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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; 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);
Regards,, Aravind. "Dovie'andi
se tovya sagain" -Mat Cauthon (WoT). From:
kernelnewbies-bounce@xxxxxxxxxxxx [mailto:kernelnewbies-bounce@xxxxxxxxxxxx] On Behalf Of Wang Yu Hi,all |
