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). > 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. 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_<OPERATION>_<OPERAND-TYPE>. Arguably, 2 syscalls for this is cleaner: pid_compare_namespaces(ns_fd1, ns_fd2); pid_translate(pid, ns_fd1, nds_fd2); > 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). 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 don't need a pid to compare to pid-ns, so user has to set that to 0. More comments below... > This command allows to retrieve file descriptors for processes and removes > the dependency of pidfds and thereby the pidfd_send_signal() syscall on > procfs. First, multiple people have expressed a desire to do this even when > pidfd_send_signal() was merged. It is even recorded in the commit message > for pidfd_send_signal() itself > (cf. commit 3eb39f47934f9d5a3027fe00d906a45fe3a15fad): > Q-06: (Andrew Morton [1]) > Is there a cleaner way of obtaining the fd? Another syscall perhaps. > A-06: Userspace can already trivially retrieve file descriptors from procfs > so this is something that we will need to support anyway. Hence, > there's no immediate need to add another syscalls just to make > pidfd_send_signal() not dependent on the presence of procfs. However, > adding a syscalls to get such file descriptors is planned for a > future patchset (cf. [1]). > Alexey made a similar request (cf. [2]). > Additionally, Andy made an additional, idependent argument that we should > go forward with non-proc-dirfd file descriptors for the sake of security > and extensibility (cf. [3]). > > The pidfds are not associated with a specific pid namespaces but rather > only with struct pid. What the pidctl() syscall enforces is that when a > caller wants to retrieve a pidfd file descriptor for a pid in a given > target pid namespace the caller > - must have been given access to two file descriptors referring > to target and source pid namespace > - the source pid namespace must be an ancestor of the target pid > namespace > - the pid must be translatable from the source pid namespace into the > target pid namespace > > 1. pidctl(PIDCMD_GET_PIDFD, 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 pidfd > > 2. pidctl(PIDCMD_GET_PIDFD, pid, -1, target_fd, 0) > - retrieve callers pidns > - retrieve struct pid identifed by pid in callers pidns > - retrieve pidns identified by target_fd > - return pidfd > > 3. pidctl(PIDCMD_GET_PIDFD, 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 pidfd > > 4. pidctl(PIDCMD_GET_PIDFD, 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 pidfd > > These pidfds are allocated using anon_inode_getfd(), are O_CLOEXEC by > default and can be used with the pidfd_send_signal() syscall. They are not > dirfds and as such have the advantage that we can make them pollable or > readable in the future if we see a need to do so (which I'm pretty sure we > will eventually). Currently they do not support any advanced operations. > > /* References */ > [1]: https://lore.kernel.org/lkml/20181228233725.722tdfgijxcssg76@xxxxxxxxxx/ > [2]: https://lore.kernel.org/lkml/20190320203910.GA2842@avx2/ > [3]: https://lore.kernel.org/lkml/CALCETrXO=V=+qEdLDVPf8eCgLZiB9bOTrUfe0V-U-tUZoeoRDA@xxxxxxxxxxxxxx/ > [4]: https://lore.kernel.org/lkml/20181109034919.GA21681@xxxxxxxxxxxx/ [snip] > diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h > index e446806a561f..a4c8c59f7c8f 100644 > --- a/include/linux/syscalls.h > +++ b/include/linux/syscalls.h > @@ -929,6 +929,8 @@ asmlinkage long sys_clock_adjtime32(clockid_t which_clock, > struct old_timex32 __user *tx); > asmlinkage long sys_syncfs(int fd); > asmlinkage long sys_setns(int fd, int nstype); > +asmlinkage long sys_pidctl(unsigned int cmd, pid_t pid, int source, int target, > + unsigned int flags); > asmlinkage long sys_sendmmsg(int fd, struct mmsghdr __user *msg, > unsigned int vlen, unsigned flags); > asmlinkage long sys_process_vm_readv(pid_t pid, > diff --git a/include/uapi/linux/wait.h b/include/uapi/linux/wait.h > index ac49a220cf2a..e9564ec06b07 100644 > --- a/include/uapi/linux/wait.h > +++ b/include/uapi/linux/wait.h > @@ -18,5 +18,19 @@ > #define P_PID 1 > #define P_PGID 2 > > +/* Commands to pass to pidctl() */ > +enum pidcmd { > + PIDCMD_QUERY_PID = 0, /* Get pid in target pid namespace */ > + PIDCMD_QUERY_PIDNS = 1, /* Determine relationship between pid namespaces */ > + PIDCMD_GET_PIDFD = 2, /* Get pidfd for a process */ > +}; > + > +/* Return values of PIDCMD_QUERY_PIDNS */ > +enum pidcmd_query_pidns { > + PIDNS_UNRELATED = 0, /* The pid namespaces are unrelated */ > + PIDNS_EQUAL = 1, /* The pid namespaces are equal */ > + PIDNS_SOURCE_IS_ANCESTOR = 2, /* Source pid namespace is ancestor of target pid namespace */ > + PIDNS_TARGET_IS_ANCESTOR = 3, /* Target pid namespace is ancestor of source pid namespace */ > +}; > > #endif /* _UAPI_LINUX_WAIT_H */ > diff --git a/kernel/pid.c b/kernel/pid.c > index 20881598bdfa..3213a137a63e 100644 > --- a/kernel/pid.c > +++ b/kernel/pid.c > @@ -26,6 +26,7 @@ > * > */ > > +#include <linux/anon_inodes.h> > #include <linux/mm.h> > #include <linux/export.h> > #include <linux/slab.h> > @@ -40,6 +41,7 @@ > #include <linux/proc_fs.h> > #include <linux/sched/task.h> > #include <linux/idr.h> > +#include <linux/wait.h> > > struct pid init_struct_pid = { > .count = ATOMIC_INIT(1), > @@ -451,6 +453,165 @@ struct pid *find_ge_pid(int nr, struct pid_namespace *ns) > return idr_get_next(&ns->idr, &nr); > } > > +static int pidfd_release(struct inode *inode, struct file *file) > +{ > + struct pid *pid = file->private_data; > + > + if (pid) { > + file->private_data = NULL; > + put_pid(pid); > + } > + > + return 0; > +} > + > +const struct file_operations pidfd_fops = { > + .release = pidfd_release, > +}; > + > +static int pidfd_create_fd(struct pid *pid, unsigned int o_flags) > +{ > + int fd; > + > + fd = anon_inode_getfd("pidfd", &pidfd_fops, get_pid(pid), O_RDWR | o_flags); > + if (fd < 0) > + put_pid(pid); > + > + return fd; > +} > + > +static struct pid_namespace *get_pid_ns_by_fd(int fd) > +{ > + struct pid_namespace *pidns = ERR_PTR(-EINVAL); > + > + if (fd >= 0) { > +#ifdef CONFIG_PID_NS > + struct ns_common *ns; > + struct file *file = proc_ns_fget(fd); > + if (IS_ERR(file)) > + return ERR_CAST(file); > + > + ns = get_proc_ns(file_inode(file)); > + if (ns->ops->type == CLONE_NEWPID) { > + pidns = container_of(ns, struct pid_namespace, ns); > + get_pid_ns(pidns); > + } > + > + fput(file); > +#endif > + } else { > + pidns = task_active_pid_ns(current); > + get_pid_ns(pidns); > + } > + > + return pidns; > +} > + > +static int pidns_related(struct pid_namespace *source, > + struct pid_namespace *target) > +{ > + int query; > + > + query = pidnscmp(source, target); > + switch (query) { > + case 0: > + return PIDNS_EQUAL; > + case 1: > + return PIDNS_SOURCE_IS_ANCESTOR; > + } > + > + query = pidnscmp(target, source); > + if (query == 1) > + return PIDNS_TARGET_IS_ANCESTOR; > + > + return PIDNS_UNRELATED; > +} > + > +/* > + * pidctl - perform operations on pids > + * @cmd: command to execute > + * @pid: pid for translation > + * @source: pid-ns file descriptor or -1 for active namespace > + * @target: pid-ns file descriptor or -1 for active namesapce > + * @flags: flags to pass > + * > + * If cmd is PIDCMD_QUERY_PID translates pid between pid namespaces > + * identified by @source and @target. Returns pid if process has pid in > + * @target, -ESRCH if process does not have a pid in @source, -ENOENT if > + * process has no pid in @target. > + * > + * If cmd is PIDCMD_QUERY_PIDNS determines the relations between two pid > + * namespaces. Returns 2 if @source is an ancestor pid namespace > + * of @target, 1 if @source and @target refer to the same pid namespace, > + * 3 if @target is an ancestor pid namespace of @source, 0 if they have > + * no parent-child relationship in either direction. > + * > + * If cmd is PIDCMD_GET_PIDFD returns pidfd for process in @target pid > + * namespace. Returns pidfd if process has pid in @target, -ESRCH if > + * process does not have a pid in @source, -ENOENT if process does not > + * have a pid in @target pid namespace. > + * > + */ > +SYSCALL_DEFINE5(pidctl, unsigned int, cmd, pid_t, pid, int, source, int, target, > + unsigned int, flags) flags seems not needed since it is unused, but I get it that you may want to have flags in the future? If yes, give example of future flags? > +{ > + struct pid_namespace *source_ns = NULL, *target_ns = NULL; Setting these to NULL is no longer needed. > + struct pid *struct_pid; > + pid_t result; > + > + if (flags) > + return -EINVAL; > + > + switch (cmd) { > + case PIDCMD_QUERY_PID: > + break; > + case PIDCMD_QUERY_PIDNS: > + if (pid) > + return -EINVAL; > + break; > + case PIDCMD_GET_PIDFD: > + break; > + default: > + return -EOPNOTSUPP; > + } > + > + 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); > + } > + > + if (cmd == PIDCMD_QUERY_PIDNS) { > + result = pidns_related(source_ns, target_ns); > + } else { > + 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 (cmd == PIDCMD_GET_PIDFD && (result > 0)) > + result = pidfd_create_fd(struct_pid, O_CLOEXEC); pidfd_create_fd already does put_pid on errors.. > + > + if (!result) > + result = -ENOENT; > + > + put_pid(struct_pid); so on error you would put_pid twice which seems odd.. I would suggest, don't release the pid ref from within pidfd_create_fd, release the ref from the caller. Speaking of which, I added to my list to convert the pid->count to refcount_t at some point :) > + } > + > + put_pid_ns(target_ns); > + put_pid_ns(source_ns); This part looks more clean than before so good. thanks, - Joel > + return result; > +} > + > void __init pid_idr_init(void) > { > /* Verify no one has done anything silly: */ > diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c > index aa6e72fb7c08..1c863fb3d55a 100644 > --- a/kernel/pid_namespace.c > +++ b/kernel/pid_namespace.c > @@ -429,6 +429,31 @@ static struct ns_common *pidns_get_parent(struct ns_common *ns) > return &get_pid_ns(pid_ns)->ns; > } > > +/** > + * pidnscmp - Determine if @ancestor is ancestor of @descendant > + * @ancestor: pidns suspected to be the ancestor of @descendant > + * @descendant: pidns suspected to be the descendant of @ancestor > + * > + * Returns -1 if @ancestor is not an ancestor of @descendant, > + * 0 if @ancestor is the same pidns as @descendant, 1 if @ancestor > + * is an ancestor of @descendant. > + */ > +int pidnscmp(struct pid_namespace *ancestor, struct pid_namespace *descendant) > +{ > + if (ancestor == descendant) > + return 0; > + > + for (;;) { > + if (!descendant) > + return -1; > + if (descendant == ancestor) > + break; > + descendant = descendant->parent; > + } > + > + return 1; > +} > + > static struct user_namespace *pidns_owner(struct ns_common *ns) > { > return to_pid_ns(ns)->user_ns; > -- > 2.21.0 >
