Re: Kernel stack

From: suthambhara nagaraj (
Date: 10/12/04

  • Next message: Herbert Poetzl: "Re: [patch 2/3] lsm: add bsdjail module"
    Date:	Tue, 12 Oct 2004 12:21:37 +0530
    To: "Dhiman, Gaurav" <>

    The problem is each process does not have a TSS of its own.Only one
    TSSper processor is present and the process dependant features (Like
    esp) are stored
    in another structure( struct thread_struct ).A kernel stack of size 8k
    (By default)
    is actully shared by processes running on a processor. There is a func named
    load_tss (or something similiar) which loads the TSS from the
    thread_struct structure during task switch .

    A Process does not have an SS entry in its thread_struct but only an
    esp (and esp0) entry. This made me believe that the stack base is the
    Correct me


    On Tue, 12 Oct 2004 11:55:24 +0530, Dhiman, Gaurav <> wrote:
    > > I have not understood how the common kernel stack in the
    > > init_thread_union(2.6 ,init_task_union in case of 2.4) works for all
    > > the processes which run on the same processor
    > As far as I know, Kernel do not have any common stack for all the
    > processes running over it. Whenever we enter the kernel mode thru system
    > calls, we go thru system gate or descriptor (0x80 entry) in IDT. This
    > entry contains the index of the descriptor in GDT (normally it points to
    > Kernel CS Segment Descriptor in GDT) and the offset (pointer) to the
    > code to be executed in kernel mode (which is system_call() function in
    > Kernel).
    > Now the descriptor entry in GDT pointed out by the system gate entry in
    > IDT, contains 2 bit field known as DPL (Desired Privelege Level). If
    > this DPL is less than the CPL (Current Prevelege Level) of CPU then CPU
    > switches to the process specific kernel stack segement by refferring the
    > TSS of current running process. This stack switch is automatic by CPUand
    > there is no assembly intruction required for it.
    > This stack switch is done at the time when we enter from user space to
    > the kernel space, this is done because we can not trust and share the
    > user process stack (stack used by user process in user mode). That is
    > why every process has atleast two and can even have four stacks. In each
    > process, stack for every CPU level (ring level) is defined. So whenever
    > the process runs in user mode (ring 3), its user mode stack is used, but
    > when it enters the kernel mode (ring 0) its stack is switched to the
    > kernel stack of that process. All the stacks of a process for different
    > levels of CPU are tracked thru TSS defined for that process.
    > To read more on IDT, GDT, TSS and System Calls invocation, refer to the
    > Intels System Programmer's Guide. Her is the Link:
    > Correct me if I am wrong somewhere.
    > Cheers !!
    > Gaurav
    > -----Original Message-----
    > From:
    > [] On Behalf Of suthambhara
    > nagaraj
    > Sent: Tuesday, October 12, 2004 10:31 AM
    > To: kernel
    > Subject: Kernel stack
    > Hi all,
    > I have not understood how the common kernel stack in the
    > init_thread_union(2.6 ,init_task_union in case of 2.4) works for all
    > the processes which run on the same processor. The scheduling is round
    > robin and yet the things on the stack (saved during SAVE_ALL) have to
    > be maintained after a switch without them getting erased. I am
    > familiar with only the i386 arch implementation.
    > Please help
    > regards,
    > Suthambhara
    > --
    > Kernelnewbies: Help each other learn about the Linux kernel.
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  • Next message: Herbert Poetzl: "Re: [patch 2/3] lsm: add bsdjail module"

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