I know it is the kernel which makes syscalls and libc possible.
Do you? You don't seem to want to accept the implications.
All the fuzzy details of how things work are laid bare inside the kernel, and it must keep track of a lot more stuff than you do. That's why it's done inside the kernel, so you don't have to.
So you want to open a file inside the kernel. Great. Do open("/path/to/file", O_RDWR) call inside the kernel -- whoops, there isn't one.
Which open would you like?
Assuming you can even find a real_ultimate_kernel_mode_open_which_acts_just_like_userspace_open() call somewhere in that list, how do you use it? When you call open() from a process, the kernel knows what process you're calling it from, so it can alter your process' open file table. You don't have a process. How's it supposed to track it?
Turns out the kernel has its own global file table, which is all well and good, but means your real_ultimate_kernel_mode_open_which_acts_just_like_userspace_open() call won't work -- without a process, many important data structures are missing. You need to alter the global table and avoid needing any userspace one.
So you'd need to find something which alters the kernel's own global file table instead. call it kernel_internal_open(). Okay, good. Now consider whether it's safe to call kernel_internal_open() from your device driver. Might the kernel want you to lock some mutex first? What if something else has already locked it -- is it really okay for your kernel code to sit waiting? Or is the other thing going to sit waiting forever, waiting for you while you're waiting for it? And what kind of waiting should you do? Is a spinlock good enough, or do you have to tell the kernel to pre-empt yourself somehow so other things can happen while you're waiting?
What if kernel_internal_open is already the function that's calling you? Wouldn't all the data structures you want already be busy? It might happen -- that's how device drivers work, big central kernel functions are switchboards for choosing which device driver callbacks to call...
If you have a process, this is an easy fix. The calling process is suspended until the kernel has the right conditions to deal with it, plus the kernel automatically knows all kinds of things about you. But when you don't have a process, suddenly there's all kinds of things you need to worry about.
The concept of "process" is an artificial thing. It pretends very hard to have ideal conditions all the time even when the kernel might be flailing like mad behind the scenes. It's a very tranquil and safe place. The kernel is not. That's why its the kernel.
Last edited by Corona688; 11-16-2011 at 10:48 AM..
Thanks for your patient and zeal. I will study the kernel earnestly. One years ago, I know nothing about linux. It is UNIX.COM who accompanies my way to linux. Thank you all. Thanks for the great forum.
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