linux-kernel - Re: [PATCH 2/2] samples: make pidfd-metadata fail gracefully on older kernels

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Message-ID: <20190624095047.2ixebkmurvv4qjrk@brauner.io>
Date:   Mon, 24 Jun 2019 11:50:48 +0200
From:   Christian Brauner <christian@...uner.io>
To:     "Dmitry V. Levin" <ldv@...linux.org>
Cc:     Jann Horn <jannh@...gle.com>, Oleg Nesterov <oleg@...hat.com>,
        Arnd Bergmann <arnd@...db.de>, linux-api@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] samples: make pidfd-metadata fail gracefully on
 older kernels

On Sun, Jun 23, 2019 at 02:28:00PM +0300, Dmitry V. Levin wrote:
> Initialize pidfd to an invalid descriptor, to fail gracefully on
> those kernels that do not implement CLONE_PIDFD and leave pidfd
> unchanged.
> 
> Signed-off-by: Dmitry V. Levin <ldv@...linux.org>

Reviewed-by: Christian Brauner <christian@...uner.io>

Thank you Dmitry, queueing this up for rc7.

> ---
>  samples/pidfd/pidfd-metadata.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/samples/pidfd/pidfd-metadata.c b/samples/pidfd/pidfd-metadata.c
> index 14b454448429..c459155daf9a 100644
> --- a/samples/pidfd/pidfd-metadata.c
> +++ b/samples/pidfd/pidfd-metadata.c
> @@ -83,7 +83,7 @@ static int pidfd_metadata_fd(pid_t pid, int pidfd)
>  
>  int main(int argc, char *argv[])
>  {
> -	int pidfd = 0, ret = EXIT_FAILURE;
> +	int pidfd = -1, ret = EXIT_FAILURE;
>  	char buf[4096] = { 0 };
>  	pid_t pid;
>  	int procfd, statusfd;
> @@ -91,7 +91,11 @@ int main(int argc, char *argv[])
>  
>  	pid = pidfd_clone(CLONE_PIDFD, &pidfd);
>  	if (pid < 0)
> -		exit(ret);
> +		err(ret, "CLONE_PIDFD");
> +	if (pidfd == -1) {
> +		warnx("CLONE_PIDFD is not supported by the kernel");
> +		goto out;
> +	}
>  
>  	procfd = pidfd_metadata_fd(pid, pidfd);
>  	close(pidfd);
> -- 
> ldv