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

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description
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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.

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