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‎"I have no special talents. I am only passionately curious." - Albert Einstein

Thursday, April 19, 2012

How does 64bit Linux Kernel runs 32bit application?

I like questions which makes me think and challenge my technical ability. During a telephonic interview, interviewer asked me "How does 64bit Linux Kernel runs 32bit application?" How would you have answered this question? After taking couple of seconds, I said "That’s really a good question (was buying time J to think). Hmmm. I am not sure but I believe either 32bit C/C++ library or System call handler for 32bit will make adjustment, according to 64bit Data model, to the input/output parameters before passing them to the kernel ". Read further if you are curious to know the correct answer :):

Whenever System Call is invoked, the System Call handler method gets called. The handler validates input/output parameters and calls appropriate function which actually performs the requested operation. I.e. the handler is a kind of wrapper over actual method which processes the request.

64 bit Linux Kernel supports LP64 data model which means "long" and "pointers" will be of 64bit. I.e. The size of "Long" or "Pointer" data types should be of 64bit when a 64bit application invokes System Calls. This causes problem for 32bit applications as they consider "long" and "pointers" of 32 bit.

Because of the above mentioned difference, the 64bit kernel has to have separate system call handler method 1) for 64 bit System calls and 2) for 32bit System calls. Please note that the kernel maintain two versions of core methods which actually processes requests like opening a file. Let’s look the below picture:

The core methods of 64bit kernel expect "long" and "pointers" data types to be of 64bit size. This is ok for 64bit System Calls. However, 32bit system call has to make 32bit "long" and "pointers" types to 64bit "long" and "pointers" before passing them to kernel’s core methods. For this conversion/adjustment, Linux Kernel uses a compat (compatibility) Layer. This layer has to be the part of kernel.

In short, it is the System call handler (not C/C++ library) for 32bit which makes adjustment/conversion, according to 64bit Data model, to the input/output parameters before passing them to the kernel

Please leave comment to improve my writing skillsJ.