linux-kernel - Re: [PATCH v2 4/4] samples: Add example of using PTRACE

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Date:   Mon, 9 Dec 2019 20:30:00 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Sargun Dhillon <sargun@...gun.me>
Cc:     linux-kernel@...r.kernel.org,
        containers@...ts.linux-foundation.org, linux-api@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, tycho@...ho.ws, jannh@...gle.com,
        cyphar@...har.com, christian.brauner@...ntu.com,
        luto@...capital.net, viro@...iv.linux.org.uk
Subject: Re: [PATCH v2 4/4] samples: Add example of using PTRACE_GETFD in
 conjunction with user trap

On 12/09, Sargun Dhillon wrote:
>
> +#define CHILD_PORT_TRY_BIND	80
> +#define CHILD_PORT_ACTUAL_BIND	4998

...

> +static int handle_req(int listener)
> +{
> +	struct sockaddr_in addr = {
> +		.sin_family	= AF_INET,
> +		.sin_port	= htons(4998),

then I think
		.sin_port = htons(CHILD_PORT_ACTUAL_BIND);

would be more clear...

> +		.sin_addr	= {
> +			.s_addr	= htonl(INADDR_LOOPBACK)
> +		}
> +	};
> +	struct ptrace_getfd_args getfd_args = {
> +		.options = PTRACE_GETFD_O_CLOEXEC
> +	};
> +	struct seccomp_notif_sizes sizes;
> +	struct seccomp_notif_resp *resp;
> +	struct seccomp_notif *req;
> +	int fd, ret = 1;
> +
> +	if (seccomp(SECCOMP_GET_NOTIF_SIZES, 0, &sizes) < 0) {
> +		perror("seccomp(GET_NOTIF_SIZES)");
> +		goto out;
> +	}
> +	req = malloc(sizes.seccomp_notif);
> +	if (!req)
> +		goto out;
> +	memset(req, 0, sizeof(*req));
> +
> +	resp = malloc(sizes.seccomp_notif_resp);
> +	if (!resp)
> +		goto out_free_req;
> +	memset(resp, 0, sizeof(*resp));
> +
> +	if (ioctl(listener, SECCOMP_IOCTL_NOTIF_RECV, req)) {
> +		perror("ioctl recv");
> +		goto out;
> +	}
> +	printf("Child tried to call bind with fd: %lld\n", req->data.args[0]);
> +	getfd_args.fd = req->data.args[0];
> +	fd = ptrace_getfd(req->pid, &getfd_args);

and iiuc otherwise you do not need to ptrace the child. So you could remove
ptrace(PTRACE_SEIZE) in main() and just do

	ptrace(PTRACE_SEIZE, req->pid);
	fd = ptrace_getfd(req->pid, &getfd_args);
	ptrace(PTRACE_DETACH, req->pid);

here. However, PTRACE_DETACH won't work, it needs the stopped tracee. We can
add PTRACE_DETACH_ASYNC, but this makes me think that PTRACE_GETFD has nothing
to do with ptrace.

May be a new syscall which does ptrace_may_access() + get_task_file() will make
more sense?

Oleg.