[PATCH v8 1/2] fork: extend clone3() to support setting a PID

[PATCH v8 1/2] fork: extend clone3() to support setting a PID

[Adrian Reber]

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.

The original version of this change was using a single value for
set_tid. At the 2019 LPC, after presenting set_tid, it was, however,
decided to change set_tid to an array to enable setting the PID of a
process in multiple PID namespaces at the same time. If a process is
created in a PID namespace it is possible to influence the PID inside
and outside of the PID namespace. Details also in the corresponding
selftest.

To create a process with the following PIDs:

      PID NS level         Requested PID
        0 (host)              31496
        1                        42
        2                         1

For that example the two newly introduced parameters to struct
clone_args (set_tid and set_tid_size) would need to be:

  set_tid[0] = 1;
  set_tid[1] = 42;
  set_tid[2] = 31496;
  set_tid_size = 3;

If only the PIDs of the two innermost nested PID namespaces should be
defined it would look like this:

  set_tid[0] = 1;
  set_tid[1] = 42;
  set_tid_size = 2;

The PID of the newly created process would then be the next available
free PID in the PID namespace level 0 (host) and 42 in the PID namespace
at level 1 and the PID of the process in the innermost PID namespace
would be 1.

The set_tid array is used to specify the PID of a process starting
from the innermost nested PID namespaces up to set_tid_size PID namespaces.

set_tid_size cannot be larger then the current PID namespace level.

Signed-off-by: Adrian Reber <areber@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)

v5:
 - make set_tid checks earlier in alloc_pid() (Christian)
 - remove unnecessary comment and struct size check (Christian)

v6:
 - remove CLONE_SET_TID from description (Christian)
 - add clone3() tests for different clone_args sizes (Christian)
 - move more set_tid checks to alloc_pid() (Oleg)
 - make all set_tid checks lockless (Oleg)

v7:
 - changed set_tid to be an array to set the PID of a process
   in multiple nested PID namespaces at the same time as discussed
   at LPC 2019 (container MC)

v8:
 - skip unnecessary memset() (Rasmus)
 - replace set_tid copy loop with a single copy (Christian)
 - more parameter error checking (Christian)
 - cache set_tid in alloc_pid() (Oleg)
 - move code in "else" branch (Oleg)
---
 include/linux/pid.h           |  3 +-
 include/linux/pid_namespace.h |  2 ++
 include/linux/sched/task.h    |  3 ++
 include/uapi/linux/sched.h    |  2 ++
 kernel/fork.c                 | 24 ++++++++++++-
 kernel/pid.c                  | 64 +++++++++++++++++++++++++++--------
 kernel/pid_namespace.c        |  2 --
 7 files changed, 82 insertions(+), 18 deletions(-)

diff --git a/include/linux/pid.h b/include/linux/pid.h
index 9645b1194c98..034b7df25888 100644
--- a/include/linux/pid.h
+++ b/include/linux/pid.h
@@ -120,7 +120,8 @@ 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,
+			     size_t set_tid_size);
 extern void free_pid(struct pid *pid);
 extern void disable_pid_allocation(struct pid_namespace *ns);
 
diff --git a/include/linux/pid_namespace.h b/include/linux/pid_namespace.h
index 49538b172483..2ed6af88794b 100644
--- a/include/linux/pid_namespace.h
+++ b/include/linux/pid_namespace.h
@@ -12,6 +12,8 @@
 #include <linux/ns_common.h>
 #include <linux/idr.h>
 
+/* MAX_PID_NS_LEVEL is needed for limiting size of 'struct pid' */
+#define MAX_PID_NS_LEVEL 32
 
 struct fs_pin;
 
diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index 4b1c3b664f51..f1879884238e 100644
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -26,6 +26,9 @@ struct kernel_clone_args {
 	unsigned long stack;
 	unsigned long stack_size;
 	unsigned long tls;
+	pid_t *set_tid;
+	/* Number of elements in *set_tid */
+	size_t set_tid_size;
 };
 
 /*
diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h
index 25b4fa00bad1..13f74c40a629 100644
--- a/include/uapi/linux/sched.h
+++ b/include/uapi/linux/sched.h
@@ -72,6 +72,8 @@ struct clone_args {
 	__aligned_u64 stack;
 	__aligned_u64 stack_size;
 	__aligned_u64 tls;
+	__aligned_u64 set_tid;
+	__aligned_u64 set_tid_size;
 };
 #endif
 
diff --git a/kernel/fork.c b/kernel/fork.c
index 55af6931c6ec..6f9e6443e12c 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2026,7 +2026,8 @@ 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);
+		pid = alloc_pid(p->nsproxy->pid_ns_for_children, args->set_tid,
+				args->set_tid_size);
 		if (IS_ERR(pid)) {
 			retval = PTR_ERR(pid);
 			goto bad_fork_cleanup_thread;
@@ -2529,6 +2530,7 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
 {
 	int err;
 	struct clone_args args;
+	pid_t *kset_tid = kargs->set_tid;
 
 	if (unlikely(usize > PAGE_SIZE))
 		return -E2BIG;
@@ -2539,6 +2541,15 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
 	if (err)
 		return err;
 
+	if (unlikely(args.set_tid_size > MAX_PID_NS_LEVEL))
+		return -EINVAL;
+
+	if (unlikely(!args.set_tid && args.set_tid_size > 0))
+		return -EINVAL;
+
+	if (unlikely(args.set_tid && args.set_tid_size == 0))
+		return -EINVAL;
+
 	/*
 	 * Verify that higher 32bits of exit_signal are unset and that
 	 * it is a valid signal
@@ -2556,8 +2567,16 @@ 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_size	= args.set_tid_size,
 	};
 
+	if (args.set_tid &&
+		copy_from_user(kset_tid, u64_to_user_ptr(args.set_tid),
+			(kargs->set_tid_size * sizeof(pid_t))))
+		return -EFAULT;
+
+	kargs->set_tid = kset_tid;
+
 	return 0;
 }
 
@@ -2631,6 +2650,9 @@ SYSCALL_DEFINE2(clone3, struct clone_args __user *, uargs, size_t, size)
 	int err;
 
 	struct kernel_clone_args kargs;
+	pid_t set_tid[MAX_PID_NS_LEVEL];
+
+	kargs.set_tid = set_tid;
 
 	err = copy_clone_args_from_user(&kargs, uargs, size);
 	if (err)
diff --git a/kernel/pid.c b/kernel/pid.c
index 0a9f2e437217..17280da48224 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -157,7 +157,8 @@ 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, pid_t *set_tid,
+		      size_t set_tid_size)
 {
 	struct pid *pid;
 	enum pid_type type;
@@ -166,6 +167,17 @@ struct pid *alloc_pid(struct pid_namespace *ns)
 	struct upid *upid;
 	int retval = -ENOMEM;
 
+	/*
+	 * set_tid_size contains the size of the set_tid array. Starting at
+	 * the most nested currently active PID namespace it tells alloc_pid()
+	 * which PID to set for a process in that most nested PID namespace
+	 * up to set_tid_size PID namespaces. It does not have to set the PID
+	 * for a process in all nested PID namespaces but set_tid_size must
+	 * never be greater than the current ns->level + 1.
+	 */
+	if (set_tid_size > ns->level + 1)
+		return ERR_PTR(-EINVAL);
+
 	pid = kmem_cache_alloc(ns->pid_cachep, GFP_KERNEL);
 	if (!pid)
 		return ERR_PTR(retval);
@@ -175,23 +187,47 @@ struct pid *alloc_pid(struct pid_namespace *ns)
 
 	for (i = ns->level; i >= 0; i--) {
 		int pid_min = 1;
+		int tid = 0;
+
+		if (set_tid_size) {
+			tid = set_tid[ns->level - i];
+			if (tid < 1 || tid >= pid_max)
+				return ERR_PTR(-EINVAL);
+			/* Also fail if a PID != 1 is requested and no PID 1 exists */
+			if (tid != 1 && !tmp->child_reaper)
+				return ERR_PTR(-EINVAL);
+			if (!ns_capable(tmp->user_ns, CAP_SYS_ADMIN))
+				return ERR_PTR(-EPERM);
+		}
 
 		idr_preload(GFP_KERNEL);
 		spin_lock_irq(&pidmap_lock);
 
-		/*
-		 * init really needs pid 1, but after reaching the maximum
-		 * wrap back to RESERVED_PIDS
-		 */
-		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 (tid) {
+			nr = idr_alloc(&tmp->idr, NULL, tid,
+				       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;
+			set_tid_size--;
+		} else {
+			/*
+			 * init really needs pid 1, but after reaching the maximum
+			 * wrap back to RESERVED_PIDS
+			 */
+			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);
+		}
 		spin_unlock_irq(&pidmap_lock);
 		idr_preload_end();
 
diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
index a6a79f85c81a..d40017e79ebe 100644
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c
@@ -26,8 +26,6 @@
 
 static DEFINE_MUTEX(pid_caches_mutex);
 static struct kmem_cache *pid_ns_cachep;
-/* MAX_PID_NS_LEVEL is needed for limiting size of 'struct pid' */
-#define MAX_PID_NS_LEVEL 32
 /* Write once array, filled from the beginning. */
 static struct kmem_cache *pid_cache[MAX_PID_NS_LEVEL];
 
-- 
2.23.0

[PATCH v8 2/2] selftests: add tests for clone3()

[Adrian Reber]

This tests clone3() with *set_tid to see if all desired PIDs are working
as expected. The tests are trying multiple invalid input parameters as
well as creating processes while specifying a certain PID in multiple
PID namespaces at the same time.

Signed-off-by: Adrian Reber <areber@redhat.com>
---
 tools/testing/selftests/clone3/.gitignore     |   1 +
 tools/testing/selftests/clone3/Makefile       |   2 +-
 .../testing/selftests/clone3/clone3_set_tid.c | 345 ++++++++++++++++++
 3 files changed, 347 insertions(+), 1 deletion(-)
 create mode 100644 tools/testing/selftests/clone3/clone3_set_tid.c

diff --git a/tools/testing/selftests/clone3/.gitignore b/tools/testing/selftests/clone3/.gitignore
index 85d9d3ba2524..d56c3c49d869 100644
--- a/tools/testing/selftests/clone3/.gitignore
+++ b/tools/testing/selftests/clone3/.gitignore
@@ -1 +1,2 @@
 clone3
+clone3_set_tid
diff --git a/tools/testing/selftests/clone3/Makefile b/tools/testing/selftests/clone3/Makefile
index ea922c014ae4..2d292545ca8e 100644
--- a/tools/testing/selftests/clone3/Makefile
+++ b/tools/testing/selftests/clone3/Makefile
@@ -2,6 +2,6 @@
 
 CFLAGS += -I../../../../usr/include/
 
-TEST_GEN_PROGS := clone3
+TEST_GEN_PROGS := clone3 clone3_set_tid
 
 include ../lib.mk
diff --git a/tools/testing/selftests/clone3/clone3_set_tid.c b/tools/testing/selftests/clone3/clone3_set_tid.c
new file mode 100644
index 000000000000..9a234fd2031e
--- /dev/null
+++ b/tools/testing/selftests/clone3/clone3_set_tid.c
@@ -0,0 +1,345 @@
+// SPDX-License-Identifier: GPL-2.0
+
+/*
+ * Based on Christian Brauner's clone3() example.
+ * These tests are assuming to be running in the host's
+ * PID namespace.
+ */
+
+#define _GNU_SOURCE
+#include <errno.h>
+#include <linux/types.h>
+#include <linux/sched.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/syscall.h>
+#include <sys/types.h>
+#include <sys/un.h>
+#include <sys/wait.h>
+#include <unistd.h>
+#include <sched.h>
+
+#include "../kselftest.h"
+
+#ifndef MAX_PID_NS_LEVEL
+#define MAX_PID_NS_LEVEL 32
+#endif
+
+static int pipe_1[2];
+static int pipe_2[2];
+
+static pid_t raw_clone(struct clone_args *args)
+{
+	return syscall(__NR_clone3, args, sizeof(struct clone_args));
+}
+
+static int call_clone3_set_tid(pid_t *set_tid,
+			       size_t set_tid_size,
+			       int flags,
+			       int expected_pid,
+			       int wait_for_it)
+{
+	int status;
+	int ret = 0;
+	pid_t pid = -1;
+	struct clone_args args = {0};
+
+	args.flags = flags;
+	args.exit_signal = SIGCHLD;
+	args.set_tid = (__u64)set_tid;
+	args.set_tid_size = set_tid_size;
+
+	pid = raw_clone(&args);
+	if (pid < 0) {
+		ksft_print_msg("%s - Failed to create new process\n",
+			       strerror(errno));
+		return -errno;
+	}
+
+	if (pid == 0) {
+		char tmp = 0;
+		ksft_print_msg("I am the child, my PID is %d (expected %d)\n",
+			       getpid(), set_tid[0]);
+		if (wait_for_it) {
+			ksft_print_msg("[%d] Child is ready and waiting\n", getpid());
+			/* Signal the parent that the child is ready */
+			close(pipe_1[0]);
+			write(pipe_1[1], &tmp, 1);
+			close(pipe_1[1]);
+			close(pipe_2[1]);
+			read(pipe_2[0], &tmp, 1);
+			close(pipe_2[0]);
+		}
+
+		if (set_tid[0] != getpid())
+			_exit(EXIT_FAILURE);
+		_exit(EXIT_SUCCESS);
+	}
+
+	if (expected_pid == 0 || expected_pid == pid)
+		ksft_print_msg("I am the parent (%d). My child's pid is %d\n",
+			       getpid(), pid);
+	else {
+		ksft_print_msg(
+			"Expected child pid %d does not match actual pid %d\n",
+			expected_pid, pid);
+		ret = -1;
+	}
+
+	if (wait(&status) < 0) {
+		ksft_print_msg("Child returned %s\n", strerror(errno));
+		return -errno;
+	}
+	if (WEXITSTATUS(status))
+		return WEXITSTATUS(status);
+
+	return ret;
+}
+
+static void test_clone3_set_tid(pid_t *set_tid,
+				size_t set_tid_size,
+				int flags,
+				int expected,
+				int expected_pid,
+				int wait_for_it)
+{
+	int ret;
+
+	ksft_print_msg(
+		"[%d] Trying clone3() with CLONE_SET_TID to %d and 0x%x\n",
+		getpid(), set_tid[0], flags);
+	ret = call_clone3_set_tid(set_tid, set_tid_size, flags, expected_pid,
+				  wait_for_it);
+	ksft_print_msg(
+		"[%d] clone3() with CLONE_SET_TID %d says :%d - expected %d\n",
+		getpid(), set_tid[0], ret, expected);
+	if (ret != expected)
+		ksft_test_result_fail(
+			"[%d] Result (%d) is different than expected (%d)\n",
+			getpid(), ret, expected);
+	else
+		ksft_test_result_pass("[%d] Result (%d) matches expectation (%d)\n",
+			getpid(), ret, expected);
+}
+int main(int argc, char *argv[])
+{
+	FILE *f;
+	char buf;
+	pid_t pid;
+	pid_t ns1;
+	pid_t ns2;
+	pid_t ns3;
+	int status;
+	char *proc;
+	int ret = -1;
+	pid_t ns_pid;
+	int pid_max = 0;
+	uid_t uid = getuid();
+	char line[1024] = {0};
+	pid_t set_tid[MAX_PID_NS_LEVEL * 2];
+	pid_t set_tid_small[1];
+
+	if (pipe(pipe_1) == -1 || pipe(pipe_2))
+		 ksft_exit_fail_msg("pipe() failed\n");
+
+	ksft_print_header();
+	ksft_set_plan(27);
+
+	f = fopen("/proc/sys/kernel/pid_max", "r");
+	if (f == NULL)
+		ksft_exit_fail_msg(
+			"%s - Could not open /proc/sys/kernel/pid_max\n",
+			strerror(errno));
+	fscanf(f, "%d", &pid_max);
+	fclose(f);
+	ksft_print_msg("/proc/sys/kernel/pid_max %d\n", pid_max);
+
+	/* Try invalid settings */
+	memset(&set_tid, 0, sizeof(set_tid));
+	test_clone3_set_tid(set_tid, MAX_PID_NS_LEVEL + 1, 0, -E2BIG, 0, 0);
+	test_clone3_set_tid(set_tid, MAX_PID_NS_LEVEL * 2, 0, -E2BIG, 0, 0);
+	test_clone3_set_tid(set_tid, MAX_PID_NS_LEVEL * 2 + 1, 0, -E2BIG, 0, 0);
+	test_clone3_set_tid(set_tid, MAX_PID_NS_LEVEL * 42, 0, -E2BIG, 0, 0);
+
+	/* small set_tid array, but maximum set_tid_size */
+	/* Find the current active PID */
+	pid = fork();
+	if (pid == 0) {
+		ksft_print_msg("Child has PID %d\n", getpid());
+		_exit(EXIT_SUCCESS);
+	}
+	(void)wait(NULL);
+	/* After the child has finished, its PID should be free. */
+	set_tid_small[0] = pid;
+	/*
+	 * There is a chance that this can return -EFAULT as the actual
+	 * set_tid array has only one entry, but we are telling the kernel
+	 * that it has the size MAX_PID_NS_LEVEL. This could lead to a
+	 * situation where copy_from_user() fails. So far it always
+	 * succeeds and copies random data (whatever is after set_tid_small).
+	 */
+	test_clone3_set_tid(set_tid_small, MAX_PID_NS_LEVEL, 0, -EINVAL, 0, 0);
+
+	/*
+	 * This can actually work if this test running in a MAX_PID_NS_LEVEL - 1
+	 * nested PID namespace.
+	 */
+	test_clone3_set_tid(set_tid, MAX_PID_NS_LEVEL - 1, 0, -EINVAL, 0, 0);
+
+	memset(&set_tid, 0xff, sizeof(set_tid));
+	test_clone3_set_tid(set_tid, MAX_PID_NS_LEVEL + 1, 0, -E2BIG, 0, 0);
+	test_clone3_set_tid(set_tid, MAX_PID_NS_LEVEL * 2, 0, -E2BIG, 0, 0);
+	test_clone3_set_tid(set_tid, MAX_PID_NS_LEVEL * 2 + 1, 0, -E2BIG, 0, 0);
+	test_clone3_set_tid(set_tid, MAX_PID_NS_LEVEL * 42, 0, -E2BIG, 0, 0);
+	/*
+	 * This can actually work if this test running in a MAX_PID_NS_LEVEL - 1
+	 * nested PID namespace.
+	 */
+	test_clone3_set_tid(set_tid, MAX_PID_NS_LEVEL - 1, 0, -EINVAL, 0, 0);
+
+	memset(&set_tid, 0, sizeof(set_tid));
+	/* Try with an invalid PID */
+	set_tid[0] = 0;
+	test_clone3_set_tid(set_tid, 1, 0, -EINVAL, 0, 0);
+	set_tid[0] = -1;
+	test_clone3_set_tid(set_tid, 1, 0, -EINVAL, 0, 0);
+	/* Claim that the set_tid array actually contains 2 elements. */
+	test_clone3_set_tid(set_tid, 2, 0, -EINVAL, 0, 0);
+	/* Try it in a new PID namespace */
+	if (uid == 0)
+		test_clone3_set_tid(set_tid, 1, CLONE_NEWPID, -EINVAL, 0, 0);
+	else
+		test_clone3_set_tid(set_tid, 1, CLONE_NEWPID, -EPERM, 0, 0);
+
+	/*
+	 * Try with a valid PID (1) but as non-root. This should fail
+	 * with -EPERM if running in the initial user namespace.
+	 * As root it should tell us -EEXIST.
+	 */
+	set_tid[0] = 1;
+	if (uid == 0)
+		test_clone3_set_tid(set_tid, 1, 0, -EEXIST, 0, 0);
+	else
+		test_clone3_set_tid(set_tid, 1, 0, -EPERM, 0, 0);
+
+	/* Try it in a new PID namespace */
+	if (uid == 0)
+		test_clone3_set_tid(set_tid, 1, CLONE_NEWPID, 0, 0, 0);
+	else
+		test_clone3_set_tid(set_tid, 1, CLONE_NEWPID, -EPERM, 0, 0);
+
+	/* pid_max should fail everywhere */
+	set_tid[0] = pid_max;
+	test_clone3_set_tid(set_tid, 1, 0, -EINVAL, 0, 0);
+	if (uid == 0)
+		test_clone3_set_tid(set_tid, 1, CLONE_NEWPID, -EINVAL, 0, 0);
+	else
+		test_clone3_set_tid(set_tid, 1, CLONE_NEWPID, -EPERM, 0, 0);
+
+	if (uid != 0) {
+		/*
+		 * All remaining tests require root. Tell the framework
+		 * that all those tests are skipped as non-root.
+		 */
+		ksft_cnt.ksft_xskip += ksft_plan - ksft_test_num();
+		goto out;
+	}
+
+	/* Find the current active PID */
+	pid = fork();
+	if (pid == 0) {
+		ksft_print_msg("Child has PID %d\n", getpid());
+		usleep(500);
+		_exit(EXIT_SUCCESS);
+	}
+	(void)wait(NULL);
+	/* After the child has finished, its PID should be free. */
+	set_tid[0] = pid;
+	test_clone3_set_tid(set_tid, 1, 0, 0, 0, 0);
+	/* This should fail as there is no PID 1 in that namespace */
+	test_clone3_set_tid(set_tid, 1, CLONE_NEWPID, -EINVAL, 0, 0);
+	set_tid[0] = 1;
+	set_tid[1] = pid;
+	test_clone3_set_tid(set_tid, 2, CLONE_NEWPID, 0, pid, 0);
+
+	ksft_print_msg("unshare PID namespace\n");
+	unshare(CLONE_NEWPID);
+	set_tid[0] = pid;
+	/* This should fail as there is no PID 1 in that namespace */
+	test_clone3_set_tid(set_tid, 1, 0, -EINVAL, 0, 0);
+
+	/* Let's create a PID 1 */
+	ns_pid = fork();
+	if (ns_pid == 0) {
+		ksft_print_msg("Child in PID namespace has PID %d\n", getpid());
+		set_tid[0] = 2;
+		test_clone3_set_tid(set_tid, 1, 0, 0, 2, 0);
+		set_tid[0] = 1;
+		set_tid[1] = 42;
+		set_tid[2] = pid;
+		/*
+		 * This should fail as there are not enough active PID
+		 * namespaces. Again assuming this is running in the host's
+		 * PID namespace. Not yet nested.
+		 */
+		test_clone3_set_tid(set_tid, 4, CLONE_NEWPID, -EINVAL, 0, 0);
+		/*
+		 * This should work and from the parent we should see
+		 * something like 'NSpid:	pid	42	1'.
+		 */
+		test_clone3_set_tid(set_tid, 3, CLONE_NEWPID, 0, 42, 1);
+		_exit(ksft_cnt.ksft_pass);
+	}
+
+	close(pipe_1[1]);
+	close(pipe_2[0]);
+	while (read(pipe_1[0], &buf, 1) > 0) {
+		ksft_print_msg("[%d] Child is ready and waiting\n", getpid());
+		break;
+	}
+
+	asprintf(&proc, "/proc/%d/status", pid);
+	f = fopen(proc, "r");
+	if (f == NULL)
+		ksft_exit_fail_msg(
+			"%s - Could not open %s\n",
+			strerror(errno), proc);
+	while (fgets(line, 1024, f)) {
+		if (strstr(line, "NSpid")) {
+			/* Verify that all generated PIDs are as expected. */
+			sscanf(line, "NSpid:\t%d\t%d\t%d", &ns3, &ns2, &ns1);
+			break;
+		}
+	}
+	fclose(f);
+	free(proc);
+	close(pipe_2[0]);
+	/* Tell the clone3()'d child to finish. */
+	write(pipe_2[1], &buf, 1);
+	close(pipe_2[1]);
+
+	if (wait(&status) < 0) {
+		ksft_print_msg("Child returned %s\n", strerror(errno));
+		ret = -errno;
+		goto out;
+	}
+	if (WEXITSTATUS(status))
+		/*
+		 * Update the number of total tests with the tests from the
+		 * child processes.
+		 */
+		ksft_cnt.ksft_pass = WEXITSTATUS(status);
+
+	if (ns3 == pid && ns2 == 42 && ns1 == 1)
+		ksft_test_result_pass(
+			"PIDs in all namespaces as expected (%d,%d,%d)\n",
+			ns3, ns2, ns1);
+	else
+		ksft_test_result_fail(
+			"PIDs in all namespaces not as expected (%d,%d,%d)\n",
+			ns3, ns2, ns1);
+out:
+	ret = 0;
+
+	return !ret ? ksft_exit_pass() : ksft_exit_fail();
+}
-- 
2.23.0

Re: [PATCH v8 1/2] fork: extend clone3() to support setting a PID

[Christian Brauner]

On Wed, Nov 13, 2019 at 09:03:00AM +0100, 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.
> 
> The original version of this change was using a single value for
> set_tid. At the 2019 LPC, after presenting set_tid, it was, however,
> decided to change set_tid to an array to enable setting the PID of a
> process in multiple PID namespaces at the same time. If a process is
> created in a PID namespace it is possible to influence the PID inside
> and outside of the PID namespace. Details also in the corresponding
> selftest.
> 
> To create a process with the following PIDs:
> 
>       PID NS level         Requested PID
>         0 (host)              31496
>         1                        42
>         2                         1
> 
> For that example the two newly introduced parameters to struct
> clone_args (set_tid and set_tid_size) would need to be:
> 
>   set_tid[0] = 1;
>   set_tid[1] = 42;
>   set_tid[2] = 31496;
>   set_tid_size = 3;
> 
> If only the PIDs of the two innermost nested PID namespaces should be
> defined it would look like this:
> 
>   set_tid[0] = 1;
>   set_tid[1] = 42;
>   set_tid_size = 2;
> 
> The PID of the newly created process would then be the next available
> free PID in the PID namespace level 0 (host) and 42 in the PID namespace
> at level 1 and the PID of the process in the innermost PID namespace
> would be 1.
> 
> The set_tid array is used to specify the PID of a process starting
> from the innermost nested PID namespaces up to set_tid_size PID namespaces.
> 
> set_tid_size cannot be larger then the current PID namespace level.
> 
> Signed-off-by: Adrian Reber <areber@redhat.com>

Adrian, when you resend, can you please add --base=<commit> with the
base commit this series applies to? This makes my life easier when
applying this series.

Other from missing kernel-doc (see below) I don't have any further
complaints atm.

> ---
> 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)
> 
> v6:
>  - remove CLONE_SET_TID from description (Christian)
>  - add clone3() tests for different clone_args sizes (Christian)
>  - move more set_tid checks to alloc_pid() (Oleg)
>  - make all set_tid checks lockless (Oleg)
> 
> v7:
>  - changed set_tid to be an array to set the PID of a process
>    in multiple nested PID namespaces at the same time as discussed
>    at LPC 2019 (container MC)
> 
> v8:
>  - skip unnecessary memset() (Rasmus)
>  - replace set_tid copy loop with a single copy (Christian)
>  - more parameter error checking (Christian)
>  - cache set_tid in alloc_pid() (Oleg)
>  - move code in "else" branch (Oleg)
> ---
>  include/linux/pid.h           |  3 +-
>  include/linux/pid_namespace.h |  2 ++
>  include/linux/sched/task.h    |  3 ++
>  include/uapi/linux/sched.h    |  2 ++
>  kernel/fork.c                 | 24 ++++++++++++-
>  kernel/pid.c                  | 64 +++++++++++++++++++++++++++--------
>  kernel/pid_namespace.c        |  2 --
>  7 files changed, 82 insertions(+), 18 deletions(-)
> 
> diff --git a/include/linux/pid.h b/include/linux/pid.h
> index 9645b1194c98..034b7df25888 100644
> --- a/include/linux/pid.h
> +++ b/include/linux/pid.h
> @@ -120,7 +120,8 @@ 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,
> +			     size_t set_tid_size);
>  extern void free_pid(struct pid *pid);
>  extern void disable_pid_allocation(struct pid_namespace *ns);
>  
> diff --git a/include/linux/pid_namespace.h b/include/linux/pid_namespace.h
> index 49538b172483..2ed6af88794b 100644
> --- a/include/linux/pid_namespace.h
> +++ b/include/linux/pid_namespace.h
> @@ -12,6 +12,8 @@
>  #include <linux/ns_common.h>
>  #include <linux/idr.h>
>  
> +/* MAX_PID_NS_LEVEL is needed for limiting size of 'struct pid' */
> +#define MAX_PID_NS_LEVEL 32
>  
>  struct fs_pin;
>  
> diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
> index 4b1c3b664f51..f1879884238e 100644
> --- a/include/linux/sched/task.h
> +++ b/include/linux/sched/task.h
> @@ -26,6 +26,9 @@ struct kernel_clone_args {
>  	unsigned long stack;
>  	unsigned long stack_size;
>  	unsigned long tls;
> +	pid_t *set_tid;
> +	/* Number of elements in *set_tid */
> +	size_t set_tid_size;
>  };
>  
>  /*
> diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h
> index 25b4fa00bad1..13f74c40a629 100644
> --- a/include/uapi/linux/sched.h
> +++ b/include/uapi/linux/sched.h
> @@ -72,6 +72,8 @@ struct clone_args {
>  	__aligned_u64 stack;
>  	__aligned_u64 stack_size;
>  	__aligned_u64 tls;
> +	__aligned_u64 set_tid;
> +	__aligned_u64 set_tid_size;

Please add kernel-doc comments for these two new fields to the top of
the like we did for all the other fields.

	Christian

Re: [PATCH v8 2/2] selftests: add tests for clone3()

[Christian Brauner]

On Wed, Nov 13, 2019 at 09:03:01AM +0100, Adrian Reber wrote:
> This tests clone3() with *set_tid to see if all desired PIDs are working
> as expected. The tests are trying multiple invalid input parameters as
> well as creating processes while specifying a certain PID in multiple
> PID namespaces at the same time.
> 
> Signed-off-by: Adrian Reber <areber@redhat.com>
> ---
>  tools/testing/selftests/clone3/.gitignore     |   1 +
>  tools/testing/selftests/clone3/Makefile       |   2 +-
>  .../testing/selftests/clone3/clone3_set_tid.c | 345 ++++++++++++++++++
>  3 files changed, 347 insertions(+), 1 deletion(-)
>  create mode 100644 tools/testing/selftests/clone3/clone3_set_tid.c
> 
> diff --git a/tools/testing/selftests/clone3/.gitignore b/tools/testing/selftests/clone3/.gitignore
> index 85d9d3ba2524..d56c3c49d869 100644
> --- a/tools/testing/selftests/clone3/.gitignore
> +++ b/tools/testing/selftests/clone3/.gitignore
> @@ -1 +1,2 @@
>  clone3
> +clone3_set_tid
> diff --git a/tools/testing/selftests/clone3/Makefile b/tools/testing/selftests/clone3/Makefile
> index ea922c014ae4..2d292545ca8e 100644
> --- a/tools/testing/selftests/clone3/Makefile
> +++ b/tools/testing/selftests/clone3/Makefile
> @@ -2,6 +2,6 @@
>  
>  CFLAGS += -I../../../../usr/include/
>  
> -TEST_GEN_PROGS := clone3
> +TEST_GEN_PROGS := clone3 clone3_set_tid

Another example, where --base would help me out. :) (Since I can already
see that this is missing the clone3_clear_sighand test that's scheduled
for 5.5. :))

>  
>  include ../lib.mk
> diff --git a/tools/testing/selftests/clone3/clone3_set_tid.c b/tools/testing/selftests/clone3/clone3_set_tid.c
> new file mode 100644
> index 000000000000..9a234fd2031e
> --- /dev/null
> +++ b/tools/testing/selftests/clone3/clone3_set_tid.c
> @@ -0,0 +1,345 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +/*
> + * Based on Christian Brauner's clone3() example.
> + * These tests are assuming to be running in the host's
> + * PID namespace.
> + */
> +
> +#define _GNU_SOURCE
> +#include <errno.h>
> +#include <linux/types.h>
> +#include <linux/sched.h>
> +#include <stdio.h>
> +#include <stdlib.h>
> +#include <sys/syscall.h>
> +#include <sys/types.h>
> +#include <sys/un.h>
> +#include <sys/wait.h>
> +#include <unistd.h>
> +#include <sched.h>
> +
> +#include "../kselftest.h"
> +
> +#ifndef MAX_PID_NS_LEVEL
> +#define MAX_PID_NS_LEVEL 32
> +#endif
> +
> +static int pipe_1[2];
> +static int pipe_2[2];
> +
> +static pid_t raw_clone(struct clone_args *args)
> +{
> +	return syscall(__NR_clone3, args, sizeof(struct clone_args));
> +}

So that function is now present in the clone3.c test you've added and
also in the clone3_clear_sighand.c under sys_clone3().
If you take [1] as your base tree you could add patches that consolidate
this definition into a header so we avoid the pointless code
duplication.
(Again, if you pass --base when doing --format-patch this will be
transparent to others.)


[1]: https://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux.git/log/?h=pidfd

> +
> +static int call_clone3_set_tid(pid_t *set_tid,
> +			       size_t set_tid_size,
> +			       int flags,
> +			       int expected_pid,
> +			       int wait_for_it)

Nit: Why is that an int and not a bool? :)

> +{
> +	int status;
> +	int ret = 0;
> +	pid_t pid = -1;
> +	struct clone_args args = {0};
> +
> +	args.flags = flags;
> +	args.exit_signal = SIGCHLD;
> +	args.set_tid = (__u64)set_tid;
> +	args.set_tid_size = set_tid_size;

Nit: You can save the {0} by just doing:

struct clone_args args = {
	.flags = flags,
	.exit_signal = SIGCHLD,
	.set_tid = (__u64)set_tid,
	.set_tid_size = set_tid_size,
};

Also, I'd prefer if the cast would be done "properly" via

#define ptr_to_u64(ptr) ((__u64)((uintptr_t)(ptr)))

struct clone_args args = {
	.flags = flags,
	.exit_signal = SIGCHLD,
	.set_tid = ptr_to_u64(set_tid),
	.set_tid_size = set_tid_size,
};

> +
> +	pid = raw_clone(&args);
> +	if (pid < 0) {
> +		ksft_print_msg("%s - Failed to create new process\n",
> +			       strerror(errno));
> +		return -errno;
> +	}
> +
> +	if (pid == 0) {
> +		char tmp = 0;
> +		ksft_print_msg("I am the child, my PID is %d (expected %d)\n",
> +			       getpid(), set_tid[0]);
> +		if (wait_for_it) {
> +			ksft_print_msg("[%d] Child is ready and waiting\n", getpid());
> +			/* Signal the parent that the child is ready */
> +			close(pipe_1[0]);
> +			write(pipe_1[1], &tmp, 1);
> +			close(pipe_1[1]);
> +			close(pipe_2[1]);
> +			read(pipe_2[0], &tmp, 1);
> +			close(pipe_2[0]);
> +		}
> +
> +		if (set_tid[0] != getpid())
> +			_exit(EXIT_FAILURE);
> +		_exit(EXIT_SUCCESS);
> +	}
> +
> +	if (expected_pid == 0 || expected_pid == pid)
> +		ksft_print_msg("I am the parent (%d). My child's pid is %d\n",
> +			       getpid(), pid);
> +	else {

If one branch needs {} all branches should have {} according to kernel
coding style.

> +		ksft_print_msg(
> +			"Expected child pid %d does not match actual pid %d\n",
> +			expected_pid, pid);
> +		ret = -1;
> +	}
> +
> +	if (wait(&status) < 0) {
> +		ksft_print_msg("Child returned %s\n", strerror(errno));
> +		return -errno;
> +	}
> +	if (WEXITSTATUS(status))
> +		return WEXITSTATUS(status);

You should probably rather check:
	if (!WIFEXITED(status))
		return -1;
	
	return WEXITSTATUS(status));

Since you can also get killed by a signal.

> +
> +	return ret;
> +}
> +
> +static void test_clone3_set_tid(pid_t *set_tid,
> +				size_t set_tid_size,
> +				int flags,
> +				int expected,
> +				int expected_pid,
> +				int wait_for_it)
> +{
> +	int ret;
> +
> +	ksft_print_msg(
> +		"[%d] Trying clone3() with CLONE_SET_TID to %d and 0x%x\n",
> +		getpid(), set_tid[0], flags);
> +	ret = call_clone3_set_tid(set_tid, set_tid_size, flags, expected_pid,
> +				  wait_for_it);
> +	ksft_print_msg(
> +		"[%d] clone3() with CLONE_SET_TID %d says :%d - expected %d\n",
> +		getpid(), set_tid[0], ret, expected);
> +	if (ret != expected)
> +		ksft_test_result_fail(
> +			"[%d] Result (%d) is different than expected (%d)\n",
> +			getpid(), ret, expected);
> +	else
> +		ksft_test_result_pass("[%d] Result (%d) matches expectation (%d)\n",
> +			getpid(), ret, expected);
> +}
> +int main(int argc, char *argv[])
> +{
> +	FILE *f;
> +	char buf;
> +	pid_t pid;
> +	pid_t ns1;
> +	pid_t ns2;
> +	pid_t ns3;

Nit:
pid_t pid, ns1, ns2, ns3, ns_pid;

> +	int status;
> +	char *proc;
> +	int ret = -1;
> +	pid_t ns_pid;
> +	int pid_max = 0;
> +	uid_t uid = getuid();
> +	char line[1024] = {0};
> +	pid_t set_tid[MAX_PID_NS_LEVEL * 2];
> +	pid_t set_tid_small[1];
> +
> +	if (pipe(pipe_1) == -1 || pipe(pipe_2))
> +		 ksft_exit_fail_msg("pipe() failed\n");

Nit: The error checking here is inconsistent. Either do:

if (pipe(pipe_1) || pipe(pipe_2))
	 ksft_exit_fail_msg("pipe() failed\n");

or

if (pipe(pipe_1) < 0 || pipe(pipe_2) < 0)
	 ksft_exit_fail_msg("pipe() failed\n");

or == -1 but not mixed, please. :)

> +
> +	ksft_print_header();
> +	ksft_set_plan(27);

Now that's ambitious testing. I like it. :)

> +
> +	f = fopen("/proc/sys/kernel/pid_max", "r");
> +	if (f == NULL)
> +		ksft_exit_fail_msg(
> +			"%s - Could not open /proc/sys/kernel/pid_max\n",
> +			strerror(errno));
> +	fscanf(f, "%d", &pid_max);
> +	fclose(f);
> +	ksft_print_msg("/proc/sys/kernel/pid_max %d\n", pid_max);
> +
> +	/* Try invalid settings */
> +	memset(&set_tid, 0, sizeof(set_tid));
> +	test_clone3_set_tid(set_tid, MAX_PID_NS_LEVEL + 1, 0, -E2BIG, 0, 0);
> +	test_clone3_set_tid(set_tid, MAX_PID_NS_LEVEL * 2, 0, -E2BIG, 0, 0);
> +	test_clone3_set_tid(set_tid, MAX_PID_NS_LEVEL * 2 + 1, 0, -E2BIG, 0, 0);
> +	test_clone3_set_tid(set_tid, MAX_PID_NS_LEVEL * 42, 0, -E2BIG, 0, 0);
> +
> +	/* small set_tid array, but maximum set_tid_size */
> +	/* Find the current active PID */
> +	pid = fork();
> +	if (pid == 0) {
> +		ksft_print_msg("Child has PID %d\n", getpid());
> +		_exit(EXIT_SUCCESS);
> +	}
> +	(void)wait(NULL);
> +	/* After the child has finished, its PID should be free. */
> +	set_tid_small[0] = pid;
> +	/*
> +	 * There is a chance that this can return -EFAULT as the actual
> +	 * set_tid array has only one entry, but we are telling the kernel
> +	 * that it has the size MAX_PID_NS_LEVEL. This could lead to a
> +	 * situation where copy_from_user() fails. So far it always
> +	 * succeeds and copies random data (whatever is after set_tid_small).
> +	 */
> +	test_clone3_set_tid(set_tid_small, MAX_PID_NS_LEVEL, 0, -EINVAL, 0, 0);

Hm, I'm not sure that test makes a lot of sense.

> +
> +	/*
> +	 * This can actually work if this test running in a MAX_PID_NS_LEVEL - 1
> +	 * nested PID namespace.
> +	 */
> +	test_clone3_set_tid(set_tid, MAX_PID_NS_LEVEL - 1, 0, -EINVAL, 0, 0);
> +
> +	memset(&set_tid, 0xff, sizeof(set_tid));
> +	test_clone3_set_tid(set_tid, MAX_PID_NS_LEVEL + 1, 0, -E2BIG, 0, 0);
> +	test_clone3_set_tid(set_tid, MAX_PID_NS_LEVEL * 2, 0, -E2BIG, 0, 0);
> +	test_clone3_set_tid(set_tid, MAX_PID_NS_LEVEL * 2 + 1, 0, -E2BIG, 0, 0);
> +	test_clone3_set_tid(set_tid, MAX_PID_NS_LEVEL * 42, 0, -E2BIG, 0, 0);
> +	/*
> +	 * This can actually work if this test running in a MAX_PID_NS_LEVEL - 1
> +	 * nested PID namespace.
> +	 */
> +	test_clone3_set_tid(set_tid, MAX_PID_NS_LEVEL - 1, 0, -EINVAL, 0, 0);
> +
> +	memset(&set_tid, 0, sizeof(set_tid));
> +	/* Try with an invalid PID */
> +	set_tid[0] = 0;
> +	test_clone3_set_tid(set_tid, 1, 0, -EINVAL, 0, 0);
> +	set_tid[0] = -1;
> +	test_clone3_set_tid(set_tid, 1, 0, -EINVAL, 0, 0);
> +	/* Claim that the set_tid array actually contains 2 elements. */
> +	test_clone3_set_tid(set_tid, 2, 0, -EINVAL, 0, 0);
> +	/* Try it in a new PID namespace */
> +	if (uid == 0)
> +		test_clone3_set_tid(set_tid, 1, CLONE_NEWPID, -EINVAL, 0, 0);
> +	else
> +		test_clone3_set_tid(set_tid, 1, CLONE_NEWPID, -EPERM, 0, 0);

[1]:
I mean, you could really just do 
if (uid == 0)
	test_clone3_set_tid(set_tid, 1, CLONE_NEWPID, -EINVAL, 0, 0);
the else branch doesn't really test anything meaningful other than you
can't create new pid namespaces as an unprivileged user without also
creating a new user namespace.

> +
> +	/*
> +	 * Try with a valid PID (1) but as non-root. This should fail
> +	 * with -EPERM if running in the initial user namespace.
> +	 * As root it should tell us -EEXIST.
> +	 */
> +	set_tid[0] = 1;
> +	if (uid == 0)
> +		test_clone3_set_tid(set_tid, 1, 0, -EEXIST, 0, 0);
> +	else
> +		test_clone3_set_tid(set_tid, 1, 0, -EPERM, 0, 0);

See [1].

> +
> +	/* Try it in a new PID namespace */
> +	if (uid == 0)
> +		test_clone3_set_tid(set_tid, 1, CLONE_NEWPID, 0, 0, 0);
> +	else
> +		test_clone3_set_tid(set_tid, 1, CLONE_NEWPID, -EPERM, 0, 0);

See [1].

> +
> +	/* pid_max should fail everywhere */
> +	set_tid[0] = pid_max;
> +	test_clone3_set_tid(set_tid, 1, 0, -EINVAL, 0, 0);
> +	if (uid == 0)
> +		test_clone3_set_tid(set_tid, 1, CLONE_NEWPID, -EINVAL, 0, 0);
> +	else
> +		test_clone3_set_tid(set_tid, 1, CLONE_NEWPID, -EPERM, 0, 0);

See [1].
> +

> +	if (uid != 0) {
> +		/*
> +		 * All remaining tests require root. Tell the framework
> +		 * that all those tests are skipped as non-root.
> +		 */
> +		ksft_cnt.ksft_xskip += ksft_plan - ksft_test_num();
> +		goto out;
> +	}
> +
> +	/* Find the current active PID */
> +	pid = fork();
> +	if (pid == 0) {
> +		ksft_print_msg("Child has PID %d\n", getpid());
> +		usleep(500);

Why the usleep(500)?

> +		_exit(EXIT_SUCCESS);
> +	}
> +	(void)wait(NULL);

\n

> +	/* After the child has finished, its PID should be free. */
> +	set_tid[0] = pid;
> +	test_clone3_set_tid(set_tid, 1, 0, 0, 0, 0);

\n

> +	/* This should fail as there is no PID 1 in that namespace */
> +	test_clone3_set_tid(set_tid, 1, CLONE_NEWPID, -EINVAL, 0, 0);

Nit: please add \n between a two tests everywhere, especially if they
have comments on top of it. This is a little hard to read. :)

> +	set_tid[0] = 1;
> +	set_tid[1] = pid;
> +	test_clone3_set_tid(set_tid, 2, CLONE_NEWPID, 0, pid, 0);

This is missing a comment.

> +
> +	ksft_print_msg("unshare PID namespace\n");
> +	unshare(CLONE_NEWPID);

Why no error checking here when the next line assumes that there is no
PID 1 in that namespace?

> +	set_tid[0] = pid;
> +	/* This should fail as there is no PID 1 in that namespace */
> +	test_clone3_set_tid(set_tid, 1, 0, -EINVAL, 0, 0);
> +
> +	/* Let's create a PID 1 */
> +	ns_pid = fork();
> +	if (ns_pid == 0) {
> +		ksft_print_msg("Child in PID namespace has PID %d\n", getpid());
> +		set_tid[0] = 2;
> +		test_clone3_set_tid(set_tid, 1, 0, 0, 2, 0);
> +		set_tid[0] = 1;
> +		set_tid[1] = 42;
> +		set_tid[2] = pid;
> +		/*
> +		 * This should fail as there are not enough active PID
> +		 * namespaces. Again assuming this is running in the host's
> +		 * PID namespace. Not yet nested.
> +		 */
> +		test_clone3_set_tid(set_tid, 4, CLONE_NEWPID, -EINVAL, 0, 0);
> +		/*
> +		 * This should work and from the parent we should see
> +		 * something like 'NSpid:	pid	42	1'.

[2]:
You could verify this...
In tools/testing/selftests/pidfd/pidfd_fdinfo_test.c in my pidfd tree
is code that does this. You could just copy it. Look for
verify_fdinfo(). Though, I'll it for you to judge whether this is too
much hazzle.

> +		 */
> +		test_clone3_set_tid(set_tid, 3, CLONE_NEWPID, 0, 42, 1);
> +		_exit(ksft_cnt.ksft_pass);

Again, the missing \n makes the different tests hard to follow.

> +	}
> +
> +	close(pipe_1[1]);
> +	close(pipe_2[0]);
> +	while (read(pipe_1[0], &buf, 1) > 0) {
> +		ksft_print_msg("[%d] Child is ready and waiting\n", getpid());
> +		break;
> +	}
> +
> +	asprintf(&proc, "/proc/%d/status", pid);

char proc_path[100];
snprintf(proc_path, sizeof(proc_path), "/proc/%d/status", pid);

should do just fine and doesn't require you to free memory.

> +	f = fopen(proc, "r");
> +	if (f == NULL)
> +		ksft_exit_fail_msg(
> +			"%s - Could not open %s\n",
> +			strerror(errno), proc);
> +	while (fgets(line, 1024, f)) {

Ugh, fgets... :) I'd feel much happier with getline().

> +		if (strstr(line, "NSpid")) {
> +			/* Verify that all generated PIDs are as expected. */
> +			sscanf(line, "NSpid:\t%d\t%d\t%d", &ns3, &ns2, &ns1);
> +			break;

Oh this is the verification code for fdinfo, I mentioned in [2]. Again,
maybe you want to copy verify_fdinfo() and move this into a helper.

> +		}
> +	}
> +	fclose(f);
> +	free(proc);
> +	close(pipe_2[0]);

\n

> +	/* Tell the clone3()'d child to finish. */
> +	write(pipe_2[1], &buf, 1);
> +	close(pipe_2[1]);
> +
> +	if (wait(&status) < 0) {
> +		ksft_print_msg("Child returned %s\n", strerror(errno));
> +		ret = -errno;
> +		goto out;
> +	}
> +	if (WEXITSTATUS(status))

Nit: This should probably also verify if (WIFEXITED(status)).

Re: [PATCH v8 1/2] fork: extend clone3() to support setting a PID

[Oleg Nesterov]

On 11/13, Adrian Reber wrote:
>
> @@ -175,23 +187,47 @@ struct pid *alloc_pid(struct pid_namespace *ns)
>  
>  	for (i = ns->level; i >= 0; i--) {
>  		int pid_min = 1;

Well, this is really minor but again, pid_min is only used in the "else"
branch below, you can move this declaration there.

> +
> +		if (set_tid_size) {
> +			tid = set_tid[ns->level - i];
> +			if (tid < 1 || tid >= pid_max)
> +				return ERR_PTR(-EINVAL);
> +			/* Also fail if a PID != 1 is requested and no PID 1 exists */
> +			if (tid != 1 && !tmp->child_reaper)
> +				return ERR_PTR(-EINVAL);
> +			if (!ns_capable(tmp->user_ns, CAP_SYS_ADMIN))
> +				return ERR_PTR(-EPERM);
> +		}
>  
>  		idr_preload(GFP_KERNEL);
>  		spin_lock_irq(&pidmap_lock);
>  
> -		/*
> -		 * init really needs pid 1, but after reaching the maximum
> -		 * wrap back to RESERVED_PIDS
> -		 */
> -		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 (tid) {
> +			nr = idr_alloc(&tmp->idr, NULL, tid,
> +				       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;
> +			set_tid_size--;
                        ^^^^^^^^^^^^^^^
May I ask you to move this decrement into the "if (set_tid_size)" block above?

Oleg.