For each process, the virtual address space looks like this:
------------------ 2^32 - 1
Stack (grows down)
v v v v v v v v v
------------------
(unmapped)
------------------ Maximum stack size.
(unmapped)
-------------------
mmap
-------------------
(unmapped)
-------------------
^^^^^^^^^^^^^^^^^^^
brk (grows up)
-------------------
BSS
-------------------
Data
-------------------
Text
-------------------
------------------- 0The kernel maintains a list of pages that belong to each process, and synchronizes that with the paging.
If the program accesses memory that does not belong to it, the kernel handles a page-fault, and decides what to do:
When an ELF file is loaded by the kernel to start a program with the
exec system call, the kernel automatically registers text, data, BSS and stack for the program.The
brk and mmap areas can be modified by request of the program through the brk and mmap system calls. But the kernel can also deny the program those areas if there is not enough memory.brk and mmap can be used to implement malloc, or the so called "heap".mmap is also used to load dynamically loaded libraries into the program's memory so that it can access and run it.Stack allocation: stackoverflow.com/questions/17671423/stack-allocation-for-process
Calculating exact addresses Things are complicated by:
- Address Space Layout Randomization.
- the fact that environment variables, CLI arguments, and some ELF header data take up initial stack space: unix.stackexchange.com/questions/145557/how-does-stack-allocation-work-in-linux/239323#239323
Why the text does not start at 0: stackoverflow.com/questions/14795164/why-do-linux-program-text-sections-start-at-0x0804800-and-stack-tops-start-at-0
Minimal example: github.com/cirosantilli/x86-bare-metal-examples/blob/5c672f73884a487414b3e21bd9e579c67cd77621/paging.S
Like everything else in programming, the only way to really understand this is to play with minimal examples.
Change password without access:
Enable SSH on boot:
sudo touch /boot/ssh
Research group of the Department of physics of the University of Oxford Updated 2025-04-24 +Created 1970-01-01
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