Kernel mode programming

[Kamal R. Prasad]

--- Zera William Holladay <zholla1 at uic.edu> wrote:

> 
> 
[snip]
> 
> My understanding is that when a system call is made
> from a user process,
> there is a trap into the kernel, the state of the
> user process is saved
> and the address of the system call is determined by
> a looking up the
> address of the system call in vector table.
> 
Not exactly. When you execute a system call on the
kernel side, you execute it in the context of the
process. There is a per-process kernel stack -so you
don't save a copy of the process to execute the sys
call.

> I assume that a kernel module would at least have to
> push the parameters
> of the system call, push some of its registers, jump
> to the address space
> of the system call and return.  Further I assume,

When you call another function from within the kernel,
it is like calling another function within userspace.
The args get pushed onto stack and there is space for
retval etc.

> the kernel would have to
> copy the parameters of the system call to another
> address, since it is
> possible for the call to be interrupted and some of

If the syscall is interrupted, it won't affect the
params as they will essentially be local variables in
the syscall. An interrupt handler won't corrupt the
args to the syscall for sure.

> the parameters to be
> corrupted.
> 
It is possible that the userspace may pass a pointer
which gets freed before the kernel is finished with
accessing it. That is why, a system call
implementation does a copyin() on entry and when it
wants to reflect the data to userspace, it does a
copyout().

> I am really asking a question out of blind ignorance
> in an attempt to
> learn more about what goes on with the kernel.  What
> is the difference
> between a system call made from the kernel and a
> system call made from
> user code?
> 

The differece is that in one, you do a context switch
in one and in another you don't. The function names
change eg:- open() is syscall name, and underlying
function in kernel would be sys_open() -so from within
kernel, you call sys_open() instead of open() as in
userspace.

regards
-kamal


 > Thanks, Zera
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