Re: [PATCH v2 4/4] samples: Add example of using PTRACE_GETFD in conjunction with user trap

On December 9, 2019 8:30:00 PM GMT+01:00, Oleg Nesterov [off-list ref] wrote:

On 12/09, Sargun Dhillon wrote:

quoted

+#define CHILD_PORT_TRY_BIND	80
+#define CHILD_PORT_ACTUAL_BIND	4998

...

quoted

+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...

quoted

+		.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]);

quoted

+	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.

Once more since this annoying app uses html by default...

But we can already do this right now and this is just an improvement.
That's a bit rich for a new syscall imho...

Christian