[lxc-devel] [PATCH] start.c: handle potential signal flood

[Stéphane Graber]

On Thu, Aug 29, 2013 at 10:44:41AM -0500, Serge Hallyn wrote:
> Signalfd does not guarantee that we'll get an event for every signal.
> So if 3 tasks exit at the same time, we may get only one sigchld
> event.  Therefore, in signal_handler(), always check whether init has
> exited.  Do with with WNOWAIT so that we can still wait4 to cleanup
> the init after lxc_poll() exists (rather than complicating the code).
> 
> Note - there is still a race in the kernel which can cause the
> container init to become a defunct child of the host init (!).  This
> doesn't solve that, but is a potential (if very unlikely) race which
> apw pointed out while we were trying to create a reproducer for the
> kernel bug.
> 
> Signed-off-by: Serge Hallyn <serge.hallyn at ubuntu.com>

Acked-by: Stéphane Graber <stgraber at ubuntu.com>

> ---
>  src/lxc/start.c | 15 ++++++++++++---
>  1 file changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/src/lxc/start.c b/src/lxc/start.c
> index f552e35..a574a8d 100644
> --- a/src/lxc/start.c
> +++ b/src/lxc/start.c
> @@ -162,8 +162,10 @@ static int signal_handler(int fd, void *data,
>  			   struct lxc_epoll_descr *descr)
>  {
>  	struct signalfd_siginfo siginfo;
> +	siginfo_t info;
>  	int ret;
>  	pid_t *pid = data;
> +	bool init_died = false;
>  
>  	ret = read(fd, &siginfo, sizeof(siginfo));
>  	if (ret < 0) {
> @@ -176,16 +178,23 @@ static int signal_handler(int fd, void *data,
>  		return -1;
>  	}
>  
> +	// check whether init is running
> +	info.si_pid = 0;
> +	ret = waitid(P_PID, *pid, &info, WEXITED | WNOWAIT | WNOHANG);
> +	if (ret == 0 && info.si_pid == *pid) {
> +		init_died = true;
> +	}
> +
>  	if (siginfo.ssi_signo != SIGCHLD) {
>  		kill(*pid, siginfo.ssi_signo);
>  		INFO("forwarded signal %d to pid %d", siginfo.ssi_signo, *pid);
> -		return 0;
> +		return init_died ? 1 : 0;
>  	}
>  
>  	if (siginfo.ssi_code == CLD_STOPPED ||
>  	    siginfo.ssi_code == CLD_CONTINUED) {
>  		INFO("container init process was stopped/continued");
> -		return 0;
> +		return init_died ? 1 : 0;
>  	}
>  
>  	/* more robustness, protect ourself from a SIGCHLD sent
> @@ -193,7 +202,7 @@ static int signal_handler(int fd, void *data,
>  	 */
>  	if (siginfo.ssi_pid != *pid) {
>  		WARN("invalid pid for SIGCHLD");
> -		return 0;
> +		return init_died ? 1 : 0;
>  	}
>  
>  	DEBUG("container init process exited");
> -- 
> 1.8.3.2
> 
> 
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-- 
Stéphane Graber
Ubuntu developer
http://www.ubuntu.com
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