Skip to content

Commit

Permalink
ELF part 1
Browse files Browse the repository at this point in the history 
Signed-off-by: innocentzero <[email protected]>
  • Loading branch information
@InnocentZero
InnocentZero committed Jan 8, 2025
1 parent 512f8c6 commit 21b9018
Show file tree
Hide file tree
Showing 2 changed files with 180 additions and 1 deletion.
9 changes: 8 additions & 1 deletion content/_typos.toml
View file
Original file line number Diff line number Diff line change
@@ -1,3 +1,10 @@
[default]
extend-ignore-identifiers-re = [
# *sigh* this just isn't worth the cost of fixing
"0+.*",
"0x.*"
]
[default.extend-words]
# Don't correct the surname "Teh"
IIT = "IIT"
RELA = "RELA"
rela = ".rela"
172 changes: 172 additions & 0 deletions content/posts/understanding_elfs.md
View file
Original file line number Diff line number Diff line change
@@ -0,0 +1,172 @@
+++
title = "Understanding ELFs, part 1"
date = 2025-01-08
authors = ["InnocentZero"]
+++

In this post we analyze the header and sections of an ELF binary on disk.

## The header

ELF files have a header section that can be read with `readelf -h executable` which gives you quite
a bit of information about the binary.

```
ELF Header:
Magic: 7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00
Class: ELF64
Data: 2's complement, little endian
Version: 1 (current)
OS/ABI: UNIX - System V
ABI Version: 0
Type: DYN (Position-Independent Executable file)
Machine: Advanced Micro Devices X86-64
Version: 0x1
Entry point address: 0x1040
Start of program headers: 64 (bytes into file)
Start of section headers: 13520 (bytes into file)
Flags: 0x0
Size of this header: 64 (bytes)
Size of program headers: 56 (bytes)
Number of program headers: 13
Size of section headers: 64 (bytes)
Number of section headers: 30
Section header string table index: 29
```

Needless to say, a lot of this is just metadata about the binary that is read by the OS to load
the binary.

## The sections
ELF sections comprise all all the information that is needed to build an executable from an
object file. They are only needed during compile time and not runtime. However, some of these
sections may get mapped to segments during runtime. `readelf -S executable` tells you the
sections.

Some of the more important ones are:

- `.text`: The instructions of the binary are contained here. They are executed and `rip` moves
through this section.
- `.data/.rodata`: This are the sections that contain initialized global data. _ro_ stands for
read-only.
- `.bss`: This is the section for uninitialized global variables.
- `.interp`: This holds the runtime linker, also known as the /interpreter/ of the program.
- Some linker scripts may also contain preallocated space for stack and heap, although it's not
really the job of ELF sections to define them.

For an example /hello world/ binary in C, the following was the output for `readelf -S`

```
There are 30 section headers, starting at offset 0x34d0:
Section Headers:
[Nr] Name Type Address Offset
Size EntSize Flags Link Info Align
[ 0] NULL 0000000000000000 00000000
0000000000000000 0000000000000000 0 0 0
[ 1] .interp PROGBITS 0000000000000318 00000318
000000000000001c 0000000000000000 A 0 0 1
[ 2] .note.gnu.pr[...] NOTE 0000000000000338 00000338
0000000000000040 0000000000000000 A 0 0 8
[ 3] .note.gnu.bu[...] NOTE 0000000000000378 00000378
0000000000000024 0000000000000000 A 0 0 4
[ 4] .note.ABI-tag NOTE 000000000000039c 0000039c
0000000000000020 0000000000000000 A 0 0 4
[ 5] .gnu.hash GNU_HASH 00000000000003c0 000003c0
000000000000001c 0000000000000000 A 6 0 8
[ 6] .dynsym DYNSYM 00000000000003e0 000003e0
00000000000000a8 0000000000000018 A 7 1 8
[ 7] .dynstr STRTAB 0000000000000488 00000488
000000000000008f 0000000000000000 A 0 0 1
[ 8] .gnu.version VERSYM 0000000000000518 00000518
000000000000000e 0000000000000002 A 6 0 2
[ 9] .gnu.version_r VERNEED 0000000000000528 00000528
0000000000000030 0000000000000000 A 7 1 8
[10] .rela.dyn RELA 0000000000000558 00000558
00000000000000c0 0000000000000018 A 6 0 8
[11] .rela.plt RELA 0000000000000618 00000618
0000000000000018 0000000000000018 AI 6 23 8
[12] .init PROGBITS 0000000000001000 00001000
000000000000001b 0000000000000000 AX 0 0 4
[13] .plt PROGBITS 0000000000001020 00001020
0000000000000020 0000000000000010 AX 0 0 16
[14] .text PROGBITS 0000000000001040 00001040
0000000000000141 0000000000000000 AX 0 0 16
[15] .fini PROGBITS 0000000000001184 00001184
000000000000000d 0000000000000000 AX 0 0 4
[16] .rodata PROGBITS 0000000000002000 00002000
0000000000000015 0000000000000000 A 0 0 4
[17] .eh_frame_hdr PROGBITS 0000000000002018 00002018
0000000000000024 0000000000000000 A 0 0 4
[18] .eh_frame PROGBITS 0000000000002040 00002040
000000000000007c 0000000000000000 A 0 0 8
[19] .init_array INIT_ARRAY 0000000000003dd0 00002dd0
0000000000000008 0000000000000008 WA 0 0 8
[20] .fini_array FINI_ARRAY 0000000000003dd8 00002dd8
0000000000000008 0000000000000008 WA 0 0 8
[21] .dynamic DYNAMIC 0000000000003de0 00002de0
00000000000001e0 0000000000000010 WA 7 0 8
[22] .got PROGBITS 0000000000003fc0 00002fc0
0000000000000028 0000000000000008 WA 0 0 8
[23] .got.plt PROGBITS 0000000000003fe8 00002fe8
0000000000000020 0000000000000008 WA 0 0 8
[24] .data PROGBITS 0000000000004008 00003008
0000000000000010 0000000000000000 WA 0 0 8
[25] .bss NOBITS 0000000000004018 00003018
0000000000000008 0000000000000000 WA 0 0 1
[26] .comment PROGBITS 0000000000000000 00003018
0000000000000036 0000000000000001 MS 0 0 1
[27] .symtab SYMTAB 0000000000000000 00003050
0000000000000240 0000000000000018 28 6 8
[28] .strtab STRTAB 0000000000000000 00003290
000000000000012a 0000000000000000 0 0 1
[29] .shstrtab STRTAB 0000000000000000 000033ba
0000000000000116 0000000000000000 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
L (link order), O (extra OS processing required), G (group), T (TLS),
C (compressed), x (unknown), o (OS specific), E (exclude),
D (mbind), l (large), p (processor specific)
```


- `Nr`, `Name`, and `Size` should be obvious.
- `EntSize` contains the size of the entries, if the entries in the section have fixed sizes. Like
symbol tables.
- `Type` will be explained later. `Address` contains the starting address of the section in the
binary. This depends on the previous sections and the alignment requirements of the section.
- `Offset` and `Align` should also be obvious. The fields below `Address` will be explained below.

The types of sections:

- `NULL`: This marks an empty section. It is the first section of the binary for demarcation
purposes. Acts as a placeholder.
- `PROGBITS`: These just have program-defined info, like the instructions (`.text`), the global
data (`.data/.rodata`), the `.interp` section (defines the interpreter).
- `DYNAMIC`: Holds dynamic linking information. It is actually a dynamic table that has tags
and name/value mapping of sorts that helps the runtime linker load shared libs and stuff.
- `INIT_ARRAY`: This contains an array of pointers to functions that must be executed before
`main`. Only for `.init_array`.
- `FINI_ARRAY`: This contains an array of pointers to functions that must be executed on `exit`.
Only for `.fini_array`.
- `GNU_HASH`: This is a sort of hash table for faster symbol lookup used by the dynamic linker.
Used for `.gnu.hash`
- `NOBITS`: Used for `.bss`, which is *zeroed out* upon loading. This contains the section having
undefined global variables.
- `DYNSYM`: Used for `.dynsym` section, contains the dynamic symbol table.
- `STRTAB`: As the name suggests, it contains a string table. Usually it's indexed. For sections
`.strtab` and `.dynstr`, which are obviously static and dynamic string tables.
- `SYMTAB`: Once again, symbol table for `.symtab`. Larger than `.dynsym` as it's more detailed, but
not required at runtime.
- `RELA`: Contains reloc tables. These specify how to to modify certain addresses in the program
to account for the layout of shared libraries or changes in addresses during linking.

I'm not covering specific sections like `.got` and `.plt` in detail as they require a separate post
of their own.

The flags for each section have been added below in the `readelf` output.

The link section is an index to another section to indicate a dependency.

The info section is an index to another section to indicate additional
information.

0 comments on commit 21b9018

Please sign in to comment.