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Tue, 26 Mar 2019 12:19:46 -0700 (PDT) Received: from brauner.io ([2a02:8109:b6bf:d24a:b136:35b0:7c8c:280a]) by smtp.gmail.com with ESMTPSA id c9sm6216691eds.92.2019.03.26.12.19.44 (version=TLS1_3 cipher=AEAD-AES256-GCM-SHA384 bits=256/256); Tue, 26 Mar 2019 12:19:45 -0700 (PDT) Date: Tue, 26 Mar 2019 20:19:43 +0100 From: Christian Brauner To: Joel Fernandes Cc: jannh@google.com, khlebnikov@yandex-team.ru, luto@kernel.org, dhowells@redhat.com, serge@hallyn.com, ebiederm@xmission.com, linux-api@vger.kernel.org, linux-kernel@vger.kernel.org, arnd@arndb.de, keescook@chromium.org, adobriyan@gmail.com, tglx@linutronix.de, mtk.manpages@gmail.com, bl0pbl33p@gmail.com, ldv@altlinux.org, akpm@linux-foundation.org, oleg@redhat.com, nagarathnam.muthusamy@oracle.com, cyphar@cyphar.com, viro@zeniv.linux.org.uk, dancol@google.com Subject: Re: [PATCH v1 2/4] pid: add pidctl() Message-ID: <20190326191942.jyqpk3ehpbvhgvyz@brauner.io> References: <20190326155513.26964-1-christian@brauner.io> <20190326155513.26964-3-christian@brauner.io> <20190326170601.GA101741@google.com> <20190326172231.daa5a53lxf6nz6jn@brauner.io> <20190326181012.GA138478@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20190326181012.GA138478@google.com> User-Agent: NeoMutt/20180716 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Mar 26, 2019 at 02:10:12PM -0400, Joel Fernandes wrote: > On Tue, Mar 26, 2019 at 06:22:33PM +0100, Christian Brauner wrote: > > On Tue, Mar 26, 2019 at 01:06:01PM -0400, Joel Fernandes wrote: > > > On Tue, Mar 26, 2019 at 04:55:11PM +0100, Christian Brauner wrote: > > > > The pidctl() syscalls builds on, extends, and improves translate_pid() [4]. > > > > I quote Konstantins original patchset first that has already been acked and > > > > picked up by Eric before and whose functionality is preserved in this > > > > syscall: > > > > > > > > "Each process have different pids, one for each pid namespace it belongs. > > > > When interaction happens within single pid-ns translation isn't required. > > > > More complicated scenarios needs special handling. > > > > > > > > For example: > > > > - reading pid-files or logs written inside container with pid namespace > > > > - writing logs with internal pids outside container for pushing them into > > > > - attaching with ptrace to tasks from different pid namespace > > > > > > > > Generally speaking, any cross pid-ns API with pids needs translation. > > > > > > > > Currently there are several interfaces that could be used here: > > > > > > > > Pid namespaces are identified by device and inode of /proc/[pid]/ns/pid. > > > > > > > > Pids for nested pid namespaces are shown in file /proc/[pid]/status. > > > > In some cases pid translation could be easily done using this information. > > > > Backward translation requires scanning all tasks and becomes really > > > > complicated for deeper namespace nesting. > > > > > > > > Unix socket automatically translates pid attached to SCM_CREDENTIALS. > > > > This requires CAP_SYS_ADMIN for sending arbitrary pids and entering > > > > into pid namespace, this expose process and could be insecure." > > > > > > > > The original patchset allowed two distinct operations implicitly: > > > > - discovering whether pid namespaces (pidns) have a parent-child > > > > relationship > > > > - translating a pid from a source pidns into a target pidns > > > > > > > > Both tasks are accomplished in the original patchset by passing a pid > > > > along. If the pid argument is passed as 1 the relationship between two pid > > > > namespaces can be discovered. > > > > The syscall will gain a lot clearer syntax and will be easier to use for > > > > userspace if the task it is asked to perform is passed through a > > > > command argument. Additionally, it allows us to remove an intrinsic race > > > > caused by using the pid argument as a way to discover the relationship > > > > between pid namespaces. > > > > This patch introduces three commands: > > > > > > > > /* PIDCMD_QUERY_PID */ > > > > PIDCMD_QUERY_PID allows to translate a pid between pid namespaces. > > > > Given a source pid namespace fd return the pid of the process in the target > > > > namespace: > > > > > > Could we call this PIDCMD_TRANSLATE_PID please? QUERY is confusing/ambigious > > > IMO (see below). > > > > Yes, doesn't matter to me too much what we call it. > > Ok. > > > > > 1. pidctl(PIDCMD_QUERY_PID, pid, source_fd, -1, 0) > > > > - retrieve pidns identified by source_fd > > > > - retrieve struct pid identifed by pid in pidns identified by source_fd > > > > - retrieve callers pidns > > > > - return pid in callers pidns > > > > > > > > 2. pidctl(PIDCMD_QUERY_PID, pid, -1, target_fd, 0) > > > > - retrieve callers pidns > > > > - retrieve struct pid identifed by pid in callers pidns > > > > - retrieve pidns identified by target_fd > > > > - return pid in pidns identified by target_fd > > > > > > > > 3. pidctl(PIDCMD_QUERY_PID, 1, source_fd, -1, 0) > > > > - retrieve pidns identified by source_fd > > > > - retrieve struct pid identifed by init task in pidns identified by source_fd > > > > - retrieve callers pidns > > > > - return pid of init task of pidns identified by source_fd in callers pidns > > > > > > > > 4. pidctl(PIDCMD_QUERY_PID, pid, source_fd, target_fd, 0) > > > > - retrieve pidns identified by source_fd > > > > - retrieve struct pid identifed by pid in pidns identified by source_fd > > > > - retrieve pidns identified by target_fd > > > > - check whether struct pid can be found in pidns identified by target_fd > > > > - return pid in pidns identified by target_fd > > > > > > > > /* PIDCMD_QUERY_PIDNS */ > > > > PIDCMD_QUERY_PIDNS allows to determine the relationship between pid > > > > namespaces. > > > > In the original version of the pachset passing pid as 1 would allow to > > > > deterimine the relationship between the pid namespaces. This is inherhently > > > > racy. If pid 1 inside a pid namespace has died it would report false > > > > negatives. For example, if pid 1 inside of the target pid namespace already > > > > died, it would report that the target pid namespace cannot be reached from > > > > the source pid namespace because it couldn't find the pid inside of the > > > > target pid namespace and thus falsely report to the user that the two pid > > > > namespaces are not related. This problem is simple to avoid. In the new > > > > version we simply walk the list of ancestors and check whether the > > > > namespace are related to each other. By doing it this way we can reliably > > > > report what the relationship between two pid namespace file descriptors > > > > looks like. > > > > > > > > 1. pidctl(PIDCMD_QUERY_PIDNS, 0, ns_fd1, ns_fd1, 0) == 0 > > > > - pidns_of(ns_fd1) and pidns_of(ns_fd2) are unrelated to each other > > > > > > > > 2. pidctl(PIDCMD_QUERY_PIDNS, 0, ns_fd1, ns_fd2, 0) == 1 > > > > - pidns_of(ns_fd1) == pidns_of(ns_fd2) > > > > > > > > 3. pidctl(PIDCMD_QUERY_PIDNS, 0, ns_fd1, ns_fd2, 0) == 2 > > > > - pidns_of(ns_fd1) is ancestor of pidns_of(ns_fd2) > > > > > > > > 4. pidctl(PIDCMD_QUERY_PIDNS, 0, ns_fd1, ns_fd2, 0) == 3 > > > > - pidns_of(ns_fd2) is ancestor of pidns_of(ns_fd1) > > > > > > Why not call this PIDCMD_COMPARE_PIDNS, since a comparison is what you're doing. > > > > Same. > > Ok, glad we agree on it. > > > > > > > Again QUERY is ambigious here. Above you called QUERY to translate something, > > > now you're calling QUERY to mean compare something. I suggest to be explicit > > > about the operation PIDCMD__. > > > > > > Arguably, 2 syscalls for this is cleaner: > > > pid_compare_namespaces(ns_fd1, ns_fd2); > > > pid_translate(pid, ns_fd1, nds_fd2); > > > > I don't think we want to send out pid_compare_namespaces() as a separate > > syscall. If that's the consensus I'd rather just drop this functionality > > completely. > > I mean, if pid-namespace fd comparison functionality is not needed in > userspace, why add it all. I suggest to drop it if not needed and can be > considered for addition later when needed. > > Then you're just left with GET_PID and TRANSLATE_PID which we know we do need. > > > > > These two commands - PIDCMD_QUERY_PID and PIDCMD_QUERY_PIDNS - cover and > > > > improve the functionality expressed implicitly in translate_pid() before. > > > > > > > > /* PIDCMD_GET_PIDFD */ > > > > > > And this can be a third syscall: > > > pidfd_translate(pid, ns_fd1, ns_fd2). > > > > Sigh, yes. But I honestly want other people other than us to come out > > and say "yes, please send a PR to Linus with three separate syscalls for > > very closely related functionality". There's a difference between "this > > is how we would ideally like to do it" and "this is what is common > > practice and will likely get accepted". But I'm really not opposed to it > > per se. > > Ok. > > > > I am actually supportive of Daniel's view that by combining too many > > > arguments into a single syscall, becomes confusing and sometimes some > > > arguments have to be forced to 0 in the single shoe-horned syscall. Like you > > > > There's a difference between an ioctl() and say seccomp() which this is > > close to: > > int seccomp(unsigned int operation, unsigned int flags, void *args); > > The point is that the functionality is closely related not just randomly > > unrelated stuff. But as I said I'm more than willing to compromise. > > Sounds great, yeah whatever makes sense. One option is to do the following which removes the cmd argument all together in favor of a flag GET_PIDFD: +SYSCALL_DEFINE4(pidfd_translate, pid_t, pid, int, source, int, target, + unsigned int, flags) +{ + struct pid_namespace *source_ns, *target_ns; + struct pid *struct_pid; + pid_t result; + + if (flags & ~GET_PIDFD) + return -EINVAL; + + source_ns = get_pid_ns_by_fd(source); + if (IS_ERR(source_ns)) + return PTR_ERR(source_ns); + + target_ns = get_pid_ns_by_fd(target); + if (IS_ERR(target_ns)) { + put_pid_ns(source_ns); + return PTR_ERR(target_ns); + } + + rcu_read_lock(); + struct_pid = get_pid(find_pid_ns(pid, source_ns)); + rcu_read_unlock(); + + if (struct_pid) + result = pid_nr_ns(struct_pid, target_ns); + else + result = -ESRCH; + + if ((flags & GET_PIDFD) && (result > 0)) + result = pidfd_create_fd(struct_pid, O_CLOEXEC); + + if (!result) + result = -ENOENT; + + put_pid(struct_pid); + put_pid_ns(target_ns); + put_pid_ns(source_ns); + + return result; +}