How does copy_from_user from the Linux kernel work internally?

linux linux-kernel memory-mapping
Santi1986 picture Santi1986 · Nov 25, 2011 · Viewed 26.9k times · Source

How exactly does the copy_from_user() function work internally? Does it use any buffers or is there any memory mapping done, considering the fact that kernel does have the privilege to access the user memory space?

Answer

caf picture caf · Nov 25, 2011

The implementation of copy_from_user() is highly dependent on the architecture.

On x86 and x86-64, it simply does a direct read from the userspace address and write to the kernelspace address, while temporarily disabling SMAP (Supervisor Mode Access Prevention) if it is configured. The tricky part of it is that the copy_from_user() code is placed into a special region so that the page fault handler can recognise when a fault occurs within it. A memory protection fault that occurs in copy_from_user() doesn't kill the process like it would if it is triggered by any other process-context code, or panic the kernel like it would if it occured in interrupt context - it simply resumes execution in a code path which returns -EFAULT to the caller.

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