Skeleton for building a ELF loader

Introduction

This project contains a skeleton for building an ELF binary on-demand loader in Linux. The loader will provide two methods, defined in the loader.h header:

• int so_init_loader(void); - initializes the on-demand loader
• int so_execute(char *path, char *argv[]); - executes the binary located in path with the required argv arguments.

Content

The project contains of three components, each of them in its own directory:

• loader - a dynamic library that can be used to run ELF binaries. It consists of the following files:
  • exec_parser.c - Implements an ELF binary parser.
  • exec_parser.h - The header exposed by the ELF parser.
  • loader.h - The interface of the loader, described in the Introduction section.
  • loader.c - Contains functions to implement the loader:
    • 'so_execute' - call so_execute function which parse an executable speficied in path; - open the file specified in path; - allocate memory for "data" parameter of a segment (data is implemented as a data structure which contains an array of pages with values 0 and 1, that tells if a page has been mapped or not, and an integer that stores the number of pages) and for each array of pages; - run the first instruction of the executable.
    • 'so_init_loader' - initializes the loader and calls the handler for SIGSEGV signal.
    • 'segv_handler' - contains the actual loader impelentation; - verify if the received signal is SIGSEV, if not - call the default handler; - call function 'find_segment_with_segv' and store in variable index_segment the index of the segment where SIGSEGV happened - knowing the segment, find the page where SIGSEGV happned; - knowing the page, verify if it's mapped, if it is - call the default handler - if the page is not mapped, use the mmap function to do so, and copy the information from the file opened in so_execute function (three cases: first when the whole page is copied, second when just a part of it is copied, third when there's nothing to be copied) and set the permissions
  • debug.h - header for the dprintf function that can be used for logging and debugging.

Notes

This skeleton is provided by the Operating System team from the University Politehnica of Bucharest to their students to help them complete their Executable Loader assignment.