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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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NAME | DESCRIPTION | ALTERNATIVES | SEE ALSO | COLOPHON |
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ERROR::DWARF(7stap) ERROR::DWARF(7stap)
error::dwarf - dwarf debuginfo quality problems
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
$PR15123_ASSUME_MFENTRY=1.
linux kbuild damaging debuginfo quality
An upstream kernel commit #2062afb4f804a put
-fno-var-tracking-assignments into KCFLAGS, dramatically
reducing debuginfo quality, which can cause debuginfo fail‐
ures. The simplest fix is to erase, excise, nay, eradicate
this line from the top level linux Makefile:
KBUILD_CFLAGS += [...] -fno-var-tracking-assignments [...]
function inlining
Even modern gcc sometimes has problems with parameters for
inlined functions. It may be necessary to change the
script to probe at a slightly different place. Try a
.statement() probe, instead of a .function() probe, some‐
where a few source lines into the body of the inlined func‐
tion. Or try putting a probe at the call site of the in‐
lined function. Or use the if @defined($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 sys‐
temtap 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 wild‐
card 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 appro‐
priate 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 decompres‐
sion 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.
In order to reduce reliance on ELF/DWARF debuginfo, consider the
use of statically compiled-in instrumentation, such as kernel tra‐
cepoints, 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.
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)
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 [email protected].
This page was obtained from the project's upstream Git repository
⟨git://sourceware.org/git/systemtap.git⟩ on 2025-08-11. (At that
time, the date of the most recent commit that was found in the
repository was 2025-07-24.) 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
[email protected]
ERROR::DWARF(7stap)
Pages that refer to this page: error::pass2(7stap), error::sdt(7stap), warning::debuginfo(7stap), warning::pass5(7stap), warning::symbols(7stap)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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