Re: [PATCH v5 1/2] fork: extend clone3() to support CLONE_SET_TID

[Christian Brauner]

On August 11, 2019 10:33:26 PM GMT+02:00, Adrian Reber <areber@xxxxxxxxxx> 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 <areber@xxxxxxxxxx>
>---
>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)
>
>v5:
> - make set_tid checks earlier in alloc_pid() (Christian)
> - remove unnecessary comment and struct size check (Christian)
>---
> include/linux/pid.h        |  2 +-
> include/linux/sched/task.h |  1 +
> include/uapi/linux/sched.h |  1 +
> kernel/fork.c              | 22 ++++++++++++++++++++--
> kernel/pid.c               | 36 +++++++++++++++++++++++++++++-------
> 5 files changed, 52 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..0365041243b1 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 +2548,13 @@ 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)))
>+	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;
> 
>@@ -2571,6 +2588,7 @@ noinline static int
>copy_clone_args_from_user(struct kernel_clone_args *kargs,
> 		.stack		= args.stack,
> 		.stack_size	= args.stack_size,
> 		.tls		= args.tls,
>+		.set_tid	= args.set_tid,
> 	};
> 
> 	return 0;
>diff --git a/kernel/pid.c b/kernel/pid.c
>index 0a9f2e437217..c3ce82f8206b 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;
>@@ -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();
> 

Ah, one more nit, since I reckon you'll resend anyway because of the additional tests:
The subject of the patch is not accurate anymore.
Since we don't add a new flag but rather only a new argument referencing CLONE_SET_TID seems misleading. :)

Christian