Tracing Arbitrary Instructions

You can use the pid provider to trace any instruction in any user function. Upon demand, the pid provider will create a probe for every instruction in a function. The name of each probe is the offset of its corresponding instruction in the function expressed as a hexadecimal integer. For example, to enable a probe associated with the instruction at offset 0x1c in function foo of module bar.so in the process with PID 123, you can use the following command:

# dtrace -n pid123:bar.so:foo:1c

To enable all of the probes in the function foo, including the probe for each instruction, you can use the command:

# dtrace -n pid123:bar.so:foo:

This command demonstrates an extremely powerful technique for debugging and analyzing user applications. Infrequent errors can be difficult to debug because they can be difficult to reproduce. Often, you can identify a problem after the failure has occurred, too late to reconstruct the code path. The following example demonstrates how to combine the pid provider with speculative tracing (see Chapter 7, Speculative Tracing) to solve this problem by tracing every instruction in a function.

pid$1::$2:entry
{
        self->spec = speculation();
        speculate(self->spec);
        printf("%x %x %x %x %x", arg0, arg1, arg2, arg3, arg4);
}

pid$1::$2:
/self->spec/
{
        speculate(self->spec);
}

pid$1::$2:return
/self->spec && arg1 == 0/
{
        discard(self->spec);
        self->spec = 0;
}

pid$1::$2:return
/self->spec && arg1 != 0/
{
        commit(self->spec);
        self->spec = 0;
}

Executing errorpath.d results in output similar to the following example:

# ./errorpath.d 100461 _chdir
dtrace: script './errorpath.d' matched 19 probes
CPU     ID                    FUNCTION:NAME
  0  25253                     _chdir:entry 81e08 6d140 ffbfcb20 656c73 0
  0  25253                     _chdir:entry
  0  25269                         _chdir:0
  0  25270                         _chdir:4
  0  25271                         _chdir:8
  0  25272                         _chdir:c
  0  25273                        _chdir:10
  0  25274                        _chdir:14
  0  25275                        _chdir:18
  0  25276                        _chdir:1c
  0  25277                        _chdir:20
  0  25278                        _chdir:24
  0  25279                        _chdir:28
  0  25280                        _chdir:2c
  0  25268                    _chdir:return