error::buildid(7stap) [centos man page]

ERROR::BUILDID(7stap)													     ERROR::BUILDID(7stap)

NAME

       error::buildid - build-id verification failures

DESCRIPTION

       Because	systemtap's script translation / execution stages may be executed at different times and places, it is sometimes necessary to ver-
       ify certain invariants.	One such invariant is that if a script was informed by translate-time analysis of  executables,  then  those  same
       executables  need  to  be  used	at run time.  This checking is done based upon the build-id, a binary hash that modern (post-2007) compil-
       ers/toolchains add as an NT_GNU_BUILD_ID ELF note to object files and executables.  Use the readelf -n command to examine the build-ids	of
       binaries, if you are interested.

       Only  scripts  are sensitive to executables' build-ids: generally those that perform deep analysis of the binaries or their debuginfo.  For
       example, scripts that place .function or .statement probes, or use stack  backtrace-related  tapset  functions  may  be	sensitive.   Other
       scripts	that  rely  only  on  process.mark  or kernel.trace probes do not require debuginfo.  See the DWARF DEBUGINFO section in the stap-
       probes(3stap) man page.

       During translation, systemtap saves a copy of the relevant files' build-ids within the compiled modules.  At run-time, the modules  compare
       the saved ones to the actual run-time build-ids in memory.  The error message indicates that they did not match, so the module will decline
       placing a probe that was computed based upon obsolete data.  This is important for safety, as placing  them  at	an  inappropriate  address
       could crash the programs.  However, this is not necessarily a fatal error, since probes unrelated to the mismatching binaries may operate.

       A build-id mismatch could be caused by a few different situations.  The main one is where the executable versions or architecture were dif-
       ferent between the systemtap translation and execution times/places.  For example, one may run a stap-server on a slightly  different  ver-
       sion of the OS distribution.  The kernel running on the workstation may be slightly different from the version being targeted - perhaps due
       to a pending kernel upgrade leaving different files on disk versus running in memory.  If your  OS  distribution  uses  separate  debuginfo
       packages, the split .IR .debug files may not exactly match the main binaries.

SEE ALSO

       http://fedoraproject.org/wiki/Releases/FeatureBuildId,
       stap(1),
       stapprobes(3stap),
       warning::debuginfo(7stap),
       error::reporting(7stap)

															     ERROR::BUILDID(7stap)

ERROR::SDT(7stap)														 ERROR::SDT(7stap)

NAME

       error::sdt - <sys/sdt.h> marker failures

DESCRIPTION

       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, 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 op-
       erand,  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" con-
       straint.

       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  can-
	      not.   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 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  indi-
	      vidual  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.

SEE ALSO

       stap(1),
       stapprobes(3stap),
       error::dwarf(7stap),
       http://gcc.gnu.org/onlinedocs/gcc/Constraints.html,
       http://sourceware.org/systemtap/wiki/UserSpaceProbeImplementation,
       error::reporting(7stap)

																 ERROR::SDT(7stap)