On Sun, Aug 11, 2019 at 08:51:48AM +0200, Christian Brauner wrote: > On Sat, Aug 10, 2019 at 07:59:18AM +0200, Adrian Reber wrote: > > On Sat, Aug 10, 2019 at 03:10:34AM +0200, Christian Brauner wrote: > > > On Thu, Aug 08, 2019 at 11:22:21PM +0200, Adrian Reber wrote: > > > > The main motivation to add set_tid to clone3() is CRIU. > > > > > > > > To restore a process with the same PID/TID CRIU currently uses > > > > /proc/sys/kernel/ns_last_pid. It writes the desired (PID - 1) to > > > > ns_last_pid and then (quickly) does a clone(). This works most of the > > > > time, but it is racy. It is also slow as it requires multiple syscalls. > > > > > > > > Extending clone3() to support set_tid makes it possible restore a > > > > process using CRIU without accessing /proc/sys/kernel/ns_last_pid and > > > > race free (as long as the desired PID/TID is available). > > > > > > > > This clone3() extension places the same restrictions (CAP_SYS_ADMIN) > > > > on clone3() with set_tid as they are currently in place for ns_last_pid. > > > > > > > > Signed-off-by: Adrian Reber <are...@redhat.com> > > > > --- > > > > v2: > > > > - Removed (size < sizeof(struct clone_args)) as discussed with > > > > Christian and Dmitry > > > > - Added comment to ((set_tid != 1) && idr_get_cursor() <= 1) (Oleg) > > > > - Use idr_alloc() instead of idr_alloc_cyclic() (Oleg) > > > > > > > > v3: > > > > - Return EEXIST if PID is already in use (Christian) > > > > - Drop CLONE_SET_TID (Christian and Oleg) > > > > - Use idr_is_empty() instead of idr_get_cursor() (Oleg) > > > > - Handle different `struct clone_args` sizes (Dmitry) > > > > > > > > v4: > > > > - Rework struct size check with defines (Christian) > > > > - Reduce number of set_tid checks (Oleg) > > > > - Less parentheses and more robust code (Oleg) > > > > - Do ns_capable() on correct user_ns (Oleg, Christian) > > > > --- > > > > include/linux/pid.h | 2 +- > > > > include/linux/sched/task.h | 1 + > > > > include/uapi/linux/sched.h | 1 + > > > > kernel/fork.c | 25 +++++++++++++++++++++++-- > > > > kernel/pid.c | 34 +++++++++++++++++++++++++++------- > > > > 5 files changed, 53 insertions(+), 10 deletions(-) > > > > > > > > diff --git a/include/linux/pid.h b/include/linux/pid.h > > > > index 2a83e434db9d..052000db0ced 100644 > > > > --- a/include/linux/pid.h > > > > +++ b/include/linux/pid.h > > > > @@ -116,7 +116,7 @@ extern struct pid *find_vpid(int nr); > > > > extern struct pid *find_get_pid(int nr); > > > > extern struct pid *find_ge_pid(int nr, struct pid_namespace *); > > > > > > > > -extern struct pid *alloc_pid(struct pid_namespace *ns); > > > > +extern struct pid *alloc_pid(struct pid_namespace *ns, pid_t set_tid); > > > > extern void free_pid(struct pid *pid); > > > > extern void disable_pid_allocation(struct pid_namespace *ns); > > > > > > > > diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h > > > > index 0497091e40c1..4f2a80564332 100644 > > > > --- a/include/linux/sched/task.h > > > > +++ b/include/linux/sched/task.h > > > > @@ -26,6 +26,7 @@ struct kernel_clone_args { > > > > unsigned long stack; > > > > unsigned long stack_size; > > > > unsigned long tls; > > > > + pid_t set_tid; > > > > }; > > > > > > > > /* > > > > diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h > > > > index b3105ac1381a..e1ce103a2c47 100644 > > > > --- a/include/uapi/linux/sched.h > > > > +++ b/include/uapi/linux/sched.h > > > > @@ -45,6 +45,7 @@ struct clone_args { > > > > __aligned_u64 stack; > > > > __aligned_u64 stack_size; > > > > __aligned_u64 tls; > > > > + __aligned_u64 set_tid; > > > > }; > > > > > > > > /* > > > > diff --git a/kernel/fork.c b/kernel/fork.c > > > > index 2852d0e76ea3..2a03f0e201e9 100644 > > > > --- a/kernel/fork.c > > > > +++ b/kernel/fork.c > > > > @@ -117,6 +117,13 @@ > > > > */ > > > > #define MAX_THREADS FUTEX_TID_MASK > > > > > > > > +/* > > > > + * Different sizes of struct clone_args > > > > + */ > > > > +#define CLONE3_ARGS_SIZE_V0 64 > > > > +/* V1 includes set_tid */ > > > > +#define CLONE3_ARGS_SIZE_V1 72 > > > > + > > > > /* > > > > * Protected counters by write_lock_irq(&tasklist_lock) > > > > */ > > > > @@ -2031,7 +2038,13 @@ static __latent_entropy struct task_struct > > > > *copy_process( > > > > stackleak_task_init(p); > > > > > > > > if (pid != &init_struct_pid) { > > > > - pid = alloc_pid(p->nsproxy->pid_ns_for_children); > > > > + if (args->set_tid && !ns_capable( > > > > + > > > > p->nsproxy->pid_ns_for_children->user_ns, > > > > + CAP_SYS_ADMIN)) { > > > > + retval = -EPERM; > > > > + goto bad_fork_cleanup_thread; > > > > + } > > > > + pid = alloc_pid(p->nsproxy->pid_ns_for_children, > > > > args->set_tid); > > > > if (IS_ERR(pid)) { > > > > retval = PTR_ERR(pid); > > > > goto bad_fork_cleanup_thread; > > > > @@ -2535,9 +2548,14 @@ noinline static int > > > > copy_clone_args_from_user(struct kernel_clone_args *kargs, > > > > if (unlikely(size > PAGE_SIZE)) > > > > return -E2BIG; > > > > > > > > - if (unlikely(size < sizeof(struct clone_args))) > > > > + /* The struct needs to be at least the size of the original > > > > struct. */ > > > > > > I don't think you need that comment. I think the macro is pretty > > > self-explanatory. If you want it to be even clearer you could even make > > > it CLONE3_ARGS_SIZE_MIN but V0 is good enough. :) > > > > Will remove the comment. > > > > > > + if (unlikely(size < CLONE3_ARGS_SIZE_V0)) > > > > return -EINVAL; > > > > > > > > + if (size < sizeof(struct clone_args)) > > > > + memset((void *)&args + size, 0, > > > > + sizeof(struct clone_args) - size); > > > > + > > > > if (unlikely(!access_ok(uargs, size))) > > > > return -EFAULT; > > > > > > > > @@ -2573,6 +2591,9 @@ noinline static int > > > > copy_clone_args_from_user(struct kernel_clone_args *kargs, > > > > .tls = args.tls, > > > > }; > > > > > > > > + if (size >= CLONE3_ARGS_SIZE_V1) > > > > + kargs->set_tid = args.set_tid; > > > > > > Hm, the if-condition is not needed though, right? At this point we will > > > have already copied from struct clone_args __user *uargs into struct > > > clone_args args. If we hit that codepath that means the kernel > > > definitely has a field for set_tid in its struct clone_args. :) So this > > > could probably just be: > > > > > > .tls = args.tls, > > > .set_tid = args.set_tid, > > > } > > > > > > ? > > > > Right. > > > > > > + > > > > return 0; > > > > } > > > > > > > > diff --git a/kernel/pid.c b/kernel/pid.c > > > > index 0a9f2e437217..9ce89c35c5be 100644 > > > > --- a/kernel/pid.c > > > > +++ b/kernel/pid.c > > > > @@ -157,7 +157,7 @@ void free_pid(struct pid *pid) > > > > call_rcu(&pid->rcu, delayed_put_pid); > > > > } > > > > > > > > -struct pid *alloc_pid(struct pid_namespace *ns) > > > > +struct pid *alloc_pid(struct pid_namespace *ns, int set_tid) > > > > { > > > > struct pid *pid; > > > > enum pid_type type; > > > > @@ -186,12 +186,32 @@ struct pid *alloc_pid(struct pid_namespace *ns) > > > > if (idr_get_cursor(&tmp->idr) > RESERVED_PIDS) > > > > pid_min = RESERVED_PIDS; > > > > > > > > - /* > > > > - * Store a null pointer so find_pid_ns does not find > > > > - * a partially initialized PID (see below). > > > > - */ > > > > - nr = idr_alloc_cyclic(&tmp->idr, NULL, pid_min, > > > > - pid_max, GFP_ATOMIC); > > > > + if (set_tid) { > > > > + /* > > > > + * Also fail if a PID != 1 is requested > > > > + * and no PID 1 exists. > > > > + */ > > > > + nr = -EINVAL; > > > > + if (set_tid < pid_max && set_tid > 0 && > > > > > > Hm, you're already in the if-branch hat verified if (set_tid) so the > > > set_tid > 0 conjunct seems redundant. :) > > > > Yes, but I dropped all checks to see if set_tid is negative as suggested > > by Oleg and moved it here. > > > > > > + (set_tid == 1 || !idr_is_empty(&tmp->idr))) > > > > + nr = idr_alloc(&tmp->idr, NULL, set_tid, > > > > + set_tid + 1, GFP_ATOMIC); > > > > > > I'm confused, shouldn't this be > > > > > > if (set_tid < pid_max || (set_tid == 1 && !idr_is_emtpy(&tmp->idf))) > > > > Now I am also confused ;). This does not work. This will always return > > true if set_tid is less than pid_max. So pid_max needs to be something > > like 1 for the check after || to make sense, right? But you really got > > me confused here right now. Right now I still think what I did is > > correct. > > I missed the part where you reset set_tid to 0 below and mis-parsed the > rightmost conjunct in the if statement. > > One thing I dislike is that you do > > if (set_tid) { > if ([...] && set_tid > 0 && [...]) > }
Thinking about this a bit more you probably did the explicit set_tid > 0 to catch the case where it is negative? But also, I don't understand why we do any work at all before having verified that set_tid is sensible, i.e. why do we call kmem_cache_calloc(), do idr_preload(), and take a spinlock, before even verifying that our parameters are sane? If there's no specific reason for this I suggest to patch alloc_pid() like this: -struct pid *alloc_pid(struct pid_namespace *ns) +struct pid *alloc_pid(struct pid_namespace *ns, int set_tid) { struct pid *pid; enum pid_type type; @@ -166,6 +166,9 @@ struct pid *alloc_pid(struct pid_namespace *ns) struct upid *upid; int retval = -ENOMEM; + if (set_tid < 0 || set_tid >= pid_max) + return ERR_PTR(-EINVAL); + pid = kmem_cache_alloc(ns->pid_cachep, GFP_KERNEL); if (!pid) return ERR_PTR(retval); @@ -186,12 +189,31 @@ struct pid *alloc_pid(struct pid_namespace *ns) if (idr_get_cursor(&tmp->idr) > RESERVED_PIDS) pid_min = RESERVED_PIDS; - /* - * Store a null pointer so find_pid_ns does not find - * a partially initialized PID (see below). - */ - nr = idr_alloc_cyclic(&tmp->idr, NULL, pid_min, - pid_max, GFP_ATOMIC); + if (set_tid) { + /* + * Also fail if a PID != 1 is requested + * and no PID 1 exists. + */ + nr = -EINVAL; + if (set_tid == 1 || !idr_is_empty(&tmp->idr)) + nr = idr_alloc(&tmp->idr, NULL, set_tid, + set_tid + 1, GFP_ATOMIC); + /* + * If ENOSPC is returned it means that the PID is + * alreay in use. Return EEXIST in that case. + */ + if (nr == -ENOSPC) + nr = -EEXIST; + /* Only use set_tid for one PID namespace. */ + set_tid = 0; + } else { + /* + * Store a null pointer so find_pid_ns does not find + * a partially initialized PID (see below). + */ + nr = idr_alloc_cyclic(&tmp->idr, NULL, pid_min, + pid_max, GFP_ATOMIC); + } spin_unlock_irq(&pidmap_lock); idr_preload_end() This makes things a lot more clearer in my opinion. First, verify that the pre-conditions are met. Second, verify that the conditions are met which depend on the state of the pid namespace, i.e. there's either already a pid 1 or pid 1 is requested. We should also do this since alloc_pid() is exported in a header file and so we can't and shouldn't rely on the fact that all callers will pass in something sensible for set_tid. > > For consistency this should rather be: > > if (set_tid) { > if ([...] && set_tid && [...]) > } > > Overall I'd rather write the second if as: > > if (set_tid && set_tid < pid_max && (set_tid == 1 || > !idr_is_empty(&tmp->idr))) > > to place the most basic condition (i.e. set_tid is non-zero) leftmost. > > The right-most conjunct (set_tid == 1 || !idr_is_empty(&tmp->idr)) is > non-trivial to parse. Someone not familiar with that code would need > time to figure out why and how exactly 1 is special-cased. > I think overall this is begging for alloc_pid() to be documented now if > you don't mind. :) > > Christian