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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 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.



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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Time: 17:41:48 GMT, December 22, 2014