Implement a Simple Program Loader

Topic Overview

OS Loaders Map Programs into Memory

The kernel's loader (ELF interpreter ld-linux-x86-64.so.2, Windows PE loader) reads the executable header, maps segments into virtual memory, sets permissions (RX for code, RW for data), and jumps to the entry point.

Load Process

Parse ELF PT_LOAD segments. mmap each at preferred or ASLR-randomized address. Zero BSS. Apply relocations for PIE. Transfer control to _start, which calls __libc_start_main then main.

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• ASLR randomizes base addresses per run
• Stack and heap grow after mapped segments
• Interpreter itself is loaded for dynamic binaries
• execve is the syscall that starts loading

Connecting to real compilers

Production compilers embed this step inside a longer pipeline. GCC flows through cpp, cc1, assembly, and ld; Clang uses the driver, Sema, LLVM IR passes, and a target backend. LLVM bitcode, JVM bytecode, and WASM are other familiar IRs at the same layer. The exercise isolates one pass so you can test it alone before chaining it to the next stage.

Why for this exercise

You will implement a simple program loader that maps segments and jumps to entry. This exercise requires parsing load headers, simulating memory mapping, and transferring control to the program start address.

Your Task

Play the role of the kernel's ELF loader: map an executable's LOAD segments into memory page by page with the right permissions, copy their bytes from the file, zero-fill the part that exists only in memory (.bss), check memory accesses against the result, and build the initial System V process stack the program starts with.

Input (any order)

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• Pages are 4096 bytes. A segment maps every page from vaddr rounded down to vaddr + memsz rounded up, with permissions r/w/x from its flags. Two segments mapping the same page is an error: print error: segments overlap at page 0xADDR and stop.
• The first filesz bytes come from the file at offset; the remaining memsz - filesz bytes (and the rest of each page) are zero.

Stack (System V AMD64)

With the stack top at 0x7ffffffff000:

1. The argument strings then the environment strings, each with its terminating NUL, are packed together in that order so the last byte ends right below the top.
2. Below them go 8-byte words: argc, the argv pointers, a NULL, the envp pointers, a NULL, and the auxiliary vector terminator AT_NULL (two zero words). rsp is placed so these words end at or below the start of the strings, and rounded down to a multiple of 16.

Output

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Reads and writes are printed in input order, after all segments are mapped.

Example

Input:

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Output:

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Requirements

1. A page table (array of mapped pages with permissions and 4 KiB of contents each).
2. Compute the stack layout from sizes first, then print it top-down from rsp.

Hidden tests cover a .bss that spans several pages, reads crossing into an unmapped page, an entry point in a non-executable segment, overlapping segments, and several environment variables.