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Date:   Thu, 20 Jun 2019 12:31:06 +0200
From:   Christian Brauner <christian@brauner.io>
To:     "Dmitry V. Levin" <ldv@altlinux.org>
Cc:     linux-kernel@vger.kernel.org
Subject: Re: [PATCH] samples: make pidfd-metadata fail gracefully on older
 kernels
Message-ID: <20190620103105.cdxgqfelzlnkmblv@brauner.io>
References: <20190620031144.GA32182@altlinux.org>
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On Thu, Jun 20, 2019 at 06:11:44AM +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@altlinux.org>
> ---
>  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..ff109fdac3a5 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;

Hm, that currently won't work since we added a check in fork.c for
pidfd == 0. It it isn't you'll get EINVAL. This was done to ensure that
we can potentially extend CLONE_PIDFD by passing in flags through the
return argument.
However, I find this increasingly unlikely. Especially since the
interface would be horrendous and an absolute last resort.
If clone3() gets merged for 5.3 (currently in linux-next) we also have
no real need anymore to extend legacy clone() this way. So either wait
until (if) we merge clone3() where the check I mentioned is gone anyway,
or remove the pidfd == 0 check from fork.c in a preliminary patch.
Thoughts?

Thanks!
Christian