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crond: reap orphaned grandchildren to prevent zombie buildup

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If a cron job launches a background task, e.g. `sh -c "sleep 5 &"`,
the shell exits immediately and the `sleep` process is re-parented to
PID 1.  When BusyBox `crond` itself happens to be PID 1 (common in a
minimal container), those orphans become direct children of `crond`.
Because `crond` only calls waitpid() for the PIDs it explicitly tracks,
these processes remain forever in Z state and the container slowly
fills with zombies.

Add a small `while (waitpid(-1, NULL, WNOHANG) > 0)` sweep at the end
of check_completions() so any stray children are reaped.  When `crond`
is not PID 1 the loop returns -ECHILD immediately, so behaviour and
overhead on a normal system are unchanged.

Size impact: +12 bytes on x86-64 (gcc 13.3.0 -Os, static)

Signed-off-by: Valentin Lab <valentin.lab@kalysto.org>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
This commit is contained in:
Valentin Lab 2025-05-31 09:56:09 +08:00 committed by Denys Vlasenko
parent 427f808732
commit 41f677ec5d
1 changed files with 4 additions and 0 deletions
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4
miscutils/crond.c
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@ -1001,6 +1001,10 @@ static int check_completions(void)
/* else: r == 0: "process is still running" */
file->cf_has_running = 1;
}
/* Reap any other children we don't actively track */
while (waitpid(-1, NULL, WNOHANG) > 0);
//FIXME: if !file->cf_has_running && file->deleted: delete it!
//otherwise deleted entries will stay forever, right?
num_still_running += file->cf_has_running;