debuginfo server progress update

[Aaron Merey <amerey@redhat.com>]

Hello,

The debuginfo server, a web service that offers lightweight on-demand
lookups of debuginfo-related data,
along with its elfutils client have begun to take shape so I want to
provide an update on where we currently
are in its development and how to use it.

We plan on shipping debuginfo server with elfutils and it is currently
available on the branch 'fche/dbgserver'
of the elfutils git repo. The http web server itself is able to scan a
directory tree for any debuginfo, executable
and source files and index them by build-id. The elfutils client hooks into
libdwfl and provides a fallback so
that if the library is not able to locate a debuginfo file, the client, if
enabled, will attempt to query debuginfo
servers for debuginfo containing a particular build-id.

As an example, suppose we are trying to trace some program with systemtap.
One of the stap probes we
want to run requires DWARF debuginfo but the executable has been stripped
of this information and the
corresponding debuginfo file is not in any of the default directories that
stap checks.

    $ stap -e 'probe process("/path/to/exec").function("*") { [...] }'
    semantic error: while resolving probe point: identifier 'process' at
t.stp:1:7
            source: probe process("/path/to/exec").function("*") {
    semantic error: no match (similar functions: exit, open, textdomain,
tfind, dup)

Systemtap uses elfutils to locate and read debuginfo files, so it is able
to make use of debuginfo server's
elfutils client. First we run debuginfo server and point it to the
directory containing the stripped debuginfo.

    $ dbgserver -vvv -p 8002 -F ~/debuginfo_dir &
    [Tue Jul  2 15:00:37] (20459/20462): file-path scanning
/home/amerey/debuginfo_dir
    [Tue Jul  2 15:00:37] (20459/20459): Started http server on port 8002,
database path /home/amerey/.dbgserver.sqlite
    [Tue Jul  2 15:00:37] (20459/20462): fts traversing
/home/amerey/debuginfo_dir
    [Tue Jul  2 15:00:37] (20459/20462): fts traversing
/home/amerey/debuginfo_dir/debuginfo
    [Tue Jul  2 15:00:37] (20459/20462): recorded
buildid=0a0cd15e690a378ec77359bb2eeb76ea0f8d67f8 \
        file=/home/amerey/debuginfo_dir/debuginfo mtime=1561060856 as
executable and debuginfo
    [...]

In order to enable the elfutils client, there must be a $DBGSERVER_URLS
environment variable containing
at least one URL of a debuginfo server (multiple URLs should be separated
by spaces). When we run stap
again, elfutils will query the debuginfo server for debuginfo with the
build-id of the executable.

    $ export DBGSERVER_URLS=localhost:8002
    $ stap -v -e 'probe process("/path/to/exec").function("*") { [...] }'
    [...]
    Pass 5: starting run

This time systemtap successfully acquires the debuginfo and begins running
the probe. Although in this
example everything is running locally, the server and the debuginfo could
just as well be on a remote host.

Debuginfo server development is ongoing. Some of the features we have
planned include a gdb client
that could query debuginfo servers for debuginfo or source code and the
ability for the server to
automatically search rpms for any of the files requested by a client. We
also are planning a talk/demo
at GNU Cauldron this September in Montréal, Canada (
https://gcc.gnu.org/wiki/cauldron2019).

Hopefully this helped give you a sense of how to use debuginfo server,
where it is in its development,
and what is to come. Feedback is always appreciated!

Aaron Merey