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.\" t
.TH ERROR::DWARF 7stap
.SH NAME
error::dwarf \- dwarf debuginfo quality problems
.SH 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.
.PP
Common conditions that trigger these problems include:
.TP
debuginfo missing or mismatching
Sometimes debuginfo is installed, but does not match the binaries being
probed. See the \fIwarning::debuginfo\fP man page for more help for
locating correct debuginfo.
.TP
compiler version
Prior to GCC version 4.5, debuginfo quality was fairly limited.
Often developers were advised to build their programs with
.IR "-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
.IR "-O2 -g"
optimization. It is not perfect, but much better than before.
Note that, due to another gcc bug (PR51358)
.IR "-O0 -g"
can actually sometimes make debuginfo quality worse than for
.IR "-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
.IR "-grecord-gcc-switches"
CFLAGS, and/or try rerunning systemtap with
.nh
.IR $PR15123_ASSUME_MFENTRY=1 .
.hy
.TP
linux kbuild damaging debuginfo quality
An upstream kernel commit #2062afb4f804a put
\%-fno-var-tracking-assignments into KCFLAGS, dramatically reducing
debuginfo quality, which can cause debuginfo failures. The simplest
fix is to \fBerase, excise, nay, eradicate\fP this line from the top
level linux Makefile:
.SAMPLE
KBUILD_CFLAGS += [...] -fno-var-tracking-assignments [...]
.ESAMPLE
.TP
function inlining
Even modern gcc sometimes has problems with parameters for inlined functions.
It may be necessary to change the script to probe at a slightly different place.
Try a
.IR .statement() " probe, instead of a " .function() " probe,"
somewhere a few source lines into the body of the inlined function. Or try
putting a probe at the call site of the inlined function. Or use the
.nh
.IR "if @defined($var) { ... }"
.hy
script language construct to test for the resolvability of the context
variable before using it.
.TP
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.
.TP
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 (\f2.zdebug_*\f1 ELF
sections or \f2.xz\f1 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.
.TP
debuginfo configuration
Some tools may generate debuginfo that is unsupported by systemtap, such
as the linux kernel CONFIG_DEBUG_INFO_SPLIT (\f2.dwo\f1 files) option.
Stick with plain ELF/DWARF (optinally split, Fedora-style), if possible.
.SH ALTERNATIVES
In order to reduce reliance on ELF/DWARF debuginfo, consider the use of
statically compiled-in instrumentation, such as kernel tracepoints, or
.IR <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.
.SH SEE ALSO
.nh
.nf
.IR stap (1),
.IR http://dwarfstd.org/ ,
.IR http://sourceware.org/systemtap/wiki/TipContextVariables ,
.IR http://gcc.gnu.org/wiki/Var_Tracking_Assignments ,
.IR warning::debuginfo (7stap),
.IR warning::buildid (7stap),
.IR error::reporting (7stap)
|
