Syscall

In Linux, whenever a user application wants to touch any of these:

  • files
  • any hardware or devices
  • running processes
  • networking stuff
  • date/time
  • writing to stdout it can't do so on its own - it must ask Linux Kernel to do it instead.

Syscall is a way to ask Kernel to do something for you. The data is transfered between Kernel and the application using CPU registers, and the kernel is invoked using an interrupt (trap).

As many key functions are impossible without kernel, many of the programming language features rely on syscall under the hood.

For example in 01 Inbox/Golang:

  • fmt.Println calls Fprintln with os.Stdout as an argument.
  • Fprintln calls write function on os.Stdout
  • os.Stdout is a File under the hood
  • File's write function finally invokes syscall.Write

As you can see, syscalls are often deeply abstracted, and their higher abstractions should always be used instead of syscalls directly (where available).

When invoked, syscall:

  • saves the state of the CPU registers
  • adds data about the syscall you are creating to the registers
  • issues an interrupt (trap), letting the kernel know that there is something that needs to be done
  • kernel then reads the info from the register, performs the operation, and writes the response back into the register
  • syscall then resumes by reading the response from the register
  • and finally restores the register to initial state (i assume as cleanup?)

We can see the syscalls that a process makes by using a tool called strace: strace -c echo "hello world" will give us the count of all the syscalls made during execution of echo "hello world".

strace itself actually accomplishes this by using Syscalls under the hood - by relying on ptrace syscall, it can see into the process and manipulate it. This is how breakpoint debuggers work.

It's possible to limit what syscalls a process can make for security reasons. For docker containers, we can do this with seccomp.

Status: #💡

References:

  • Video - A Go Programmers guide to SyscallsVideo - A Go Programmers guide to Syscalls
    Source: GopherCon 2017: Liz Rice - A Go Programmer's Guide to Syscalls

    Status: #🛈/📹/✅

    POC: github.com/hpdobrica/go-playground/strace




    [[Syscall]] is a way for a program running in userspace ...
  • syscall man page