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Why does the kernel have access to all memory?
To the kernel there isn’t much difference technically, it’s just memory. Why? Because the kernel, which is running in the most privileged CPU mode, can access all memory. What are the different ways I can write in kernel address space from user space?
How to access kernel space from user mode?
The kernel (or one of its drivers) may, however, copy data from the user mode application’s memory to the kernel memory. is there any way we can access a physical address that is present in the kernel space and perform operations on it?
Where is the memory mapping in the Linux kernel?
A driver that supports the mmap () operation must complete and initialize the associated struct vm_area_struct. The most important fields of this structure are: vm_start, vm_end – the beginning and the end of the memory area, respectively (these fields also appear in /proc/ /maps );
How is the virtual address space divided into user and kernel space?
For efficiency reasons, the virtual address space is divided into user space and kernel space. For the same reason, the kernel space contains a memory mapped zone, called lowmem, which is contiguously mapped in physical memory, starting from the lowest possible physical address (usually 0).
How to access kernel space from user in Linux?
Even in system calls, we send a data to kernel space, where it (driver or respective module) calls functions like copy_from_user () to copy data from user space to kernel space. Here we exactly are not writing into address space. we are just passing a user pointer which contains the data that needs to be copied into the kernel buffers.
Is it true that Linux never denies requests for more memory?
It’s sometimes said that linux by default never denies requests for more memory from application code — e.g. malloc (). 1 This is not in fact true; the default uses a heuristic whereby Obvious overcommits of address space are refused.
How to get direct memory access in Linux?
Then you can ioremap this memory which will give you a kernel virtual address, and then you can mmap it and write a nice device driver. This take us back to linux device drivers in PDF format. Have a look at chapter 15, it is describing this technique on page 443 Edit : ioremap and mmap.