[dtrace-discuss] dtrace

[Chip Bennett]

Vivek Joshi wrote:
> ...
>
> syscall::stat:entry
> {
>  printf("%s\n",stringof(copyinstr(arg0)));
> }
> ...

This will work most of the time, but you should also put off translating 
the pointer with copyinstr, until you can be sure the string is paged 
in.  Something like:

syscall::stat:entry
{
   self->statpath = arg0;
}
syscall::stat:return
/ self->statpath /
{
   printf ("%s\n", copyinstr (self->statpath));
   self->statpath = 0;
}

D clauses run in kernel mode with interrupts turned off, so they can't 
handle a page fault.  Deferring handling arg0 until the stat call is 
finished assures you that the pointer's been referenced and the string 
is in memory.

Chip