tr2: log N parent process names on Linux

In 2f732bf15e6 (tr2: log parent process name, 2021-07-21) we started
logging parent process names, but only logged all parents on Windows.
on Linux only the name of the immediate parent process was logged.

Extend the functionality added there to also log full parent chain on
Linux.

This requires us to lookup "/proc/<getppid()>/stat" instead of
"/proc/<getppid()>/comm". The "comm" file just contains the name of the
process, but the "stat" file has both that information, and the parent
PID of that process, see procfs(5). We parse out the parent PID of our
own parent, and recursively walk the chain of "/proc/*/stat" files all
the way up the chain. A parent PID of 0 indicates the end of the
chain.

It's possible given the semantics of Linux's PID files that we end up
getting an entirely nonsensical chain of processes. It could happen if
e.g. we have a chain of processes like:

    1 (init) => 321 (bash) => 123 (git)

Let's assume that "bash" was started a while ago, and that as shown
the OS has already cycled back to using a lower PID for us than our
parent process. In the time it takes us to start up and get to
trace2_collect_process_info(TRACE2_PROCESS_INFO_STARTUP) our parent
process might exit, and be replaced by an entirely different process!

We'd racily look up our own getppid(), but in the meantime our parent
would exit, and Linux would have cycled all the way back to starting
an entirely unrelated process as PID 321.

If that happens we'll just silently log incorrect data in our ancestry
chain. Luckily we don't need to worry about this except in this
specific cycling scenario, as Linux does not have PID
randomization. It appears it once did through a third-party feature,
but that it was removed around 2006[1]. For anyone worried about this
edge case raising PID_MAX via "/proc/sys/kernel/pid_max" will mitigate
it, but not eliminate it.

One thing we don't need to worry about is getting into an infinite
loop when walking "/proc/*/stat". See 353d3d77f4f (trace2: collect
Windows-specific process information, 2019-02-22) for the related
Windows code that needs to deal with that, and [2] for an explanation
of that edge case.

Aside from potential race conditions it's also a bit painful to
correctly parse the process name out of "/proc/*/stat". A simpler
approach is to use fscanf(), see [3] for an implementation of that,
but as noted in the comment being added here it would fail in the face
of some weird process names, so we need our own parse_proc_stat() to
parse it out.

With this patch the "ancestry" chain for a trace2 event might look
like this:

    $ GIT_TRACE2_EVENT=/dev/stdout ~/g/git/git version | grep ancestry | jq -r .ancestry
    [
      "bash",
      "screen",
      "systemd"
    ]

And in the case of naughty process names like the following. This uses
perl's ability to use prctl(PR_SET_NAME, ...). See
Perl/perl5@7636ea95c5 (Set the legacy process name with prctl() on
assignment to $0 on Linux, 2010-04-15)[4]:

    $ perl -e '$0 = "(naughty\nname)"; system "GIT_TRACE2_EVENT=/dev/stdout ~/g/git/git version"' | grep ancestry | jq -r .ancestry
    [
      "sh",
      "(naughty\nname)",
      "bash",
      "screen",
      "systemd"
    ]

1. https://grsecurity.net/news#grsec2110
2. https://lore.kernel.org/git/48a62d5e-28e2-7103-a5bb-5db7e197a4b9@jeffhostetler.com/
3. https://lore.kernel.org/git/87o8agp29o.fsf@evledraar.gmail.com/
4. https://github.com/Perl/perl5/commit/7636ea95c57762930accf4358f7c0c2dec086b5e

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Acked-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

0 files changed, 0 insertions, 0 deletions