Avoiding Errors

The copyin() and copyinstr() subroutines cannot read from user addresses which have not yet been touched. A valid address might cause an error if the page that contains that address has not been faulted in by an access attempt. Consider the following example:

# dtrace -n syscall::open:entry'{ trace(copyinstr(arg0)); }'
dtrace: description 'syscall::open:entry' matched 1 probe
CPU     ID                    FUNCTION:NAME
dtrace: error on enabled probe ID 2 (ID 50: syscall::open:entry): invalid address
(0x9af1b) in action #1 at DIF offset 52

In the output from the previous example, the application was functioning properly and the address in arg0 was valid. However, the address in arg0 referred to a page that the corresponding process had not accessed. To resolve this issue, wait for the kernel or application to use the data before tracing the data. For example, you might wait until the system call returns to apply copyinstr(), as shown in the following example:

# dtrace -n syscall::open:entry'{ self->file = arg0; }' \
-n syscall::open:return'{ trace(copyinstr(self->file)); self->file = 0; }'
dtrace: description 'syscall::open:entry' matched 1 probe
CPU     ID                    FUNCTION:NAME
  2     51                      open:return   /dev/null