Re: [PATCH 6/6] tr2: log N parent process names on Linux

From: Eric Sunshine <sunshine@sunshineco.com>
To: "Ævar Arnfjörð Bjarmason" <avarab@gmail.com>
Cc: Git List <git@vger.kernel.org>,
	Junio C Hamano <gitster@pobox.com>,
	Emily Shaffer <emilyshaffer@google.com>
Subject: Re: [PATCH 6/6] tr2: log N parent process names on Linux
Date: Wed, 25 Aug 2021 19:49:21 -0400	[thread overview]
Message-ID: <CAPig+cShVK1OChWP+BCx-_8wPV2BKwem8vHgTdYF2gAZX0pFUQ@mail.gmail.com> (raw)
In-Reply-To: <patch-6.6-da003330800-20210825T231400Z-avarab@gmail.com>

On Wed, Aug 25, 2021 at 7:20 PM Ævar Arnfjörð Bjarmason
<avarab@gmail.com> wrote:
> [...]
> Extend the functionality added there to also log full parent chain on
> Linux. In 2f732bf15e6 it was claimed that "further ancestry info can
> be gathered with procfs, but it's unwieldy to do so.".
> [...]
> Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
> ---
> diff --git a/compat/linux/procinfo.c b/compat/linux/procinfo.c
> @@ -4,27 +4,129 @@
> +/*
> + * We need more complex parsing instat_parent_pid() and

s/instat_parent_pid/in stat_parent_pid/

> + * parse_proc_stat() below than a dumb fscanf(). That's because while
> + * the statcomm field is surrounded by parentheses, the process itself
> + * is free to insert any arbitrary byte sequence its its name. That
> + * can include newlines, spaces, closing parentheses etc. See
> + * do_task_stat() in fs/proc/array.c in linux.git, this is in contrast
> + * with the escaped version of the name found in /proc/%d/status.
> + *
> + * So instead of using fscanf() we'll read N bytes from it, look for
> + * the first "(", and then the last ")", anything in-between is our
> + * process name.
> + *
> + * How much N do we need? On Linux /proc/sys/kernel/pid_max is 2^15 by
> + * default, but it can be raised set to values of up to 2^22. So
> + * that's 7 digits for a PID. We have 2 PIDs in the first four fields
> + * we're interested in, so 2 * 7 = 14.
> + *
> + * We then have 4 spaces between those four values, which brings us up
> + * to 18. Add the two parentheses and it's 20. The "state" is then one
> + * character (now at 21).
> + *
> + * Finally the maximum length of the "comm" name itself is 15
> + * characters, e.g. a setting of "123456789abcdefg" will be truncated
> + * to "123456789abcdef". See PR_SET_NAME in prctl(2). So all in all
> + * we'd need to read 21 + 15 = 36 bytes.
> + *
> + * Let's just read 2^6 (64) instead for good measure. If PID_MAX ever
> + * grows past 2^22 we'll be future-proof. We'll then anchor at the
> + * last ")" we find to locate the parent PID.
> + */