Ubuntu Manpage: babeltrace2-filter.lttng-utils.debug-info - Babeltrace 2's debugging information filter

NAME

       babeltrace2-filter.lttng-utils.debug-info - Babeltrace 2's 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 event common context’s ip (instruction pointer) and vpid (process ID) fields 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’s 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
operating system’s native format. This format is recognized by a filter.lttng-utils.debug-
info component: it can translate the instruction pointer field of an event’s 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 executable’s source files with the -g option or with an
equivalent option, 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 do not 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, 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’s 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 is to
reduce the executable’s file size 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 is 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, you can use the target-prefix
parameter to specify a prefix directory, that is, the root of the target file system.

For example, if an instrumented executable’s path is /usr/bin/foo on the target system,
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=yes [optional boolean]
           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.

       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>).