1ERROR::DWARF(7stap) ERROR::DWARF(7stap)
2
6 error::dwarf - dwarf debuginfo quality problems
7
10 Systemtap sometimes relies on ELF/DWARF debuginfo for programs being
11 instrumented to locate places to probe, or context variables to
12 read/write, just like a symbolic debugger does. Even though examina‐
13 tion of the program's source code may show variables or lines where
14 probes may be desired, the compiler must preserve information about
15 them for systemtap (or a debugger such as gdb) to get pinpoint access
16 to the desired information. If a script requires such data, but the
17 compiler did not preserve enough of it, pass-2 errors may result.
18 Common conditions that trigger these problems include;
20 compiler version
23 Prior to GCC version 4.5, debuginfo quality was fairly limited.
24 Often developers were advised to build their programs with -O0
25 -g flags to disable optimization. GCC version 4.5 introduced a
26 facility called "variable-tracking assignments" that allows it
27 to generate high-quality debuginfo under full -O2 -g optimiza‐
28 tion. It is not perfect, but much better than before. Note
29 that, due to another gcc bug (PR51358) -O0 -g can actually some‐
30 times make debuginfo quality worse than for -O2 -g.
31 Another related problem involves debuginfo quality for the pro‐
33 logue area of a function (PR15123), wherein a program compiled
34 with CFLAGS=-mfentry (especially the kernel, for ftrace) may
35 lack accurate debuginfo for the entry instructions for gcc prior
36 to version 4.8. If able, arrange to compile your programs with
37 -grecord-gcc-switches CFLAGS, and/or try rerunning systemtap
38 with $PR15123_ASSUME_MFENTRY=1.
39 function inlining
42 Even modern gcc sometimes has problems with parameters for in‐
43 lined functions. It may be necessary to change the script to
44 probe at a slightly different place (try a .statement() probe,
45 instead of a .function() probe, somewhere a few source lines in‐
46 to the body of the inlined function. Or try putting a probe at
47 the call site of the inlined function. Or use the if
48 @defined($var) { ... } script language construct to test for the
49 resolvability of the context variable before using it.
50 instruction reordering
53 Heavily optimized code often smears the instructions from multi‐
54 ple source statements together. This can leave systemtap with
55 no place to choose to place a probe, especially a statement
56 probe specified by line number. Systemtap may advise to try a
57 nearby line number, but these may not work well either. Consid‐
58 er placing a probe by a statement wildcard or line number range.
59 elfutils configuration
62 It is possible that the DWARF debuginfo being sought is avail‐
63 able, but not in a format acceptable to the copy of elfutils
64 used by systemtap. For example, your copy of gcc might produce
65 compressed debuginfo (.zdebug_* ELF sections or .xz files) while
66 your copy of elfutils might lack appropriate decompression capa‐
67 bilities. Unfortunately, there is no easy way to tell if this
68 is the problem. If you're building your own copy of elfutils,
69 ensure all decompression library headers/libraries are available
70 at build time.
71 debuginfo configuration
74 Some tools may generate debuginfo that is unsupported by system‐
75 tap, such as the linux kernel CONFIG_DEBUG_INFO_SPLIT (.dwo
76 files) option. Stick with plain ELF/DWARF (optinally split, Fe‐
77 dora-style), if possible.
78
81 In order to reduce reliance on ELF/DWARF debuginfo, consider the use of
82 statically compiled-in instrumentation, such as kernel tracepoints, or
83 <sys/sdt.h> userspace markers. Such instrumentation hook sites are
84 relatively low cost (just one NOP instruction for sdt.h), and nearly
85 guarantee the availability of parameter data and a reliable probe site,
86 all without reliance on debuginfo.
87
90 stap(1),
91 http://dwarfstd.org/,
92 http://sourceware.org/systemtap/wiki/TipContextVariables,
93 http://gcc.gnu.org/wiki/Var_Tracking_Assignments,
94 warning::debuginfo(7stap),
95 error::reporting(7stap)
96 ERROR::DWARF(7stap)