Ubuntu Manpage: babeltrace2-filter.lttng-utils.debug-info - Babeltrace 2: Debugging information filter component class

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plucky (7) babeltrace2-filter.lttng-utils.debug-info.7.gz

NAME

       babeltrace2-filter.lttng-utils.debug-info - Babeltrace 2: Debugging information filter component class
       for LTTng traces

DESCRIPTION

A Babeltrace 2 filter.lttng-utils.debug-info message iterator creates and emits copies of upstream
messages, augmenting LTTng event messages with debugging information when it’s available and possible.

Messages without
debugging information
|
| +----------------------------+
| | flt.lttng-utils.debug-info |
| | |
'->@ in out @--> Messages with
+----------------------------+ debugging information

See babeltrace2-intro(7) to learn more about the Babeltrace 2 project and its core concepts.

A filter.lttng-utils.debug-info message iterator uses the LTTng state dump events as well as the ip
(instruction pointer) and vpid (process ID) common context fields of the event to locate and read the
corresponding debugging information. The message iterator can find the extra debugging information in an
executable file or in a directory containing debugging information which the compiler creates.

The new LTTng events (copies of the original ones with added debugging information) contain, when
possible, a new event common context structure field (besides the ip field) named debug_info by default
(you can use the debug-info-field-name parameter to choose another name). This structure field contains
the following fields:

bin [string]
Executable path or name followed with @ADDR or +ADDR, where ADDR is the address (hexadecimal) where
it was loaded while being traced.

@ADDR means ADDR is an absolute address, and +ADDR means ADDR is a relative address.

Examples: my-program@0x4b7fdd23, my-program+0x18d7c.

func [string]
Function name followed with +OFFSET, where OFFSET is the offset (hexadecimal) from the beginning of
the function symbol in the executable file.

Example: load_user_config+0x194.

src [string]
Source file path or name followed with :LINE, where LINE is the line number in this source file at
which the event occurred.

Example: user-config.c:1025.

Any of the previous fields can be an empty string if the debugging information was not available for the
analyzed original LTTng event.

A filter.lttng-utils.debug-info message iterator systematically copies the upstream messages, but it only
augments compatible LTTng event classes. This means that the message iterator copies messages of
non-LTTng trace (see “LTTng prerequisites”) without alteration.

Compile an executable for debugging information analysis
With GCC or Clang, you need to compile the program or library source files in debug mode with the -g
option. This option makes the compiler generate debugging information in the native format of the
operating system. This format is recognized by a filter.lttng-utils.debug-info component: it can
translate the instruction pointer field of an event common context to a source file and line number,
along with the name of the surrounding function.

Important
This component class only supports the debugging information in DWARF format, version 2 or later. Use
the -gdwarf or -gdwarf-VERSION (where VERSION is the DWARF version) compiler options to explicitly
generate DWARF debugging information.

If you don’t compile the source files of the executable with the -g option or with an equivalent option,
then no DWARF information is available: the message iterator uses ELF symbols from the executable file
instead. In this case, the events that the message iterator creates don’t contain the source file and
line number (see the src field), but only the name of the nearest function symbol with an offset in bytes
to the location in the executable from which the LTTng event occurred (see the func field).

If the executable file has neither ELF symbols nor DWARF information, then the filter.lttng-utils.debug-
info message iterator cannot map the event to its source location: the message iterator still copies the
upstream messages but without altering them.

LTTng prerequisites
A filter.lttng-utils.debug-info message iterator can only analyze user space events which LTTng (see
<https://lttng.org>) 2.8.0 or later generates.

To get debugging information for LTTng-UST events which occur in executables and libraries which the
system loader loads (what you can see with ldd(1)):

1. Add the ip and vpid context fields to user space event records:

$ lttng add-context --userspace --type=ip --type=vpid

See lttng-add-context(1) for more details.

2. Enable the LTTng-UST state dump events:

$ lttng enable-event --userspace 'lttng_ust_statedump:*'

See lttng-enable-event(1) and lttng-ust(3) for more details.

To get debugging information for LTTng-UST events which occur in dynamically loaded objects, for example
plugins:

1. Do the previous steps (add context fields and enable the LTTng-UST state dump events).

2. Enable the LTTng-UST dynamic linker tracing helper events:

$ lttng enable-event --userspace 'lttng_ust_dl:*'

See lttng-ust-dl(3) for more details.

3. When you are ready to trace, start your application with the LD_PRELOAD environment variable set to
liblttng-ust-dl.so:

$ LD_PRELOAD=liblttng-ust-dl.so my-app

Separate debugging information
You can store DWARF debugging information outside the executable itself, whether it’s to reduce the file
size of the executable or simply to facilitate sharing the debugging information.

This is usually achieved via one of two mechanisms, namely build ID and debug link. Their use and
operation is described in the Debugging Information in Separate Files (see
<https://sourceware.org/gdb/onlinedocs/gdb/Separate-Debug-Files.html>) section of GDB’s documentation.

A filter.lttng-utils.debug-info message iterator can find separate debugging information files
automatically, as long as they meet the requirements stated in this manual page.

The debugging information lookup order is the same as GDB’s, namely:

1. Within the executable file itself.

2. Through the build ID method in the /usr/lib/debug/.build-id directory.

3. In the various possible debug link locations.

The message iterator uses the first debugging information file that it finds.

You can use the debug-info-dir initialization parameter to override the default /usr/lib/debug directory
used in the build ID and debug link methods.

Note
It’s currently not possible to make this component search for debugging information in multiple
directories.

Target prefix
The debugging information analysis that a filter.lttng-utils.debug-info message iterator performs uses
the paths to the executables as collected during tracing as the default mechanism to resolve DWARF and
ELF information.

If the trace was recorded on a separate machine, however, then you can use the target-prefix parameter to
specify a prefix directory, that is, the root of the target file system.

For example, if the path of an instrumented executable is /usr/bin/foo on the target system, then you can
place this file at /home/user/target/usr/bin/foo on the system on which you use a filter.lttng-
utils.debug-info component. In this case, the target prefix to use is /home/user/target.

INITIALIZATION PARAMETERS

       debug-info-dir=DIR [optional string]
           Use DIR as the directory from which to load debugging information with the build ID and debug link
           methods instead of /usr/lib/debug.

       debug-info-field-name=NAME [optional string]
           Name the debugging information structure field in the common context of the created events NAME
           instead of the default debug_info.

       full-path=VAL [optional boolean]
           If VAL is true, then use the full path when writing the executable name (bin) and source file name
           (src) fields in the debug_info context field of the created events.

           Default: false.

       target-prefix=DIR [optional string]
           Use DIR as the root directory of the target file system instead of /.

PORTS

           +----------------------------+
           | flt.lttng-utils.debug-info |
           |                            |
           @ in                     out @
           +----------------------------+

   Input
       in
           Single input port.

   Output
       out
           Single output port.

BUGS

       If you encounter any issue or usability problem, please report it on the Babeltrace bug tracker (see
       <https://bugs.lttng.org/projects/babeltrace>).

RESOURCES

       The Babeltrace project shares some communication channels with the LTTng project (see
       <https://lttng.org/>).

       •   Babeltrace website (see <https://babeltrace.org/>)

       •   Mailing list (see <https://lists.lttng.org>) for support and development: lttng-dev@lists.lttng.org

       •   IRC channel (see <irc://irc.oftc.net/lttng>): #lttng on irc.oftc.net

       •   Bug tracker (see <https://bugs.lttng.org/projects/babeltrace>)

       •   Git repository (see <https://git.efficios.com/?p=babeltrace.git>)

       •   GitHub project (see <https://github.com/efficios/babeltrace>)

       •   Continuous integration (see <https://ci.lttng.org/view/Babeltrace/>)

       •   Code review (see <https://review.lttng.org/q/project:babeltrace>)

AUTHORS

       The Babeltrace 2 project is the result of hard work by many regular developers and occasional
       contributors.

       The current project maintainer is Jérémie Galarneau <mailto:jeremie.galarneau@efficios.com>.

COPYRIGHT

       This component class is part of the Babeltrace 2 project.

       Babeltrace is distributed under the MIT license (see <https://opensource.org/licenses/MIT>).