Query regarding Linux Arm pagetable setup

[Catalin Marinas]

On Thu, May 05, 2016 at 05:04:57PM +0100, Russell King - ARM Linux wrote:
> On Thu, May 05, 2016 at 04:17:08PM +0530, Dushyant Behl wrote:
> > Also, another thing which I noticed is that kernel is mapping the high
> > interrupt vectors (addr 0xFFFF0000) in the user process's page table
> > with read only permissions and user bit enabled.
> > Can you also explain why kernel is mapping the kernel's pages with the
> > user bit?
> 
> The vector page is used to provide a set of userspace helpers so that
> userspace can be more independent of the CPU than it otherwise would
> be - things like a cmpxchg implementation, memory barrier, and a TLS
> helper.  You can turn this off, provided you know for certain that
> your userspace does not make use of these facilities.
> 
> > Also are there any other kernel pages which are marked with similar
> > user bit and read only permissions in the pagetables with possible
> > aliasing in the kernel's address space too?
> 
> No.

Actually, most read-only user pages (e.g. mmap'ed code) have an alias in
the kernel linear mapping or temporarily in highmem (after kmap).

-- 
Catalin