error::sdt - <sys/sdt.h> marker failures
Systemtap's <sys/sdt.h> probes are modeled after the dtrace USDT API,
but are implemented differently. They leave a only a NOP instruction
in the userspace program's text segment, and add an ELF note to the
binary with metadata. This metadata describes the marker's name and
parameters. This encoding is designed to be parseable by multiple
tools (not just systemtap: GDB, the GNU Debugger, also contains sup‐
port). These allow the tools to find parameters and their types, wher‐
ever they happen to reside, even without DWARF debuginfo.
The reason finding parameters is tricky is because the STAP_PROBE /
DTRACE_PROBE markers store an assembly language expression for each op‐
erand, as a result of use of gcc inline-assembly directives. The com‐
piler is given a broad gcc operand constraint string ("nor") for the
operands, which usually works well. Usually, it does not force the
compiler to load the parameters into or out of registers, which would
slow down an instrumented program. However, some instrumentation sites
with some parameters do not work well with the default "nor" con‐
unresolveable at run-time
GCC may emit strings that an assembler could resolve (from the
context of compiling the original program), but a run-time tool
cannot. For example, the operand string might refer to a label
of a local symbol that is not emitted into the ELF object file
at all, which leaves no trace for the run-time. Reference to
such parameters from within systemtap can result in "SDT asm not
too complicated expression
GCC might synthesize very complicated assembly addressing modes
from complex C data types / pointer expressions. systemtap or
gdb may not be able to parse some valid but complicated expres‐
sions. Reference to such parameters from within systemtap can
result in "SDT asm not understood" errors.
overly restrictive constraint
GCC might not be able to even compile the original program with
the default "nor" constraint due to shortage of registers or
other reasons. A compile-time gcc error such as "asm operand
has impossible constraints" may result.
There are two general workarounds to this family of problems.
change the constraints
While compiling the original instrumented program, set the
STAP_SDT_ARG_CONSTRAINT macro to different constraint strings.
See the GCC manual about various options. For example, on many
machine architectures, "r" forces operands into registers, and
"g" leaves operands essentially unconstrained.
revert to debuginfo
As long as the instrumented program compiles, it may be fine
simply to keep using <sys/sdt.h> but eschew extraction of a few
individual parameters. In the worst case, disable <sys/sdt.h>
macros entirely to eschew the compiled-in instrumentation. If
DWARF debuginfo was generated and preserved, a systemtap script
could refer to the underlying source context variables instead
of the positional STAP_PROBE parameters.