NAME | DESCRIPTION | ALTERNATIVES | SEE ALSO | COLOPHON

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

NAME         top

       error::dwarf - dwarf debuginfo quality problems

DESCRIPTION         top

       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 generate 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.

              Another related problem involves debuginfo quality for the
              prologue area of a function (PR15123), wherein a program
              compiled with CFLAGS=-mfentry (especially the kernel, for
              ftrace) may lack accurate debuginfo for the entry instructions
              for gcc prior to version 4.8.  If able, arrange to compile
              your programs with -grecord-gcc-switches CFLAGS, and/or try
              rerunning systemtap with $PR15123_ASSUME_MFENTRY=1.

       function inlining
              Even modern gcc sometimes has problems with parameters for in‐
              lined 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 mul‐
              tiple source statements together.  This can leave systemtap
              with no place to choose to place a probe, especially a state‐
              ment 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 num‐
              ber range.

       elfutils configuration
              It is possible that the DWARF debuginfo being sought is avail‐
              able, but not in a format acceptable to the copy of elfutils
              used by systemtap.  For example, your copy of gcc might pro‐
              duce compressed debuginfo (.zdebug_* ELF sections or .xz
              files) while your copy of elfutils might lack appropriate de‐
              compression capabilities.  Unfortunately, there is no easy way
              to tell if this is the problem.  If you're building your own
              copy of elfutils, ensure all decompression library headers/li‐
              braries are available at build time.

       debuginfo configuration
              Some tools may generate debuginfo that is unsupported by sys‐
              temtap, such as the linux kernel CONFIG_DEBUG_INFO_SPLIT (.dwo
              files) option.  Stick with plain ELF/DWARF (optinally split,
              Fedora-style), if possible.

ALTERNATIVES         top

       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         top

       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)

COLOPHON         top

       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 systemtap@sourceware.org.  This page was
       obtained from the project's upstream Git repository 
       ⟨git://sourceware.org/git/systemtap.git⟩ on 2017-03-13.  If you dis‐
       cover 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 man-pages@man7.org

                                                         ERROR::DWARF(7stap)