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 debuginfo missing or mismatching
23 Sometimes debuginfo is installed, but does not match the bina‐
24 ries being probed. See the warning::debuginfo man page for more
25 help for locating correct debuginfo.
26 compiler version
29 Prior to GCC version 4.5, debuginfo quality was fairly limited.
30 Often developers were advised to build their programs with -O0
31 -g flags to disable optimization. GCC version 4.5 introduced a
32 facility called "variable-tracking assignments" that allows it
33 to generate high-quality debuginfo under full -O2 -g optimiza‐
34 tion. It is not perfect, but much better than before. Note
35 that, due to another gcc bug (PR51358) -O0 -g can actually some‐
36 times make debuginfo quality worse than for -O2 -g.
37 Another related problem involves debuginfo quality for the pro‐
39 logue area of a function (PR15123), wherein a program compiled
40 with CFLAGS=-mfentry (especially the kernel, for ftrace) may
41 lack accurate debuginfo for the entry instructions for gcc prior
42 to version 4.8. If able, arrange to compile your programs with
43 -grecord-gcc-switches CFLAGS, and/or try rerunning systemtap
44 with $PR15123_ASSUME_MFENTRY=1.
45 linux kbuild damaging debuginfo quality
48 An upstream kernel commit #2062afb4f804a put
49 -fno-var-tracking-assignments into KCFLAGS, dramatically reduc‐
50 ing debuginfo quality, which can cause debuginfo failures. The
51 simplest fix is to erase, excise, nay, eradicate this line from
52 the top level linux Makefile:
53 KBUILD_CFLAGS += [...] -fno-var-tracking-assignments [...]
55 function inlining
58 Even modern gcc sometimes has problems with parameters for in‐
59 lined functions. It may be necessary to change the script to
60 probe at a slightly different place. Try a .statement() probe,
61 instead of a .function() probe, somewhere a few source lines in‐
62 to the body of the inlined function. Or try putting a probe at
63 the call site of the inlined function. Or use the if
64 @defined($var) { ... } script language construct to test for the
65 resolvability of the context variable before using it.
66 instruction reordering
69 Heavily optimized code often smears the instructions from multi‐
70 ple source statements together. This can leave systemtap with
71 no place to choose to place a probe, especially a statement
72 probe specified by line number. Systemtap may advise to try a
73 nearby line number, but these may not work well either. Consid‐
74 er placing a probe by a statement wildcard or line number range.
75 elfutils configuration
78 It is possible that the DWARF debuginfo being sought is avail‐
79 able, but not in a format acceptable to the copy of elfutils
80 used by systemtap. For example, your copy of gcc might produce
81 compressed debuginfo (.zdebug_* ELF sections or .xz files) while
82 your copy of elfutils might lack appropriate decompression capa‐
83 bilities. Unfortunately, there is no easy way to tell if this
84 is the problem. If you're building your own copy of elfutils,
85 ensure all decompression library headers/libraries are available
86 at build time.
87 debuginfo configuration
90 Some tools may generate debuginfo that is unsupported by system‐
91 tap, such as the linux kernel CONFIG_DEBUG_INFO_SPLIT (.dwo
92 files) option. Stick with plain ELF/DWARF (optinally split, Fe‐
93 dora-style), if possible.
94
98 In order to reduce reliance on ELF/DWARF debuginfo, consider the use of
99 statically compiled-in instrumentation, such as kernel tracepoints, or
100 <sys/sdt.h> userspace markers. Such instrumentation hook sites are
101 relatively low cost (just one NOP instruction for sdt.h), and nearly
102 guarantee the availability of parameter data and a reliable probe site,
103 all without reliance on debuginfo.
104
107 stap(1),
108 http://dwarfstd.org/,
109 http://sourceware.org/systemtap/wiki/TipContextVariables,
110 http://gcc.gnu.org/wiki/Var_Tracking_Assignments,
111 warning::debuginfo(7stap),
112 warning::buildid(7stap),
113 error::reporting(7stap)
114 ERROR::DWARF(7stap)