How remap_pfn_range remaps kernel memory to user space?

linux-kernel linux-device-driver kernel-module virtual-address-space
Kapil Gokhale picture Kapil Gokhale · Jan 9, 2012 · Viewed 22.5k times · Source

remap_pfn_range function (used in mmap call in driver) can be used to map kernel memory to user space. How is it done? Can anyone explain precise steps? Kernel Mode is a privileged mode (PM) while user space is non privileged (NPM). In PM CPU can access all memory while in NPM some memory is restricted - cannot be accessed by CPU. When remap_pfn_range is called, how is that range of memory which was restricted only to PM is now accessible to user space?

Looking at remap_pfn_range code there is pgprot_t struct. This is protection mapping related struct. What is protection mapping? Is it the answer to above question?

Answer

gby picture gby · Jan 9, 2012

It's simple really, kernel memory (usually) simply has a page table entry with the architecture specific bit that says: "this page table entry is only valid while the CPU is in kernel mode".

What remap_pfn_range does is create another page table entry, with a different virtual address to the same physical memory page that doesn't have that bit set.

Usually, it's a bad idea btw :-)

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