Example of linux process memory management and virtual memory.
Created for a course at Blekinge Institute of Technology.

Purpose

The purpose of this example is to provide insight into virtual memory and pointers.

Using only GDB it is possible to dump a process' virtual memory and to visualize the memory layout of a process to give a concrete example of the classic diagram below.

┌─────────────────────┐
│Command line args and│
│environment variables│
├─────────────────────┤
│        Stack        │
│          │          │
│          │          │
│          ▼          │ Grows downwards
├ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ┤ 
│                     │
│                     │
│                     │
├ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ┤
│          ▲          │
│          │          │ Grows upwards
│          │          │
│        Heap         │
├─────────────────────┤
│        BSS          │
├─────────────────────┤
│        DATA         │
├─────────────────────┤
│        TEXT         │
└─────────────────────┘

Usage

Place a breakpoint on line 28 to stop execution at a point when the programme is inside a function with some local data and after some dynamic memory has been allocated and filled.

Execute info proc mappings in GDB or -exec info proc mappings if using GDB through vscode. The output will look something like:

process 6283
Mapped address spaces:

          Start Addr           End Addr       Size     Offset objfile
      0x555555554000     0x555555555000     0x1000        0x0 /home/david/dev/cpp/repos/memoryexplore/main
      0x555555555000     0x555555556000     0x1000     0x1000 /home/david/dev/cpp/repos/memoryexplore/main
      0x555555556000     0x555555557000     0x1000     0x2000 /home/david/dev/cpp/repos/memoryexplore/main
      0x555555557000     0x555555558000     0x1000     0x2000 /home/david/dev/cpp/repos/memoryexplore/main
      0x555555558000     0x555555559000     0x1000     0x3000 /home/david/dev/cpp/repos/memoryexplore/main
      0x555555559000     0x55555557a000    0x21000        0x0 [heap]
      0x7ffff7d84000     0x7ffff7d87000     0x3000        0x0 
      0x7ffff7d87000     0x7ffff7db3000    0x2c000        0x0 /usr/lib/libc.so.6
      0x7ffff7db3000     0x7ffff7f29000   0x176000    0x2c000 /usr/lib/libc.so.6
      0x7ffff7f29000     0x7ffff7f7d000    0x54000   0x1a2000 /usr/lib/libc.so.6
      0x7ffff7f7d000     0x7ffff7f7e000     0x1000   0x1f6000 /usr/lib/libc.so.6
      0x7ffff7f7e000     0x7ffff7f81000     0x3000   0x1f6000 /usr/lib/libc.so.6
      0x7ffff7f81000     0x7ffff7f84000     0x3000   0x1f9000 /usr/lib/libc.so.6
      0x7ffff7f84000     0x7ffff7f93000     0xf000        0x0 
      0x7ffff7fc0000     0x7ffff7fc4000     0x4000        0x0 [vvar]
      0x7ffff7fc4000     0x7ffff7fc6000     0x2000        0x0 [vdso]
      0x7ffff7fc6000     0x7ffff7fc8000     0x2000        0x0 /usr/lib/ld-linux-x86-64.so.2
      0x7ffff7fc8000     0x7ffff7fef000    0x27000     0x2000 /usr/lib/ld-linux-x86-64.so.2
      0x7ffff7fef000     0x7ffff7ffa000     0xb000    0x29000 /usr/lib/ld-linux-x86-64.so.2
      0x7ffff7ffb000     0x7ffff7ffd000     0x2000    0x34000 /usr/lib/ld-linux-x86-64.so.2
      0x7ffff7ffd000     0x7ffff7fff000     0x2000    0x36000 /usr/lib/ld-linux-x86-64.so.2
      0x7ffffffde000     0x7ffffffff000    0x21000        0x0 [stack]
  0xffffffffff600000 0xffffffffff601000     0x1000        0x0 [vsyscall]

The following commands can then be executed to dump pieces of virtual memory to a file:

dump memory heap 0x555555559000 0x55555557a000
dump memory proc 0x555555554000 0x555555559000
dump memory stack 0x7ffffffde000 0x7ffffffff000

Using the output from the currently running programme execution, the exact heap offset of the dynamically allocated memory can be found:

Address of allocated memory: 0x5555555592a0

print /x 0x5555555592a0 - 0x555555559000
$1 = 0x2a0

argc, argv, and environment variables can be found in the end of the stack file, as expected.

Local variables can be found inside the stack file