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.
