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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:
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.
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.
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)
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 2018-02-02. (At that
time, the date of the most recent commit that was found in the repos‐
itory was 2018-02-01.) 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::symbols(7stap)
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