Sandboxing vs. Virtualisation

Maybe I am missing something but isn't sandboxi开发者_如何学编程ng and virtualisation exactly the same concept, ie., separating the memory space for applications running in parallel. So I am wondering why they are having different names, are there maybe differences in the way they are employed?

Many thanks, Simon

These concepts address different problems: When we virtualize, we are hiding physical limitations of the machine. Sandboxing, on the other hand, sets artificial limits on access across a machine. Consider memory as a representative analogy.

Virtualization of memory is to allow every program to access every address in a 32- or 64-bit space, even when there isn't that much physical RAM.

Sandboxing of memory is to prevent one program from seeing another's data, even though they might occupy neigboring cells in memory.

The two concepts are certainly related in the common implementation of virtual memory. However, this is a convenient artifact of the implementation, since the hardware page table is only accessible by the kernel.

Consider how to implement them separately, on an x86 machine: You could isolate programs' memory using page tables without ever swapping to disk (sandboxing without virtualization). Alternatively, you could implement full virtual memory, but also give application-level access to the hardware page table so they could see whatever they wanted (virtualization without sandboxing).

There are actually 3 concepts that you are muddling up here. The first and foremost is what is provided by the OS and what it does is it separates the memory space for applications running in parallel. And it is called virtual memory.

In Virtual memory systems, the OS maps the memory address as seen by applications onto real physical memory. Thus memory space for applications can be separated so that they never collide.

The second is sandboxing. It is any technique you, the programmer, use to run untrusted code. If you, the programmer, are writing the OS then from your point of view the virtual memory system you are writing is a sandboxing mechanism. If you, the programmer, are writing a web browser then the virtual memory system, in itself, is not a sandboxing mechanism (different perspectives, you see). Instead it is a pontential mechanism for you to implement your sandbox for browser plug-ins. Google Chrome is an example of a program that uses the OS's virtual memory mechanism to implement its sandboxing mechanism.

But virtual memory is not the only way to implement sandboxing. The tcl programming language for example allows you to instantiate slave interpreters via the interp command. The slave interpreter is often used to implement a sandbox since it runs in a separate global space. From the OS's point of view the two interpreters run in the same memory space in a single process. But because, at the C level, the two interpreters never share data structures (unless explicitly programmed) they are effectively separated.

Now, the third concept is virtualization. Which is again separate from both virtual memory and sandboxing. Whereas virtual memory is a mechanism that, from the OS's perspective, sandboxes processes from each other, virtualisation is a mechanism that sandboxes operating systems from each other. Example of software that does this include: Vmware, Parallels Desktop, Xen and the kernel virtual machine.

Sandboxing means isolation only, when virtualization usually means simulating of some sort of hardware (virtual machine). Virtualization can happen with our without sandboxing.

Sandboxing is limiting access by a particular program. Virtualization is a mechanism that can be used to help do this but sandboxing is acheived with other mechanisms as well, and likewise virtualization has uses besides sandboxing. Sandboxing is a "what", virtualization is a "how".