DESCRIPTION

Errors that occur during pass 2 (elaboration) can have a variety of causes. Common types include:

unavailable probe point classes

Some types of probe points are only available on certain system versions, architectures, and configurations. For example, user-space process.* probes may require utrace or uprobes capability in the kernel for this architecture.

unavailable probe points

Some probe points may be individually unavailable even when their class is fine. For example, kprobe.function(foobar) may fail if function foobar does not exist in the kernel any more. Debugging or symbol data may be absent for some types of .function or .statement probes; check for availability of debuginfo. Try the stap-prep program to download possibly-required debuginfo. Use a wildcard parameter such as stap -l 'kprobe.function(*foo*)' to locate still-existing variants. Use ! or ? probe point suffixes to denote optional / preferred-alternatives, to let the working parts of a script continue.

typos

There might be a spelling error in the probe point name ("sycsall" vs. "syscall"). Wildcard probes may not find a match at all in the tapsets. Recheck the names using stap -l PROBEPOINT. Another common mistake is to use the . operator instead of the correct -> when dereferencing context variable subfields or pointers: $foo->bar->baz even if in C one would say foo->bar.baz.

unavailable context variables

Systemtap scripts often wish to refer to variables from the context of the probed programs using $variable notation. These variables may not always be available, depending on versions of the compiler, debugging/optimization flags used, architecture, etc. Use stap -L PROBEPOINT to list available context variables for given probes. Use the @defined() expression to test for the resolvability of a context variable expression. Consider using the stap --skip-badvars option to silently replace misbehaving context variable expressions with zero. Experiment with the stap --prologue-searching option.

module cache inconsistencies

Occasionally, the systemtap module cache ($HOME/.systemtap/cache) might contain obsolete information from a prior system configuration/version, and produce false results as systemtap attempts to reuse it. Retrying with stap --poison-cache ... forces new information to be generated. Note: this should not happen and likely represents a systemtap bug. Please report it.

GATHERING MORE INFORMATION

Increasing the verbosity of pass-2 with an option such as --vp 02 can help pinpoint the problem.