doc/debug-info.txt

   1 Babeltrace Debug Info Analysis
   2 -----------------------------
   3
   4 The babeltrace debug info analysis is a set of features which allow
   5 mapping events from a trace to their location in source code or within
   6 a binary file, based on their `ip` (instruction pointer) field.
   7
   8 Prerequisites
   9 -------------
  10
  11 In order to install a version of babeltrace with debug info support,
  12 the following libraries are required:
  13
  14     * libelf
  15     * libdw
  16
  17 Both of them are provided by the elfutils project
  18 (https://fedorahosted.org/elfutils/), and can be installed through the
  19 elfutils package on Ubuntu, Debian, RHEL, and others.
  20
  21 Compiling for Debug Info Analysis
  22 ---------------------------------
  23
  24 Traced programs on which debug info analysis is to be performed can be
  25 compiled in a few different ways, and still lead to useful results.
  26
  27 Ideally, one should compile the program in debug mode, which is
  28 achieved on gcc by simply using the `-g` flag. This generates debug
  29 information in the operating system's native format, which is then
  30 used by babeltrace to map an event's source location to a file and
  31 line number, and the name of the surrounding function.
  32
  33 Do note that only debug information in DWARF format, version 2 or
  34 later, is currently supported by babeltrace. Use the `-gdwarf` or
  35 `-gdwarf-(VERSION)` to explicitly generate DWARF debug information.
  36
  37 If the executable is not compiled with `-g` or an equivalent option
  38 enabled, and thus no DWARF information is available, babeltrace will
  39 use ELF symbols from the executable. Instead of providing source file,
  40 line number and function name, however, the analysis will provide the
  41 name of the nearest function symbol, plus an offset in bytes to the
  42 location in the executable from which the event originated.
  43
  44 If the executable has neither ELF symbols nor DWARF information,
  45 babeltrace will be unable to map an event to its source location and
  46 will simply display the instruction pointer (address), as in prior
  47 versions of babeltrace.
  48
  49 Getting the Right Tracer
  50 ------------------------
  51
  52 Debug info analysis is performed automatically by babeltrace, provided
  53 the trace contains sufficient information. In order to be able to
  54 trace all the necessary information, the following software is
  55 required:
  56
  57     * lttng-ust version 2.8.0 or later
  58     * lttng-tools, corresponding version
  59
  60 You can get these from source at:
  61
  62     * https://github.com/lttng/lttng-ust
  63     * https://github.com/lttng/lttng-tools
  64
  65 Ubuntu users also have the option of installing via the LTTng daily
  66 PPA:
  67
  68     * https://launchpad.net/~lttng/+archive/ubuntu/daily
  69
  70 Tracing for Debug Info Analysis
  71 -------------------------------
  72
  73 Babeltrace needs some extra information from contexts, namely ip and
  74 vpid, to perform its analysis. These can be enabled after the creation
  75 of a tracing session as follows:
  76
  77    $ lttng add-context --userspace --type ip --type vpid
  78
  79 The tracing can then be performed as it normally would. Once the trace
  80 is collected, it can the be read by babeltrace for analysis.
  81
  82 Analysing the Trace
  83 -------------------
  84
  85 To perform the analysis, the trace can simply be read as it normally
  86 would:
  87
  88    $ babeltrace <path/to/trace>
  89
  90 Debug info analysis is on by default and will automatically print the
  91 extra source location information if it can find it. A sample output
  92 may look like this:
  93
  94     [...]
  95     [16:18:15.845829429] (+0.000011697) colossus my_provider:my_first_tracepoint: { cpu_id = 2 }, { ip = 0x7F4D2A5D550E, debug_info = { bin = "libhello.so+0x150e", func = "foo+0xa9", src = "libhello.c:7" }, vpid = 28719 }, { my_string_field = "hello, tracer", my_integer_field = 42 }
  96     [16:18:15.845841484] (+0.000012055) colossus my_provider:my_first_tracepoint: { cpu_id = 2 }, { ip = 0x7F4D2A5D55E0, debug_info = { bin = "libhello.so+0x15e0", func = "bar+0xa9", src = "libhello.c:13" }, vpid = 28719 }, { my_string_field = "recoltes et semailles", my_integer_field = 57 }
  97     [16:18:15.845844852] (+0.000003368) colossus my_provider:my_other_tracepoint: { cpu_id = 2 }, { ip = 0x7F4D2A5D56A5, debug_info = { bin = "libhello.so+0x16a5", func = "baz+0x9c", src = "libhello.c:20" }, vpid = 28719 }, { some_field = 1729 }
  98     [...]
  99
 100 The interesting part is the debug_info section of the context:
 101
 102     debug_info = { bin = "libhello.so+0x150e", func = "foo+0xa9", src = "libhello.c:7" }
 103
 104 This is the expected output for events generated by an executable for
 105 which DWARF information is available. It shows the name of the binary
 106 and offset to the tracepoint, the name of the function containing the
 107 tracepoint instance which generated the event ("foo") and the offset
 108 within the function, and its source location ("libhello.c", line 7).
 109
 110 The second event in the sample output is of the same type
 111 ("my_first_tracepoint"), but it was generated by a different
 112 tracepoint instance, hence the different source location (line 13) and
 113 function ("bar").
 114
 115 The third event, of a different type, also shows debug information.
 116
 117 If DWARF info is absent, but ELF symbols are not stripped, the output
 118 will instead look like this:
 119
 120     [...]
 121     [16:18:15.845829429] (+0.000011697) colossus my_provider:my_first_tracepoint: { cpu_id = 2 }, { ip = 0x7F4D2A5D550E, debug_info = { bin = "libhello.so+0x150e", func = "foo+0xa9" }, vpid = 28719 }, { my_string_field = "hello, tracer", my_integer_field = 42 }
 122     [16:18:15.845841484] (+0.000012055) colossus my_provider:my_first_tracepoint: { cpu_id = 2 }, { ip = 0x7F4D2A5D55E0, debug_info = { bin = "libhello.so+0x15e0", func = "bar+0xa9" }, vpid = 28719 }, { my_string_field = "recoltes et semailles", my_integer_field = 57 }
 123     [16:18:15.845844852] (+0.000003368) colossus my_provider:my_other_tracepoint: { cpu_id = 2 }, { ip = 0x7F4D2A5D56A5, debug_info = { bin = "libhello.so+0x16a5", func = "baz+0x9c" }, vpid = 28719 }, { some_field = 1729 }
 124     [...]
 125
 126 The debug information now provides both binary and function location
 127 information, but no source location information, as this requires
 128 DWARF. The function names are in fact resolved using ELF symbols, so
 129 there may be a discrepancy with those provided by DWARF (e.g. in the
 130 case of mangling).
 131
 132 Paths to the binary and to the source location (if any) can be
 133 expanded by using the command-line option
 134 --debug-info-full-path. Otherwise, only the filename is shown.
 135
 136 Debug Info and Dynamic Loading
 137 ------------------------------
 138
 139 Babeltrace can resolve addresses of events originating from
 140 dynamically loaded libraries, provided that some extra information is
 141 collected at tracing time.
 142
 143 This can be achieved by preloading LTTng UST's libdl helper when
 144 launching the program to be traced, like so:
 145
 146     $ LD_PRELOAD="liblttng-ust-dl.so" <path/to/executable>
 147
 148 The tracing and analysis can now be performed as described in prior
 149 sections, and events from tracepoints in dlopened libraries will be
 150 resolved automatically by babeltrace.
 151
 152 Separate Debug Info
 153 -------------------
 154
 155 It is possible to store DWARF debug information separate from an
 156 executable, whether for concerns of file size, or simply to facilitate
 157 the sharing of the debug information.
 158
 159 This is usually achieved via one of two mechanisms, namely build ID
 160 and debug link. Both methods permit separate executables and debug
 161 information. Their use and operation is described in GDB's
 162 documentation at:
 163
 164     https://sourceware.org/gdb/onlinedocs/gdb/Separate-Debug-Files.html
 165
 166 Babeltrace will find separate debug files automatically, provided they
 167 follow the requirements described in the documentation above. The
 168 debug information lookup order is the same as GDB's, that is first
 169 debug info is looked for within the executable, then through the build
 170 ID method in the standard /usr/lib/debug/.build-id/ location, and
 171 finally in the various possible debug link locations. The first debug
 172 information file found is used.
 173
 174 The --debug-info-dir command-line option can be used to override the
 175 default /usr/lib/debug/ directory used in build ID and debug link
 176 lookups. Multiple debug info directories are currently not supported.
 177
 178 Target Prefix
 179 -------------
 180
 181 The debug info analysis uses the paths to the executables as collected
 182 during tracing as one mechanism to resolve DWARF or ELF
 183 information. If the trace was taken on a separate machine, for
 184 instance, it is possible to use --debug-info-target-prefix to specify
 185 a prefix directory, representing the root of the target filesystem,
 186 which will then be used for lookups. For example, if an executable was
 187 located at /usr/bin/foo on the target system, it could be placed at
 188 /home/efficios/target/usr/bin/foo on the system on which the analysis
 189 is performed. In this case, the prefix is /home/efficios/target/.