ERROR::DWARF(7stap)ERROR::DWARF(7stap)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 examina‐
tion 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 optimiza‐
tion. It is not perfect, but much better than before. Note
that, due to another gcc bug (PR51358) -O0 -g can actually some‐
times make debuginfo quality worse than for -O2 -g.
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 multi‐
ple 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. Con‐
sider placing a probe by a statement wildcard or line number
range.
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.
SEE ALSOstap(1),
http://dwarfstd.org/,
http://sourceware.org/systemtap/wiki/TipContextVariables,
http://gcc.gnu.org/wiki/Var_Tracking_Assignments,
warning::debuginfo(7stap),
error::reporting(7stap)ERROR::DWARF(7stap)