Re: Why system call need to copy the date from the userspace before using it

[David Wagner]

Hacksaw  wrote:
>What I would expect the kernel to do is this:
>
>system_call_data_prep (userdata, size){	 [...]
>      for each page from userdata to userdata+size
>      {
> 	if the page is swapped out, swap it in
>	if the page is not owned by the user process, return -ENOWAYMAN
>	otherwise, lock the page
>      }   [...]

One challenge that might make this issue a little tricky is that
you have to handle double-indirection, where the kernel copies in
a buffer that includes a pointer to some other buffer that you then
have to copy in.  I think this comes up in some of the ioctl() calls.
Because only the guts of the ioctl() implementation knows the format of
the data structure, only it knows what system_call_data_prep() calls
would be needed.  So, everywhere that currently does copy_from_user()
would have to do system_call_data_prep().  (It wouldn't be sufficient
to call system_call_data_prep() once in some standardized way at the
start of each system call, and leave it at that.)
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