Hi Mulyadi, this is a really good peace of information. I didn't understand everything you wrote yet, but I'm on it! Could you explain me, what exactly a stack segment is? Is it a memory range, where the stack data (I mean, something, that I push on the stack) is saved? best regards, Paul > --- Ursprüngliche Nachricht --- > Von: Mulyadi Santosa <mulyadi.santosa@xxxxxxxxx> > An: Paul Duplys <kernel_newbie@xxxxxx> > Kopie: kernelnewbies@xxxxxxxxxxxx > Betreff: Re: kernel stack vs. user stack > Datum: Wed, 10 Aug 2005 11:24:18 +0700 > > Hello.... > > > For security and general sanity of the system, kernel and user code > > can't use the same stack. So each process has to have 2 stacks -- one > > is in kernel memory, used by kernel code, and one is in user memory, > > used by the user code. > > it is important to note that kernel stack is located on memory area with > privilege level 0, so it can only be accessed by code which has > privilege level 0 too. Conversely, user stack has privilege level 3, so > technically it can be accessed by user mode and kernel mode. This > memory area's privilege is defined by the related segment descritp > > During the switch between user and kernel mode (e.g by issuing system > call), Stack Segment selector (SS) is also updated to point on related > segment descriptor. IIRC, on user mode, SS is equivalent to __USER_DS, > while on kernel mode, it is __KERNEL_DS. Both segment actually overlaps > (both starts on address 0 and ends at oxFFFFFFFF) , so something else > is needed to protect these two segment. > > If you read on Understanding Linux Kernel 2nd ed chapter 2, you will > realize that the protection is doubled by paging mechanism. Page frame > containing kernel stack has Supervisor flag set as 0. This means it can > only be accessed by kernel mode codes > > Perhaps the final question is "how can I determine the code is in user > mode or kernel mode?" remember that Linux kernel operates in protected > mode, thus initially codes are referenced by logical address, composed > by segment selector and offset. Inside the segment selector, there is > an RPL (requestor privilege level) field (2 bit length) which defines > CPL (current privilege level). Kernel codes always issue logical > address which has RPL=0, whereas user mode code has RPL=3. I guess it > is also the work of kernel itself which is doing switching between > segment selectors > > hope it helps > > regards > > Mulyadi > > > -- > Kernelnewbies: Help each other learn about the Linux kernel. > Archive: http://mail.nl.linux.org/kernelnewbies/ > FAQ: http://kernelnewbies.org/faq/ > -- GMX DSL = Maximale Leistung zum minimalen Preis! 2000 MB nur 2,99, Flatrate ab 4,99 Euro/Monat: http://www.gmx.net/de/go/dsl -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/
