Ubuntu Manpage: error::dwarf - dwarf debuginfo quality problems

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:

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

       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.

NAME

DESCRIPTION

ALTERNATIVES

SEE ALSO