1ERROR::SDT(7stap) ERROR::SDT(7stap)
2
6 error::sdt - <sys/sdt.h> marker failures
7
12 Systemtap's <sys/sdt.h> probes are modeled after the dtrace USDT API,
13 but are implemented differently. They leave a only a NOP instruction
14 in the userspace program's text segment, and add an ELF note to the
15 binary with metadata. This metadata describes the marker's name and
16 parameters. This encoding is designed to be parseable by multiple
17 tools (not just systemtap: GDB, the GNU Debugger, also contains sup‐
18 port). These allow the tools to find parameters and their types, wher‐
19 ever they happen to reside, even without DWARF debuginfo.
20 The reason finding parameters is tricky is because the STAP_PROBE /
23 DTRACE_PROBE markers store an assembly language expression for each op‐
24 erand, as a result of use of gcc inline-assembly directives. The com‐
25 piler is given a broad gcc operand constraint string ("nor") for the
26 operands, which usually works well. Usually, it does not force the
27 compiler to load the parameters into or out of registers, which would
28 slow down an instrumented program. However, some instrumentation sites
29 with some parameters do not work well with the default "nor" con‐
30 straint.
31 unresolveable at run-time
34 GCC may emit strings that an assembler could resolve (from the
35 context of compiling the original program), but a run-time tool
36 cannot. For example, the operand string might refer to a label
37 of a local symbol that is not emitted into the ELF object file
38 at all, which leaves no trace for the run-time. Reference to
39 such parameters from within systemtap can result in "SDT asm not
40 understood" errors.
41 too complicated expression
44 GCC might synthesize very complicated assembly addressing modes
45 from complex C data types / pointer expressions. systemtap or
46 gdb may not be able to parse some valid but complicated expres‐
47 sions. Reference to such parameters from within systemtap can
48 result in "SDT asm not understood" errors.
49 overly restrictive constraint
52 GCC might not be able to even compile the original program with
53 the default "nor" constraint due to shortage of registers or
54 other reasons. A compile-time gcc error such as "asm operand
55 has impossible constraints" may result.
56 There are two general workarounds to this family of problems.
59 change the constraints
62 While compiling the original instrumented program, set the
63 STAP_SDT_ARG_CONSTRAINT macro to different constraint strings.
64 See the GCC manual about various options. For example, on many
65 machine architectures, "r" forces operands into registers, and
66 "g" leaves operands essentially unconstrained.
67 revert to debuginfo
70 As long as the instrumented program compiles, it may be fine
71 simply to keep using <sys/sdt.h> but eschew extraction of a few
72 individual parameters. In the worst case, disable <sys/sdt.h>
73 macros entirely to eschew the compiled-in instrumentation. If
74 DWARF debuginfo was generated and preserved, a systemtap script
75 could refer to the underlying source context variables instead
76 of the positional STAP_PROBE parameters.
77
80 stap(1),
81 stapprobes(3stap),
82 error::dwarf(7stap),
83 http://gcc.gnu.org/onlinedocs/gcc/Constraints.html,
84 http://sourceware.org/systemtap/wiki/UserSpaceProbeImplementation,
85 error::reporting(7stap)
86 ERROR::SDT(7stap)