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 support). These allow the tools to find
parameters and their types, wherever 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 operand, as a result of use of gcc inline-assembly
directives. The compiler 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" constraint.
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 understood" errors.
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 expressions. Reference to such parameters
from within systemtap can result in "SDT asm not
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
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.
This page is part of the systemtap (a tracing and live-system
analysis tool) project. Information about the project can be
found at ⟨https://sourceware.org/systemtap/⟩. If you have a bug
report for this manual page, send it to firstname.lastname@example.org.
This page was obtained from the project's upstream Git repository
⟨git://sourceware.org/git/systemtap.git⟩ on 2021-08-27. (At that
time, the date of the most recent commit that was found in the
repository was 2021-08-26.) If you discover any rendering
problems in this HTML version of the page, or you believe there
is a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to