Courses/Computer Science/CPSC 457.F2014/Lecture Notes/SystemCalls

System Calls

System calls are one of the most important constructs in OS. More generally, the represent a way to transition in a standard, safe, and well-understood fashion between unprivileged code and privileged code.

We will start from a discussion of x86 assembly language programming and see how system calls are made (i.e., their calling convention on x86/Linux), how they are defined, how they can be traced, and how they differ from "normal" function calls.

Notes

• Slides: http://www.cpsc.ucalgary.ca/~locasto/teaching/2014/CPSC457/talks/syscalls2014.pdf

We looked at assembly-level function invocation (as a reminder).

We saw how we could talk directly to the kernel via the syscall interface

• now let's write a program using write(2)
• compare this with our assembly version mywrite.asm
• what is the nature of this interface? (combined API and privilege transition mechanism)
• where are the parts documented?
  • manual pages for syscall definitions (syntax and semantics)
  • show locations of syscall number definitions: /usr/include/asm/unistd_32.h
• what is the actual mechanism?
  • trace the "control flow" of making a system call
  • phrack article on IDT: http://www.phrack.com/issues.html?issue=59&id=4&mode=txt

Links to kernel code:

• http://lxr.cpsc.ucalgary.ca/lxr/#linux+v2.6.32/arch/x86/include/asm/unistd_32.h
• http://lxr.cpsc.ucalgary.ca/lxr/#linux+v2.6.32/include/linux/syscalls.h#L256 (DEFINE0 macro)
• http://lxr.cpsc.ucalgary.ca/lxr/#linux/arch/x86/kernel/syscall_table_32.S
• x86_64 syscall table data structure declaration: http://lxr.cpsc.ucalgary.ca/lxr/#linux/arch/x86/kernel/syscall_64.c
• http://lxr.cpsc.ucalgary.ca/lxr/#linux+v2.6.32/include/linux/linkage.h#L64 (ENTRY macro)
• each architecture is free to define its own system call numbers, see, e.g., Itanium (IA64): http://lxr.cpsc.ucalgary.ca/lxr/#linux+v2.6.32/arch/ia64/include/asm/unistd.h

• do_fork() system call: http://lxr.cpsc.ucalgary.ca/lxr/#linux+v2.6.32/kernel/fork.c#L1375

Scribe Notes

• s1
• s2
• s3

Readings

• MOS: 1.6: System Calls (you should have read this already)
• LKD: Chapter 5. System Calls (this describes how to add a system call to your OS, something you'll have an opportunity to practice in your tutorials later and in homeworks)