What is ELF in Linux
What is ELF in Linux?
In Linux, ELF stands for Executable and Linkable Format. It’s the standard file format for executables, object code, shared libraries, and core dumps. Linux and other UNIX-like systems use ELF as their primary format for binary files.
Here’s a breakdown of ELF’s uses and how it works:
1. Executable Files
ELF is the format for binary executables that the Linux operating system can run directly. These files contain machine code that the CPU executes.
2. Object Files
Compilers like gcc generate these intermediate files. They contain code and data not yet linked into a complete program. ELF serves as the format for these files, enabling linking tools like ld to create the final executable.
3. Shared Libraries
ELF files are used for shared libraries (.so files), allowing code reuse across different programs without statically including it in each executable.
4. Core Dumps
When a program crashes, the Linux system may generate a core dump. This ELF file contains the program’s memory and state at the time of the crash, proving useful for debugging.
Structure of an ELF File
An ELF file consists of several sections, each with specific roles:
- Header: Contains information about interpreting the rest of the file.
- Program Header: Describes segments that need loading into memory.
- Section Header: Provides details about individual sections like text (code), data, and symbol tables.
- Text Segment: Contains the actual executable code.
- Data Segment: Contains global variables and dynamic data used by the program.
ELF simplifies program development and execution by providing a unified format for both executables and libraries.
It also supports dynamic linking, allowing programs to use shared libraries at runtime. This reduces memory usage and enables easier updates.
Now that you know what ELF is, you might wonder how to view the details of ELF files. Trust me, it’s simpler than you’d think.
Display Information about ELF Files
You can use several commands and tools in Linux to display information about ELF files. Some of the most common ones are file, readelf, and objdump.
1. Using the file Command
The file command quickly identifies the type of a file, including whether it’s an ELF file, and provides basic information about it.
file <filename>
Example:
file /bin/ls
Sample Output:
/bin/ls: ELF 64-bit LSB pie executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, BuildID[sha1]=15dfff3239aa7c3b16a71e6b2e3b6e4009dab998, for GNU/Linux 3.2.0, stripped
Display Information about ELF Files
2. Using the readelf Command
readelf is a more detailed tool specifically designed for examining the contents of ELF files. You can use it to display headers, section details, and more.
Basic usage:
readelf -h <filename> # Displays ELF header information
Example:
readelf -h /bin/ls
Viewing Details of ELF File using readelf command
You can also use different flags to get more detailed information:
S: Lists the sections in the ELF file.l: Displays the program headers (used by the loader).r: Shows the relocation entries.s: Displays the symbol table (if present).
Example:
readelf -S /bin/ls # Lists all sections
3. Using the objdump Command
objdump is a more comprehensive tool that can disassemble ELF binaries and display information about them. It shows sections, disassembled code, and more.
Basic usage:
objdump -h <filename> # Displays the section headers
Example:
objdump -h /bin/ls
Find Information about ELF File using objdump Command
Other useful flags:
d: Disassembles the file and shows machine code.x: Displays all headers, including the ELF and section headers.s: Displays the contents of all sections (in hexadecimal).
Example:
objdump -d /bin/ls # Disassemble and view the assembly code
Summary of Tools
file: Quick summary of the file type and basic ELF details.readelf: Detailed ELF file structure and headers.objdump: Disassembling and more in-depth inspection of sections and headers.
These tools are typically pre-installed on most Linux distributions. If you need specific information, readelf and objdump will be your most detailed options.
Why ELF is Important in Linux
For the average Linux user, knowing how to examine ELF files using tools like file, readelf, or objdump may not seem essential at first. But, there are practical situations where this knowledge becomes useful. Here’s how it can help in everyday tasks:
1. Identifying File Types and Troubleshooting
Purpose:
Sometimes, a file might have no extension, or its extension could be misleading. Using the file command to determine whether it’s an ELF binary, script, or data file can clarify what kind of file you are dealing with.
Example:
If you downloaded a file and are unsure whether it’s a valid executable or corrupted, file will quickly tell you whether it’s a valid ELF file.
file myfile
If the file is not an ELF executable, the command can guide you in troubleshooting further (e.g., figuring out if it’s a text file or needs different handling).
2. Verifying System Executables
Purpose:
Using readelf or file allows you to inspect system binaries and libraries to verify they are in the expected format. For instance, after a system upgrade or during troubleshooting, you can ensure that your important binaries (e.g., /bin/bash, /bin/ls) are intact and correctly formatted as ELF files.
Example:
If a system utility is acting strangely, checking if the file is valid and has not been corrupted or replaced can help:
file /bin/bash
3. Understanding Program Dependencies
Purpose:
The readelf -l or objdump command helps identify the shared libraries an executable depends on. If a program fails to run due to missing libraries, this information is useful for troubleshooting missing dependencies.
Example:
If a program complains about missing libraries, running:
readelf -d /usr/bin/ls | grep NEEDED
Will show which libraries are required, helping you install any missing ones.
Sample Output:
0x0000000000000001 (NEEDED) Shared library: [libselinux.so.1]
0x0000000000000001 (NEEDED) Shared library: [libc.so.6]
4. Analyzing Security and Permissions
Purpose:
Checking whether a binary is dynamically or statically linked, or whether it has unusual headers, can be useful for advanced users concerned about security.
Example:
If you suspect that a binary has been tampered with or could contain malicious code, inspecting its ELF structure using readelf could give insight into whether it behaves unexpectedly, such as having uncommon sections or unknown dependencies.
5. Debugging and Development
Purpose:
For users doing any kind of development, including scripting or compiling, knowing the ELF structure is useful for debugging. Tools like readelf help ensure that your compiled code links properly, uses the correct libraries, and behaves as expected.
Example:
When compiling your own software, you can inspect object files (.o) or the final binary:
readelf -h myprogram
6. Diagnosing Crashes or Core Dumps
Purpose:
If a program crashes and creates a core dump (an ELF file), you can inspect the core dump to analyze the state of the program at the time of the crash, making it easier to identify the cause of the failure.
Example:
If you want to analyze a core dump, running:
readelf -h <core>
provides a starting point for understanding the crash.
7. Performance Optimization
Purpose:
Advanced users looking to optimize their systems can analyze binaries to see if they’re dynamically linked or statically linked, how many sections are loaded into memory, and other performance-related characteristics.
Example:
Using objdump to inspect the machine code or linked sections of a program can help users or developers identify inefficient code.
For an average Linux user, these commands may not be used daily, but they become handy when troubleshooting system issues, verifying file integrity, understanding program dependencies, or debugging software.
Reference
https://ostechnix.com/elf-in-linux/