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::DWARF(7stap)													       ERROR::DWARF(7stap)

NAME

       error::dwarf - dwarf debuginfo quality problems

DESCRIPTION

       Systemtap  sometimes  relies  on  ELF/DWARF  debuginfo  for  programs being instrumented to locate places to probe, or context variables to
       read/write, just like a symbolic debugger does.	Even though examination of the program's source code may show  variables  or  lines  where
       probes  may  be desired, the compiler must preserve information about them for systemtap (or a debugger such as gdb) to get pinpoint access
       to the desired information.  If a script requires such data, but the compiler did not preserve enough of it, pass-2 errors may result.

       Common conditions that trigger these problems include;

       compiler version
	      Prior to GCC version 4.5, debuginfo quality was fairly limited.  Often developers were advised to build their programs with  -O0	-g
	      flags  to disable optimization.  GCC version 4.5 introduced a facility called "variable-tracking assignments" that allows it to gen-
	      erate high-quality debuginfo under full -O2 -g optimization.  It is not perfect, but much better than before.   Note  that,  due	to
	      another gcc bug (PR51358) -O0 -g can actually sometimes make debuginfo quality worse than for -O2 -g.

       function inlining
	      Even  modern gcc sometimes has problems with parameters for inlined functions.  It may be necessary to change the script to probe at
	      a slightly different place (try a .statement() probe, instead of a .function() probe, somewhere a few source lines into the body	of
	      the inlined function.  Or try putting a probe at the call site of the inlined function.  Or use the if @defined($var) { ... } script
	      language construct to test for the resolvability of the context variable before using it.

       instruction reordering
	      Heavily optimized code often smears the instructions from multiple source statements together.  This can	leave  systemtap  with	no
	      place  to choose to place a probe, especially a statement probe specified by line number.  Systemtap may advise to try a nearby line
	      number, but these may not work well either.  Consider placing a probe by a statement wildcard or line number range.

ALTERNATIVES

       In order to reduce reliance on ELF/DWARF debuginfo, consider the use of statically compiled-in instrumentation, such as kernel tracepoints,
       or <sys/sdt.h> userspace markers.  Such instrumentation hook sites are relatively low cost (just one NOP instruction for sdt.h), and nearly
       guarantee the availability of parameter data and a reliable probe site, all without reliance on debuginfo.

SEE ALSO

       stap(1),
       http://dwarfstd.org/,
       http://sourceware.org/systemtap/wiki/TipContextVariables,
       http://gcc.gnu.org/wiki/Var_Tracking_Assignments,
       warning::debuginfo(7stap),
       error::reporting(7stap)

															       ERROR::DWARF(7stap)