF64
  Data:                              2's complement, little endian
  Version:                           1 (current)
  OS/ABI:                            UNIX - System V
  ABI Version:                       0
  Type:                              DYN (Shared object file)
  Machine:                           Advanced Micro Devices X86-64
  ...

Section Headers:
  [Nr] Name              Type            Address          Offset
       Size              EntSize         Flags  Link  Info  Align
  [ 0] .interp           PROGBITS        0000000000000258  00000258
       000000000000001c  0000000000000000   A       0     0     1
  ...

Display All the Headers Present in the ELF File

Code:

readelf --headers path/to/binary

Motivation:

Sometimes you only need the file headers to gain insight into the ELF file’s organization without the clutter of additional information from sections and segments. The --headers option is ideal for quickly checking the structural metadata of an ELF file to ensure it’s correctly formatted or to identify specific headers for closer examination.

Explanation:

  • readelf: The command to interpret ELF files.
  • --headers: This option provides a unified view of both the ELF header and the program header table, offering an overview of the executable image layout.

Example Output:

ELF Header:
  Magic:   7f 45 4c 46 ...
  Class:                             ELF64
  ...
Program Headers:
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flags  Align
  PHDR           0x00000040         0x00400040         0x00400040
                 0x00000038         0x00000038         R E    8
  ...

Display the Entries in Symbol Table Section of the ELF File

Code:

readelf --symbols path/to/binary

Motivation:

When you’re debugging or optimizing, understanding where and how symbols are used in your ELF file can be crucial. Symbols represent functions or variables found within the source code. Using --symbols can help you see all the symbols defined in the symbol table, which can aid in static analysis or when you’re trying to match symbols to specific code definitions.

Explanation:

  • readelf: The command to access ELF files.
  • --symbols: This option specifically extracts and displays entries from the ELF file’s symbol table, showing names, addresses, and other attributes associated with each symbol.

Example Output:

Symbol table '.symtab' contains 64 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     1: 00000000004003c8     0 SECTION LOCAL  DEFAULT    1
     2: 00000000004003e8     0 SECTION LOCAL  DEFAULT    2
     ...

Display ELF Header Information

Code:

readelf --file-header path/to/binary

Motivation:

If you’re interested in the fundamental metadata about the ELF file, the --file-header option provides essential information such as the file type, machine architecture, entry point address, and other vital startup information. This data is particularly useful for verifying the target platform compatibility and confirming the nature of the ELF file.

Explanation:

  • readelf: Accesses the ELF file to provide detailed insights.
  • --file-header: Displays the ELF file header, which includes basic details about the file type and architecture.

Example Output:

ELF Header:
  Magic:   7f 45 4c 46 ...
  Class:                             ELF64
  Data:                              2's complement, little endian
  Version:                           1 (current)
  OS/ABI:                            UNIX - System V
  ABI Version:                       0
  Type:                              EXEC (Executable file)
  Machine:                           Advanced Micro Devices X86-64
  ...

Display ELF Section Header Information

Code:

readelf --section-headers path/to/binary

Motivation:

Each ELF file consists of sections, and understanding these sections can be key to knowing how data and code are organized. The --section-headers option is used for viewing the section headers, which include information about individual sections like their size, type, and location within the file. This is valuable when you want to analyze the nature of each section or trace errors in specific parts of the file.

Explanation:

  • readelf: The command for accessing ELF data.
  • --section-headers: This option extracts and displays information about the file’s sections, including each section’s offset, address, size, and name.

Example Output:

Section Headers:
  [Nr] Name              Type            Address          Offset
       Size              EntSize         Flags  Link  Info  Align
  [ 0] .interp           PROGBITS        0000000000000258  00000258
       000000000000001c  0000000000000000   A       0     0     1
  [ 1] .note.ABI-tag     NOTE            0000000000000274  00000274
       0000000000000020  0000000000000000   A       0     0     4
  ...

Conclusion

The readelf command offers critical insight into ELF files by providing a variety of ways to scrutinize their structure and contents. Whether you’re assessing the overall file integrity, diving into symbol tables, or inspecting specific headers, readelf is an indispensable tool for developers and analysts working with UNIX-based systems. Through these examples, you should have a better understanding of how to leverage readelf for comprehensive binary analysis.

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