error::dwarf(7stap) — Linux manual page

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:

       debuginfo missing or mismatching
              Sometimes debuginfo is installed, but does not match the
              binaries being probed.  See the warning::debuginfo man
              page for more help for locating correct debuginfo.

       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.

       linux kbuild damaging debuginfo quality
              An upstream kernel commit #2062afb4f804a put
              -fno-var-tracking-assignments into KCFLAGS, dramatically
              reducing debuginfo quality, which can cause debuginfo
              failures. The simplest fix is to erase, excise, nay,
              eradicate this line from the top level linux Makefile:

              KBUILD_CFLAGS += [...] -fno-var-tracking-assignments [...]

       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.

       elfutils configuration
              It is possible that the DWARF debuginfo being sought is
              available, but not in a format acceptable to the copy of
              elfutils used by systemtap.  For example, your copy of gcc
              might produce compressed debuginfo (.zdebug_* ELF sections
              or .xz files) while your copy of elfutils might lack
              appropriate decompression 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/libraries are available at
              build time.

       debuginfo configuration
              Some tools may generate debuginfo that is unsupported by
              systemtap, 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),
       warning::buildid(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 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
       man-pages@man7.org

                                                     ERROR::DWARF(7stap)

Pages that refer to this page: error::pass2(7stap)error::sdt(7stap)warning::debuginfo(7stap)warning::pass5(7stap)warning::symbols(7stap)