When you make a system call from user space, the first thing that is
checked is if the address of the parameter is well within the legal
virtual address space (i.e. 0 to 3 GB for the user space). If this is
not so, the call will fail. If you want to make the same system call
from the Kernel Space( Virtual Address 3 - 4 GB) however, this address
checking has to be avoided so that the call will not fail. Now, every
process has a tak_struct associated with it and this structure contains
the legal virtual address boundaries for that process( Virtual Address
space represented by mm_segment_t). The get_fs() macro will retrieve
this boundary and the set_fs() will set it with a value. So, when you
want to access a memory region which is beyond the User Space Virtual
Address limit( i.e. falling in the Kernel Space Virtual Address region),
you first of all store the current limit by doing
mm_segment_t old_fs;
old_fs = get_fs();
Then set this limit to that of the Kernel (i.e. the whole of 4 GB) by
doing
set_fs (KERNEL_DS);
Do your memory accessing operations here (for ex: - read from a buffer
which is in the kernel space from a user context thru a system call)
.......;
Set the address limit back to the original limit that was stored in the
old_fs variable by doing.
set_fs(old_fs);
Hope this helped. Google for more answers and please let me know if you
find more details.
Regards,,
Aravind.
"Dovie'andi se tovya sagain"
-Mat Cauthon (WoT).
________________________________
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Wang Yu
Sent: Thursday, October 25, 2007 3:24 PM
To: kernelnewbies
Subject: get_fs( ) and set_fs( )
Hi,all
I have seen the following codes, but do not know what's the meaning of
them:
mm_segment_t old_fs;
old_fs = get_fs();
set_fs (KERNEL_DS);
........
set_fs(old_fs);
What is the usage of these codes and in what condition they will be
used?
Thanks!
--
National Research Center for Intelligent Computing Systems
Institute of Computing Technology, Chinese Academy of Sciences
