First, malloc a buffer from userspace and fill the buffer with all ‘A’
Then, pass the pointer of the buffer to kernel ,using netlink socket,
Finally, I can read and write the buffer, using the raw pointer directly passed from userspace.
Why ?
Why directly access to user space memory from kernel is allowed?
Linux Device Driver, Third Edition, Page 415, said that The kernel cannot directly manipulate memory that is not mapped into the kernel’s address space.
First, malloc a buffer from userspace and fill the buffer with all ‘A’ Then,
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The point is that accessing user addresses directly in kernel only sometimes work.
As long as you try to access the user address in the context of the same process that allocated it and that the process has already faulted it in and you are using a kernel with a 3:1 memory mapping (as opposed to 4:4 mapping that is sometimes used) and that the kernel did not swap out the page the allocation is in – the access will work.
The problem is that all these conditions are not always true and they can change even from run time of the program to another. Therefore the kernel driver writers needs to not count on being able to access user addresses.
The worst thing that can happen is for you to assume it works, have it always work in the lab, and have it crash at a customer site every so often. This is the reason for the book statement.