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vsyscalls 🔍🚀

1. Explain the technical concept:

  • vsyscall (virtual system call) is a technique employed by the Linux kernel to accelerate certain system calls.

  • Originally, it was designed to speed up specific system calls like gettimeofday, time, and getcpu.

  • This mechanism involves the kernel mapping a page directly into the user space, containing specific variables and the implementations of a few system calls.

  • Due to direct mapping, calls to these functions become much faster than a traditional system call transition.

  • This mechanism can be observed using the /proc/self/maps command and searching for the vsyscall keyword.

2. Curious questions:

  • What are some of the system calls that vsyscall aims to accelerate?

    • Answer: The primary system calls targeted for acceleration via vsyscall are gettimeofday, time, and getcpu.

  • Why was vDSO introduced when we already had vsyscall?

    • Answer: vDSO, or "Virtual Dynamic Shared Object", was introduced to overcome the limitations of vsyscall, particularly its security concerns and the limitation of supporting only a few system calls. vDSO is more flexible, secure due to ASLR, and can support more than just four system calls.

3. Explain the concept in simple words:

  • Imagine you're a chef 🍳 in a busy kitchen. Some ingredients are used frequently, so you keep them on the counter right beside you instead of in the pantry. This way, you can quickly grab them without walking over to the pantry every time. The vsyscall is like having those ingredients on the counter: the kernel provides quick access to specific functions without the need for the usual back-and-forth of a regular system call. But, just like you can only fit a limited number of ingredients on the counter (due to space), vsyscall has its limitations. Over time, the kitchen might introduce a new, safer, and more efficient method to store and access these ingredients, which is like the introduction of vDSO.