Update Linux_Memory_Management_Essentials.md

added PXA, PXN examples

Signed-off-by: Igor Stoppa <[email protected]>

Contributions/Linux_Memory_Management_Essentials.md

@@ -66,7 +66,7 @@ The following section presents a set of statements that can be objectively verif
    1. through the userspace mapping 
       1. this type of access is limited to few functions, like copy_to_user()/copy_from_user() and put_user()/get_user()
       2. outside the execution of the functions mentioned, this type of access is not possible, because the userspace mappings are made available only while executing such functions
-      3. the userspace memory map can implement HW protections against being misused by the kernel
+      3. the userspace memory map can implement HW protections against being misused by the kernel (e.g. ARM PXN, PXA)
       4. the kernel is able to access the pages of a user space process using the same translation mechanisms as the process (i.e. the process memory map), unless explicitly prevented by e.g. PAN or PXN
    2. through a memory buffer (e.g. a memory area where the kernel regularly needs to perform large amount of read/write operations, like a network buffer)
       1. the memory used by a buffer is well defined and delimited, rather than a generic area, and it is specifically reserved for this purpose