[PATCH 6/6] clone4: Introduce new CLONE_FD flag to get task exit notification via fd

From: Josh Triplett
Date: Thu Mar 12 2015 - 21:41:24 EST

When passed CLONE_FD, clone4 will return a file descriptor rather than a
PID. When the child process exits, it gets automatically reaped, and
the file descriptor becomes readable, producing a structure containing
the exit code and user/system time. The file descriptor also works in
epoll, poll, or select.

This allows libraries to safely launch and manage child processes on
behalf of a caller, without taking over or interfering with process-wide
signal handling. Without this, such a library would need to take over
or cooperate with the entire process's SIGCHLD handling, either via a
signal handler or a signalfd.

CLONE_FD will never return a file descriptor in the 0-2 range; thus, a 0
return from clone4 still indicates the child process.

Since a process created with CLONE_FD does not send any exit signal, the
low byte of the clone flags no longer needs to contain a signal number,
freeing it up for use as CLONE_FD-specific flags; use that to provide
the usual CLOEXEC and NONBLOCK flags.

CLONE_FD takes the value of the unused CLONE_PID, so CLONE4_VALID_ARGS
now includes CLONE_FD; CLONE_VALID_ARGS still doesn't, and sys_clone
still ignores that flag, as only clone4 can use it.

Signed-off-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
Signed-off-by: Thiago Macieira <thiago.macieira@xxxxxxxxx>
---
include/linux/sched.h | 5 ++
include/uapi/linux/sched.h | 23 ++++++++-
init/Kconfig | 11 ++++
kernel/Makefile | 1 +
kernel/clonefd.c | 123 +++++++++++++++++++++++++++++++++++++++++++++
kernel/clonefd.h | 27 ++++++++++
kernel/exit.c | 10 +++-
kernel/fork.c | 40 ++++++++++++---
8 files changed, 231 insertions(+), 9 deletions(-)
create mode 100644 kernel/clonefd.c
create mode 100644 kernel/clonefd.h

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 668c58f..55cf10bb 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1351,6 +1351,9 @@ struct task_struct {
#if defined(SPLIT_RSS_COUNTING)
struct task_rss_stat rss_stat;
#endif
+#ifdef CONFIG_CLONEFD
+ wait_queue_head_t clonefd_wqh;
+#endif
/* task state */
int exit_state;
int exit_code, exit_signal;
@@ -1372,6 +1375,8 @@ struct task_struct {
unsigned memcg_kmem_skip_account:1;
#endif

+ unsigned autoreap:1; /* Do not become a zombie on exit */
+
unsigned long atomic_flags; /* Flags needing atomic access. */

struct restart_block restart_block;
diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h
index b5b8012..d2082c61 100644
--- a/include/uapi/linux/sched.h
+++ b/include/uapi/linux/sched.h
@@ -38,10 +38,31 @@
#define CLONE_STOPPED 0x02000000

/*
+ * Flags that only work with clone4.
+ */
+#define CLONE_FD 0x00001000 /* set if we want a file descriptor rather than a PID */
+
+/*
* Valid flags for clone and for clone4
*/
#define CLONE_VALID_FLAGS (0xffffffffULL & ~(CLONE_PID | CLONE_DETACHED | CLONE_STOPPED))
-#define CLONE4_VALID_FLAGS CLONE_VALID_FLAGS
+#define CLONE4_VALID_FLAGS (CLONE_VALID_FLAGS | CLONE_FD)
+
+/*
+ * Flags passed in the low byte when using CLONE_FD, in place of the signal.
+ */
+#define CLONEFD_CLOEXEC 0x00000001 /* Used with CLONE_FD to set O_CLOEXEC on new fd */
+#define CLONEFD_NONBLOCK 0x00000002 /* Used with CLONE_FD to set O_NONBLOCK on new fd */
+
+/*
+ * Structure read from CLONE_FD file descriptor after process exits
+ */
+struct clonefd_info {
+ __s32 code;
+ __s32 status;
+ __u64 utime;
+ __u64 stime;
+};

/*
* Structure passed to clone4 for additional arguments. Initialized to 0,
diff --git a/init/Kconfig b/init/Kconfig
index 3ab6649..b444280 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1521,6 +1521,17 @@ config CLONE4

If unsure, say Y.

+config CLONEFD
+ bool "Enable CLONE_FD flag for clone4()" if EXPERT
+ depends on CLONE4
+ select ANON_INODES
+ default y
+ help
+ Enable the CLONE_FD flag for clone4(), which creates a file descriptor
+ to receive child exit events rather than receiving a signal.
+
+ If unsure, say Y.
+
# syscall, maps, verifier
config BPF_SYSCALL
bool "Enable bpf() system call" if EXPERT
diff --git a/kernel/Makefile b/kernel/Makefile
index 1408b33..368986c 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -29,6 +29,7 @@ obj-y += rcu/
obj-y += livepatch/

obj-$(CONFIG_CHECKPOINT_RESTORE) += kcmp.o
+obj-$(CONFIG_CLONEFD) += clonefd.o
obj-$(CONFIG_FREEZER) += freezer.o
obj-$(CONFIG_PROFILING) += profile.o
obj-$(CONFIG_STACKTRACE) += stacktrace.o
diff --git a/kernel/clonefd.c b/kernel/clonefd.c
new file mode 100644
index 0000000..78fb776
--- /dev/null
+++ b/kernel/clonefd.c
@@ -0,0 +1,123 @@
+/*
+ * Support functions for CLONE_FD
+ *
+ * Copyright (c) 2015 Intel Corporation
+ * Original authors: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
+ * Thiago Macieira <thiago@xxxxxxxxxxxx>
+ */
+#include <linux/anon_inodes.h>
+#include <linux/file.h>
+#include <linux/fs.h>
+#include <linux/poll.h>
+#include <linux/slab.h>
+#include "clonefd.h"
+
+static int clonefd_release(struct inode *inode, struct file *file)
+{
+ put_task_struct(file->private_data);
+ return 0;
+}
+
+static unsigned int clonefd_poll(struct file *file, poll_table *wait)
+{
+ struct task_struct *p = file->private_data;
+ poll_wait(file, &p->clonefd_wqh, wait);
+ return p->exit_state == EXIT_DEAD ? (POLLIN | POLLRDNORM) : 0;
+}
+
+static ssize_t clonefd_read(struct file *file, char __user *buf, size_t count, loff_t *ppos)
+{
+ struct task_struct *p = file->private_data;
+ int ret = 0;
+
+ /* EOF after first read */
+ if (*ppos)
+ return 0;
+
+ if (file->f_flags & O_NONBLOCK)
+ ret = -EAGAIN;
+ else
+ ret = wait_event_interruptible(p->clonefd_wqh, p->exit_state == EXIT_DEAD);
+
+ if (p->exit_state == EXIT_DEAD) {
+ struct clonefd_info info = {};
+ cputime_t utime, stime;
+ task_exit_code_status(p->exit_code, &info.code, &info.status);
+ info.code &= ~__SI_MASK;
+ task_cputime(p, &utime, &stime);
+ info.utime = cputime_to_clock_t(utime + p->signal->utime);
+ info.stime = cputime_to_clock_t(stime + p->signal->stime);
+ ret = simple_read_from_buffer(buf, count, ppos, &info, sizeof(info));
+ }
+ return ret;
+}
+
+static struct file_operations clonefd_fops = {
+ .release = clonefd_release,
+ .poll = clonefd_poll,
+ .read = clonefd_read,
+ .llseek = no_llseek,
+};
+
+/* Do process exit notification for clonefd. */
+void clonefd_do_notify(struct task_struct *p)
+{
+ if (p->autoreap)
+ wake_up_all(&p->clonefd_wqh);
+}
+
+/* Handle the CLONE_FD case for copy_process. */
+int clonefd_do_clone(u64 clone_flags, struct task_struct *p, struct clonefd_setup *setup)
+{
+ int flags;
+ struct file *file;
+ int fd;
+
+ if (!(clone_flags & CLONE_FD))
+ return 0;
+
+ p->autoreap = 1;
+ init_waitqueue_head(&p->clonefd_wqh);
+
+ get_task_struct(p);
+ flags = O_RDONLY | FMODE_ATOMIC_POS
+ | (clone_flags & CLONEFD_CLOEXEC ? O_CLOEXEC : 0)
+ | (clone_flags & CLONEFD_NONBLOCK ? O_NONBLOCK : 0);
+ file = anon_inode_getfile("[process]", &clonefd_fops, p, flags);
+ if (IS_ERR(file)) {
+ put_task_struct(p);
+ return PTR_ERR(file);
+ }
+
+ /*
+ * We avoid allocating a low fd so that clone can still return 0 in the
+ * child; the child shouldn't have to change just because the parent
+ * used CLONE_FD.
+ */
+ fd = alloc_fd(3, flags);
+ if (fd < 0) {
+ fput(file);
+ return fd;
+ }
+
+ setup->fd = fd;
+ setup->file = file;
+
+ return 0;
+}
+
+/* Clean up clonefd information after a partially complete clone */
+void clonefd_cleanup_failed_clone(struct task_struct *p, struct clonefd_setup *setup)
+{
+ if (setup->fd)
+ put_unused_fd(setup->fd);
+ if (setup->file)
+ fput(setup->file);
+}
+
+/* Finish setting up the clonefd */
+int clonefd_install_fd(struct task_struct *p, struct clonefd_setup *setup)
+{
+ fd_install(setup->fd, setup->file);
+ return setup->fd;
+}
diff --git a/kernel/clonefd.h b/kernel/clonefd.h
new file mode 100644
index 0000000..07bd31f
--- /dev/null
+++ b/kernel/clonefd.h
@@ -0,0 +1,27 @@
+/*
+ * Support functions for CLONE_FD
+ *
+ * Copyright (c) 2015 Intel Corporation
+ * Original authors: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
+ * Thiago Macieira <thiago@xxxxxxxxxxxx>
+ */
+#pragma once
+
+#include <linux/sched.h>
+
+#ifdef CONFIG_CLONEFD
+struct clonefd_setup {
+ int fd;
+ struct file *file;
+};
+int clonefd_do_clone(u64 clone_flags, struct task_struct *p, struct clonefd_setup *setup);
+void clonefd_cleanup_failed_clone(struct task_struct *p, struct clonefd_setup *setup);
+int clonefd_install_fd(struct task_struct *p, struct clonefd_setup *setup);
+void clonefd_do_notify(struct task_struct *p);
+#else /* CONFIG_CLONEFD */
+struct clonefd_setup {};
+static inline int clonefd_do_clone(u64 clone_flags, struct task_struct *p, struct clonefd_setup *setup) { return 0; }
+static inline void clonefd_cleanup_failed_clone (struct task_struct *p, struct clonefd_setup *setup) {}
+static inline int clonefd_install_fd(struct task_struct *p, struct clonefd_setup *setup) { return -EINVAL; }
+static inline void clonefd_do_notify(struct task_struct *p) {}
+#endif /* CONFIG_CLONEFD */
diff --git a/kernel/exit.c b/kernel/exit.c
index feff10b..a2c8520 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -59,6 +59,8 @@
#include <asm/pgtable.h>
#include <asm/mmu_context.h>

+#include "clonefd.h"
+
static void exit_mm(struct task_struct *tsk);

static void __unhash_process(struct task_struct *p, bool group_dead)
@@ -598,7 +600,9 @@ static void exit_notify(struct task_struct *tsk, int group_dead)
if (group_dead)
kill_orphaned_pgrp(tsk->group_leader, NULL);

- if (unlikely(tsk->ptrace)) {
+ if (tsk->autoreap) {
+ autoreap = true;
+ } else if (unlikely(tsk->ptrace)) {
int sig = thread_group_leader(tsk) &&
thread_group_empty(tsk) &&
!ptrace_reparented(tsk) ?
@@ -612,8 +616,10 @@ static void exit_notify(struct task_struct *tsk, int group_dead)
}

tsk->exit_state = autoreap ? EXIT_DEAD : EXIT_ZOMBIE;
- if (tsk->exit_state == EXIT_DEAD)
+ if (tsk->exit_state == EXIT_DEAD) {
list_add(&tsk->ptrace_entry, &dead);
+ clonefd_do_notify(tsk);
+ }

/* mt-exec, de_thread() is waiting for group leader */
if (unlikely(tsk->signal->notify_count < 0))
diff --git a/kernel/fork.c b/kernel/fork.c
index e29edea..00cab05 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -87,6 +87,8 @@
#define CREATE_TRACE_POINTS
#include <trace/events/task.h>

+#include "clonefd.h"
+
/*
* Protected counters by write_lock_irq(&tasklist_lock)
*/
@@ -321,6 +323,8 @@ static struct task_struct *dup_task_struct(struct task_struct *orig)
if (err)
goto free_ti;

+ tsk->autoreap = 0;
+
tsk->stack = ti;
#ifdef CONFIG_SECCOMP
/*
@@ -1193,7 +1197,8 @@ static struct task_struct *copy_process(u64 clone_flags,
int __user *child_tidptr,
struct pid *pid,
int trace,
- unsigned long tls)
+ unsigned long tls,
+ struct clonefd_setup *clonefd_setup)
{
int retval;
struct task_struct *p;
@@ -1244,6 +1249,16 @@ static struct task_struct *copy_process(u64 clone_flags,
return ERR_PTR(-EINVAL);
}

+ /*
+ * If using CLONE_FD, the low byte is used for additional flags; check
+ * for unknown flags.
+ */
+ if (clone_flags & CLONE_FD) {
+ if (!IS_ENABLED(CONFIG_CLONEFD) ||
+ (clone_flags & CSIGNAL & ~(CLONEFD_CLOEXEC | CLONEFD_NONBLOCK)))
+ return ERR_PTR(-EINVAL);
+ }
+
retval = security_task_create(clone_flags);
if (retval)
goto fork_out;
@@ -1416,6 +1431,10 @@ static struct task_struct *copy_process(u64 clone_flags,
goto bad_fork_cleanup_io;
}

+ retval = clonefd_do_clone(clone_flags, p, clonefd_setup);
+ if (retval)
+ goto bad_fork_free_pid;
+
p->set_child_tid = (clone_flags & CLONE_CHILD_SETTID) ? child_tidptr : NULL;
/*
* Clear TID on mm_release()?
@@ -1456,7 +1475,9 @@ static struct task_struct *copy_process(u64 clone_flags,
p->group_leader = current->group_leader;
p->tgid = current->tgid;
} else {
- if (clone_flags & CLONE_PARENT)
+ if (clone_flags & CLONE_FD)
+ p->exit_signal = 0;
+ else if (clone_flags & CLONE_PARENT)
p->exit_signal = current->group_leader->exit_signal;
else
p->exit_signal = (clone_flags & CSIGNAL);
@@ -1508,7 +1529,7 @@ static struct task_struct *copy_process(u64 clone_flags,
spin_unlock(&current->sighand->siglock);
write_unlock_irq(&tasklist_lock);
retval = -ERESTARTNOINTR;
- goto bad_fork_free_pid;
+ goto bad_fork_cleanup_clonefd;
}

if (likely(p->pid)) {
@@ -1560,6 +1581,8 @@ static struct task_struct *copy_process(u64 clone_flags,

return p;

+bad_fork_cleanup_clonefd:
+ clonefd_cleanup_failed_clone(p, clonefd_setup);
bad_fork_free_pid:
if (pid != &init_struct_pid)
free_pid(pid);
@@ -1617,7 +1640,7 @@ static inline void init_idle_pids(struct pid_link *links)
struct task_struct *fork_idle(int cpu)
{
struct task_struct *task;
- task = copy_process(CLONE_VM, 0, 0, NULL, &init_struct_pid, 0, 0);
+ task = copy_process(CLONE_VM, 0, 0, NULL, &init_struct_pid, 0, 0, NULL);
if (!IS_ERR(task)) {
init_idle_pids(task->pids);
init_idle(task, cpu);
@@ -1643,6 +1666,7 @@ static long _do_fork(
struct task_struct *p;
int trace = 0;
long nr;
+ struct clonefd_setup clonefd_setup = {};

/*
* Determine whether and which event to report to ptracer. When
@@ -1653,7 +1677,8 @@ static long _do_fork(
if (!(clone_flags & CLONE_UNTRACED)) {
if (clone_flags & CLONE_VFORK)
trace = PTRACE_EVENT_VFORK;
- else if ((clone_flags & CSIGNAL) != SIGCHLD)
+ else if ((clone_flags & CLONE_FD) ||
+ (clone_flags & CSIGNAL) != SIGCHLD)
trace = PTRACE_EVENT_CLONE;
else
trace = PTRACE_EVENT_FORK;
@@ -1663,7 +1688,7 @@ static long _do_fork(
}

p = copy_process(clone_flags, stack_start, stack_size,
- child_tidptr, NULL, trace, tls);
+ child_tidptr, NULL, trace, tls, &clonefd_setup);
/*
* Do this prior waking up the new thread - the thread pointer
* might get invalid after that point, if the thread exits quickly.
@@ -1686,6 +1711,9 @@ static long _do_fork(
get_task_struct(p);
}

+ if (clone_flags & CLONE_FD)
+ nr = clonefd_install_fd(p, &clonefd_setup);
+
wake_up_new_task(p);

/* forking complete and child started to run, tell ptracer */
--
2.1.4

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