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Materials for HTS (Harmonisation Technologique Système)

Author: Gabriel Laskar

Contributor: Alizée Penel

Repository

You must push your code to:

<login>@git.cri.epita.fr:p/2022-sys/hts-<login>.git

You must change <login> with yours. You have to upload your ssh-key to accounts.cri.epita.net.

Deadline

You work is due on the 31th of July 2021, 11:42 PM.

Rules

  • An exercise which does not compile will not be marked.
  • Your Git history will be reviewed and will be taken into account in the mark.
  • Cheating implies a zero to the course.
  • Your work will be tested on Archlinux.

Exercises

You have to complete the three following exercises.

hello world

The objective is to print the famous Hello World! string on the standard output, where the only allowed function is printf from the libc but you are forbidden to call printf directly: you have to find it manually.

Implementation restrictions
  • you can not use assembly code
  • you code should be able to work on multiple architectures and versions of the libc
  • you can not use any function provided by the libc except the ones you will gather manually.
Build restrictions
  • your work must be located in a folder called hello
  • binary will be named hello
  • you must use autotools, meson or make to generate your code
Git restrictions
  • commits messages should be prefixed with hello:␣
Steps
  • find the link map of your process (auxv, phdr, dynamic, r_debug, link_map)
  • get the dynamic segment
  • get the symtab, strtab and hash
  • find the address of printf inside them
  • call it

Note

The description of the elf format (with the description of the hash table) can be find here.

readelf

You have to write a small readelf clone that outputs json. You can find a sample output here.

Implementation restrictions
  • you should handle elf binaries in the same ‘size’ as your machine (look into link.h for the macro ElfW)
Test restrictions
  • use check and/or make check to test your code (do not forget to check that it is installed)
Build restrictions
  • you must use autotools or meson to generate your code
  • the directory for your sources is readelf
  • binary should be named simple-readelf
Hint

I recommend you to clone strace source code and take a look to the use of xlat. You will find an example of xlat use on LSE Github.

Git restrictions
  • commits should be prefixed with readelf:␣
Bonus
  • Support 32-bit and 64-bit architecture
  • Support little and big endianess

small linker

You must implement our own linker.

You will find the provided files on LSE Github.

Implementation restrictions

  • Command line usage:

    $ my_ld -o output_file input_file.o

  • There is only 3 relocation types to handle:

    • R_X86_64_32 (only for the debug versions)
    • R_X86_64_64
    • R_X86_64_PC32

  • entry_point address must be the address of _start

Build restrictions
  • You need to be able to build all the binaires in the tests directory 
    $ make my_ld
$ make -C tests MY_LD=../my_ld
  • you must use autotools, gcov and check to test your code (do not forget to check that it is installed)
  • you should have a complete coverage of your code
  • in your directory, expected results should be outputted on: 
    $ ./autogen.sh && ./configure --enable-code-coverage && make distcheck

Note

AX_CODE_COVERAGE macro is your friend (either copy the macro inside your repository, or install autoconf-archive)

Test descriptions

There are 3 samples:

  • simple: a really simple hello world with no relocation
  • hello: a relocation in the .rodata
  • vars: multiple relocations

And 2 variants, one with debug symbols, and one without.

Because of the time constrain, the debug versions are not mandatory.

Hints

  • instead of trying to handle all cases, assert that your are in a known code path. (for example, do not try to handle all the relocations, but write an error message for unhandled ones)

  • we only need at most 3 PHDR:

    • one with RX permissions (for the section marked with AX flags)
    • one with R permissions (for the section marked with A flags)
    • one with RW permissions (for the section marked with AW flags)
    • drop every other sections.
  • sections are useful only to be launched inside a debugger
  • for debugging, strace and dmesg will help

Note

  • have a look to the elf.h header (/usr/include/elf.h)
  • for the relocations, you can look into x86_64 ABI section 4.4