[mirror_ubuntu-bionic-kernel.git] / fs / proc / array.c

// SPDX-License-Identifier: GPL-2.0
/*
 *  linux/fs/proc/array.c
 *
 *  Copyright (C) 1992  by Linus Torvalds
 *  based on ideas by Darren Senn
 *
 * Fixes:
 * Michael. K. Johnson: stat,statm extensions.
 *                      <johnsonm@stolaf.edu>
 *
 * Pauline Middelink :  Made cmdline,envline only break at '\0's, to
 *                      make sure SET_PROCTITLE works. Also removed
 *                      bad '!' which forced address recalculation for
 *                      EVERY character on the current page.
 *                      <middelin@polyware.iaf.nl>
 *
 * Danny ter Haar    :  added cpuinfo
 *                      <dth@cistron.nl>
 *
 * Alessandro Rubini :  profile extension.
 *                      <rubini@ipvvis.unipv.it>
 *
 * Jeff Tranter      :  added BogoMips field to cpuinfo
 *                      <Jeff_Tranter@Mitel.COM>
 *
 * Bruno Haible      :  remove 4K limit for the maps file
 *                      <haible@ma2s2.mathematik.uni-karlsruhe.de>
 *
 * Yves Arrouye      :  remove removal of trailing spaces in get_array.
 *                      <Yves.Arrouye@marin.fdn.fr>
 *
 * Jerome Forissier  :  added per-CPU time information to /proc/stat
 *                      and /proc/<pid>/cpu extension
 *                      <forissier@isia.cma.fr>
 *                      - Incorporation and non-SMP safe operation
 *                      of forissier patch in 2.1.78 by
 *                      Hans Marcus <crowbar@concepts.nl>
 *
 * aeb@cwi.nl        :  /proc/partitions
 *
 *
 * Alan Cox          :  security fixes.
 *                      <alan@lxorguk.ukuu.org.uk>
 *
 * Al Viro           :  safe handling of mm_struct
 *
 * Gerhard Wichert   :  added BIGMEM support
 * Siemens AG           <Gerhard.Wichert@pdb.siemens.de>
 *
 * Al Viro & Jeff Garzik :  moved most of the thing into base.c and
 *                       :  proc_misc.c. The rest may eventually go into
 *                       :  base.c too.
 */

#include <linux/types.h>
#include <linux/errno.h>
#include <linux/time.h>
#include <linux/kernel.h>
#include <linux/kernel_stat.h>
#include <linux/tty.h>
#include <linux/string.h>
#include <linux/mman.h>
#include <linux/sched/mm.h>
#include <linux/sched/numa_balancing.h>
#include <linux/sched/task_stack.h>
#include <linux/sched/task.h>
#include <linux/sched/cputime.h>
#include <linux/proc_fs.h>
#include <linux/ioport.h>
#include <linux/uaccess.h>
#include <linux/io.h>
#include <linux/mm.h>
#include <linux/hugetlb.h>
#include <linux/pagemap.h>
#include <linux/swap.h>
#include <linux/smp.h>
#include <linux/signal.h>
#include <linux/highmem.h>
#include <linux/file.h>
#include <linux/fdtable.h>
#include <linux/times.h>
#include <linux/cpuset.h>
#include <linux/rcupdate.h>
#include <linux/delayacct.h>
#include <linux/seq_file.h>
#include <linux/pid_namespace.h>
#include <linux/ptrace.h>
#include <linux/tracehook.h>
#include <linux/string_helpers.h>
#include <linux/user_namespace.h>
#include <linux/fs_struct.h>

#include <asm/pgtable.h>
#include <asm/processor.h>
#include "internal.h"

static inline void task_name(struct seq_file *m, struct task_struct *p)
{
        char *buf;
        size_t size;
        char tcomm[sizeof(p->comm)];
        int ret;

        get_task_comm(tcomm, p);

        seq_puts(m, "Name:\t");

        size = seq_get_buf(m, &buf);
        ret = string_escape_str(tcomm, buf, size, ESCAPE_SPACE | ESCAPE_SPECIAL, "\n\\");
        seq_commit(m, ret < size ? ret : -1);

        seq_putc(m, '\n');
}

/*
 * The task state array is a strange "bitmap" of
 * reasons to sleep. Thus "running" is zero, and
 * you can test for combinations of others with
 * simple bit tests.
 */
static const char * const task_state_array[] = {

        /* states in TASK_REPORT: */
        "R (running)",          /* 0x00 */
        "S (sleeping)",         /* 0x01 */
        "D (disk sleep)",       /* 0x02 */
        "T (stopped)",          /* 0x04 */
        "t (tracing stop)",     /* 0x08 */
        "X (dead)",             /* 0x10 */
        "Z (zombie)",           /* 0x20 */
        "P (parked)",           /* 0x40 */

        /* states beyond TASK_REPORT: */
        "I (idle)",             /* 0x80 */
};

static inline const char *get_task_state(struct task_struct *tsk)
{
        BUILD_BUG_ON(1 + ilog2(TASK_REPORT_MAX) != ARRAY_SIZE(task_state_array));
        return task_state_array[task_state_index(tsk)];
}

static inline int get_task_umask(struct task_struct *tsk)
{
        struct fs_struct *fs;
        int umask = -ENOENT;

        task_lock(tsk);
        fs = tsk->fs;
        if (fs)
                umask = fs->umask;
        task_unlock(tsk);
        return umask;
}

static inline void task_state(struct seq_file *m, struct pid_namespace *ns,
                                struct pid *pid, struct task_struct *p)
{
        struct user_namespace *user_ns = seq_user_ns(m);
        struct group_info *group_info;
        int g, umask;
        struct task_struct *tracer;
        const struct cred *cred;
        pid_t ppid, tpid = 0, tgid, ngid;
        unsigned int max_fds = 0;

        rcu_read_lock();
        ppid = pid_alive(p) ?
                task_tgid_nr_ns(rcu_dereference(p->real_parent), ns) : 0;

        tracer = ptrace_parent(p);
        if (tracer)
                tpid = task_pid_nr_ns(tracer, ns);

        tgid = task_tgid_nr_ns(p, ns);
        ngid = task_numa_group_id(p);
        cred = get_task_cred(p);

        umask = get_task_umask(p);
        if (umask >= 0)
                seq_printf(m, "Umask:\t%#04o\n", umask);

        task_lock(p);
        if (p->files)
                max_fds = files_fdtable(p->files)->max_fds;
        task_unlock(p);
        rcu_read_unlock();

        seq_printf(m, "State:\t%s", get_task_state(p));

        seq_put_decimal_ull(m, "\nTgid:\t", tgid);
        seq_put_decimal_ull(m, "\nNgid:\t", ngid);
        seq_put_decimal_ull(m, "\nPid:\t", pid_nr_ns(pid, ns));
        seq_put_decimal_ull(m, "\nPPid:\t", ppid);
        seq_put_decimal_ull(m, "\nTracerPid:\t", tpid);
        seq_put_decimal_ull(m, "\nUid:\t", from_kuid_munged(user_ns, cred->uid));
        seq_put_decimal_ull(m, "\t", from_kuid_munged(user_ns, cred->euid));
        seq_put_decimal_ull(m, "\t", from_kuid_munged(user_ns, cred->suid));
        seq_put_decimal_ull(m, "\t", from_kuid_munged(user_ns, cred->fsuid));
        seq_put_decimal_ull(m, "\nGid:\t", from_kgid_munged(user_ns, cred->gid));
        seq_put_decimal_ull(m, "\t", from_kgid_munged(user_ns, cred->egid));
        seq_put_decimal_ull(m, "\t", from_kgid_munged(user_ns, cred->sgid));
        seq_put_decimal_ull(m, "\t", from_kgid_munged(user_ns, cred->fsgid));
        seq_put_decimal_ull(m, "\nFDSize:\t", max_fds);

        seq_puts(m, "\nGroups:\t");
        group_info = cred->group_info;
        for (g = 0; g < group_info->ngroups; g++)
                seq_put_decimal_ull(m, g ? " " : "",
                                from_kgid_munged(user_ns, group_info->gid[g]));
        put_cred(cred);
        /* Trailing space shouldn't have been added in the first place. */
        seq_putc(m, ' ');

#ifdef CONFIG_PID_NS
        seq_puts(m, "\nNStgid:");
        for (g = ns->level; g <= pid->level; g++)
                seq_put_decimal_ull(m, "\t", task_tgid_nr_ns(p, pid->numbers[g].ns));
        seq_puts(m, "\nNSpid:");
        for (g = ns->level; g <= pid->level; g++)
                seq_put_decimal_ull(m, "\t", task_pid_nr_ns(p, pid->numbers[g].ns));
        seq_puts(m, "\nNSpgid:");
        for (g = ns->level; g <= pid->level; g++)
                seq_put_decimal_ull(m, "\t", task_pgrp_nr_ns(p, pid->numbers[g].ns));
        seq_puts(m, "\nNSsid:");
        for (g = ns->level; g <= pid->level; g++)
                seq_put_decimal_ull(m, "\t", task_session_nr_ns(p, pid->numbers[g].ns));
#endif
        seq_putc(m, '\n');
}

void render_sigset_t(struct seq_file *m, const char *header,
                                sigset_t *set)
{
        int i;

        seq_puts(m, header);

        i = _NSIG;
        do {
                int x = 0;

                i -= 4;
                if (sigismember(set, i+1)) x |= 1;
                if (sigismember(set, i+2)) x |= 2;
                if (sigismember(set, i+3)) x |= 4;
                if (sigismember(set, i+4)) x |= 8;
                seq_putc(m, hex_asc[x]);
        } while (i >= 4);

        seq_putc(m, '\n');
}

static void collect_sigign_sigcatch(struct task_struct *p, sigset_t *ign,
                                    sigset_t *catch)
{
        struct k_sigaction *k;
        int i;

        k = p->sighand->action;
        for (i = 1; i <= _NSIG; ++i, ++k) {
                if (k->sa.sa_handler == SIG_IGN)
                        sigaddset(ign, i);
                else if (k->sa.sa_handler != SIG_DFL)
                        sigaddset(catch, i);
        }
}

static inline void task_sig(struct seq_file *m, struct task_struct *p)
{
        unsigned long flags;
        sigset_t pending, shpending, blocked, ignored, caught;
        int num_threads = 0;
        unsigned long qsize = 0;
        unsigned long qlim = 0;

        sigemptyset(&pending);
        sigemptyset(&shpending);
        sigemptyset(&blocked);
        sigemptyset(&ignored);
        sigemptyset(&caught);

        if (lock_task_sighand(p, &flags)) {
                pending = p->pending.signal;
                shpending = p->signal->shared_pending.signal;
                blocked = p->blocked;
                collect_sigign_sigcatch(p, &ignored, &caught);
                num_threads = get_nr_threads(p);
                rcu_read_lock();  /* FIXME: is this correct? */
                qsize = atomic_read(&__task_cred(p)->user->sigpending);
                rcu_read_unlock();
                qlim = task_rlimit(p, RLIMIT_SIGPENDING);
                unlock_task_sighand(p, &flags);
        }

        seq_put_decimal_ull(m, "Threads:\t", num_threads);
        seq_put_decimal_ull(m, "\nSigQ:\t", qsize);
        seq_put_decimal_ull(m, "/", qlim);

        /* render them all */
        render_sigset_t(m, "\nSigPnd:\t", &pending);
        render_sigset_t(m, "ShdPnd:\t", &shpending);
        render_sigset_t(m, "SigBlk:\t", &blocked);
        render_sigset_t(m, "SigIgn:\t", &ignored);
        render_sigset_t(m, "SigCgt:\t", &caught);
}

static void render_cap_t(struct seq_file *m, const char *header,
                        kernel_cap_t *a)
{
        unsigned __capi;

        seq_puts(m, header);
        CAP_FOR_EACH_U32(__capi) {
                seq_printf(m, "%08x",
                           a->cap[CAP_LAST_U32 - __capi]);
        }
        seq_putc(m, '\n');
}

static inline void task_cap(struct seq_file *m, struct task_struct *p)
{
        const struct cred *cred;
        kernel_cap_t cap_inheritable, cap_permitted, cap_effective,
                        cap_bset, cap_ambient;

        rcu_read_lock();
        cred = __task_cred(p);
        cap_inheritable = cred->cap_inheritable;
        cap_permitted   = cred->cap_permitted;
        cap_effective   = cred->cap_effective;
        cap_bset        = cred->cap_bset;
        cap_ambient     = cred->cap_ambient;
        rcu_read_unlock();

        render_cap_t(m, "CapInh:\t", &cap_inheritable);
        render_cap_t(m, "CapPrm:\t", &cap_permitted);
        render_cap_t(m, "CapEff:\t", &cap_effective);
        render_cap_t(m, "CapBnd:\t", &cap_bset);
        render_cap_t(m, "CapAmb:\t", &cap_ambient);
}

static inline void task_seccomp(struct seq_file *m, struct task_struct *p)
{
        seq_put_decimal_ull(m, "NoNewPrivs:\t", task_no_new_privs(p));
#ifdef CONFIG_SECCOMP
        seq_put_decimal_ull(m, "\nSeccomp:\t", p->seccomp.mode);
#endif
        seq_putc(m, '\n');
}

static inline void task_context_switch_counts(struct seq_file *m,
                                                struct task_struct *p)
{
        seq_put_decimal_ull(m, "voluntary_ctxt_switches:\t", p->nvcsw);
        seq_put_decimal_ull(m, "\nnonvoluntary_ctxt_switches:\t", p->nivcsw);
        seq_putc(m, '\n');
}

static void task_cpus_allowed(struct seq_file *m, struct task_struct *task)
{
        seq_printf(m, "Cpus_allowed:\t%*pb\n",
                   cpumask_pr_args(&task->cpus_allowed));
        seq_printf(m, "Cpus_allowed_list:\t%*pbl\n",
                   cpumask_pr_args(&task->cpus_allowed));
}

static inline void task_core_dumping(struct seq_file *m, struct mm_struct *mm)
{
        seq_printf(m, "CoreDumping:\t%d\n", !!mm->core_state);
}

int proc_pid_status(struct seq_file *m, struct pid_namespace *ns,
                        struct pid *pid, struct task_struct *task)
{
        struct mm_struct *mm = get_task_mm(task);

        task_name(m, task);
        task_state(m, ns, pid, task);

        if (mm) {
                task_mem(m, mm);
                task_core_dumping(m, mm);
                mmput(mm);
        }
        task_sig(m, task);
        task_cap(m, task);
        task_seccomp(m, task);
        task_cpus_allowed(m, task);
        cpuset_task_status_allowed(m, task);
        task_context_switch_counts(m, task);
        return 0;
}

static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
                        struct pid *pid, struct task_struct *task, int whole)
{
        unsigned long vsize, eip, esp, wchan = 0;
        int priority, nice;
        int tty_pgrp = -1, tty_nr = 0;
        sigset_t sigign, sigcatch;
        char state;
        pid_t ppid = 0, pgid = -1, sid = -1;
        int num_threads = 0;
        int permitted;
        struct mm_struct *mm;
        unsigned long long start_time;
        unsigned long cmin_flt = 0, cmaj_flt = 0;
        unsigned long  min_flt = 0,  maj_flt = 0;
        u64 cutime, cstime, utime, stime;
        u64 cgtime, gtime;
        unsigned long rsslim = 0;
        char tcomm[sizeof(task->comm)];
        unsigned long flags;

        state = *get_task_state(task);
        vsize = eip = esp = 0;
        permitted = ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS | PTRACE_MODE_NOAUDIT);
        mm = get_task_mm(task);
        if (mm) {
                vsize = task_vsize(mm);
                /*
                 * esp and eip are intentionally zeroed out.  There is no
                 * non-racy way to read them without freezing the task.
                 * Programs that need reliable values can use ptrace(2).
                 *
                 * The only exception is if the task is core dumping because
                 * a program is not able to use ptrace(2) in that case. It is
                 * safe because the task has stopped executing permanently.
                 */
                if (permitted && (task->flags & PF_DUMPCORE)) {
                        eip = KSTK_EIP(task);
                        esp = KSTK_ESP(task);
                }
        }

        get_task_comm(tcomm, task);

        sigemptyset(&sigign);
        sigemptyset(&sigcatch);
        cutime = cstime = utime = stime = 0;
        cgtime = gtime = 0;

        if (lock_task_sighand(task, &flags)) {
                struct signal_struct *sig = task->signal;

                if (sig->tty) {
                        struct pid *pgrp = tty_get_pgrp(sig->tty);
                        tty_pgrp = pid_nr_ns(pgrp, ns);
                        put_pid(pgrp);
                        tty_nr = new_encode_dev(tty_devnum(sig->tty));
                }

                num_threads = get_nr_threads(task);
                collect_sigign_sigcatch(task, &sigign, &sigcatch);

                cmin_flt = sig->cmin_flt;
                cmaj_flt = sig->cmaj_flt;
                cutime = sig->cutime;
                cstime = sig->cstime;
                cgtime = sig->cgtime;
                rsslim = READ_ONCE(sig->rlim[RLIMIT_RSS].rlim_cur);

                /* add up live thread stats at the group level */
                if (whole) {
                        struct task_struct *t = task;
                        do {
                                min_flt += t->min_flt;
                                maj_flt += t->maj_flt;
                                gtime += task_gtime(t);
                        } while_each_thread(task, t);

                        min_flt += sig->min_flt;
                        maj_flt += sig->maj_flt;
                        thread_group_cputime_adjusted(task, &utime, &stime);
                        gtime += sig->gtime;
                }

                sid = task_session_nr_ns(task, ns);
                ppid = task_tgid_nr_ns(task->real_parent, ns);
                pgid = task_pgrp_nr_ns(task, ns);

                unlock_task_sighand(task, &flags);
        }

        if (permitted && (!whole || num_threads < 2))
                wchan = get_wchan(task);
        if (!whole) {
                min_flt = task->min_flt;
                maj_flt = task->maj_flt;
                task_cputime_adjusted(task, &utime, &stime);
                gtime = task_gtime(task);
        }

        /* scale priority and nice values from timeslices to -20..20 */
        /* to make it look like a "normal" Unix priority/nice value  */
        priority = task_prio(task);
        nice = task_nice(task);

        /* convert nsec -> ticks */
        start_time = nsec_to_clock_t(task->real_start_time);

        seq_printf(m, "%d (%s) %c", pid_nr_ns(pid, ns), tcomm, state);
        seq_put_decimal_ll(m, " ", ppid);
        seq_put_decimal_ll(m, " ", pgid);
        seq_put_decimal_ll(m, " ", sid);
        seq_put_decimal_ll(m, " ", tty_nr);
        seq_put_decimal_ll(m, " ", tty_pgrp);
        seq_put_decimal_ull(m, " ", task->flags);
        seq_put_decimal_ull(m, " ", min_flt);
        seq_put_decimal_ull(m, " ", cmin_flt);
        seq_put_decimal_ull(m, " ", maj_flt);
        seq_put_decimal_ull(m, " ", cmaj_flt);
        seq_put_decimal_ull(m, " ", nsec_to_clock_t(utime));
        seq_put_decimal_ull(m, " ", nsec_to_clock_t(stime));
        seq_put_decimal_ll(m, " ", nsec_to_clock_t(cutime));
        seq_put_decimal_ll(m, " ", nsec_to_clock_t(cstime));
        seq_put_decimal_ll(m, " ", priority);
        seq_put_decimal_ll(m, " ", nice);
        seq_put_decimal_ll(m, " ", num_threads);
        seq_put_decimal_ull(m, " ", 0);
        seq_put_decimal_ull(m, " ", start_time);
        seq_put_decimal_ull(m, " ", vsize);
        seq_put_decimal_ull(m, " ", mm ? get_mm_rss(mm) : 0);
        seq_put_decimal_ull(m, " ", rsslim);
        seq_put_decimal_ull(m, " ", mm ? (permitted ? mm->start_code : 1) : 0);
        seq_put_decimal_ull(m, " ", mm ? (permitted ? mm->end_code : 1) : 0);
        seq_put_decimal_ull(m, " ", (permitted && mm) ? mm->start_stack : 0);
        seq_put_decimal_ull(m, " ", esp);
        seq_put_decimal_ull(m, " ", eip);
        /* The signal information here is obsolete.
         * It must be decimal for Linux 2.0 compatibility.
         * Use /proc/#/status for real-time signals.
         */
        seq_put_decimal_ull(m, " ", task->pending.signal.sig[0] & 0x7fffffffUL);
        seq_put_decimal_ull(m, " ", task->blocked.sig[0] & 0x7fffffffUL);
        seq_put_decimal_ull(m, " ", sigign.sig[0] & 0x7fffffffUL);
        seq_put_decimal_ull(m, " ", sigcatch.sig[0] & 0x7fffffffUL);

        /*
         * We used to output the absolute kernel address, but that's an
         * information leak - so instead we show a 0/1 flag here, to signal
         * to user-space whether there's a wchan field in /proc/PID/wchan.
         *
         * This works with older implementations of procps as well.
         */
        if (wchan)
                seq_puts(m, " 1");
        else
                seq_puts(m, " 0");

        seq_put_decimal_ull(m, " ", 0);
        seq_put_decimal_ull(m, " ", 0);
        seq_put_decimal_ll(m, " ", task->exit_signal);
        seq_put_decimal_ll(m, " ", task_cpu(task));
        seq_put_decimal_ull(m, " ", task->rt_priority);
        seq_put_decimal_ull(m, " ", task->policy);
        seq_put_decimal_ull(m, " ", delayacct_blkio_ticks(task));
        seq_put_decimal_ull(m, " ", nsec_to_clock_t(gtime));
        seq_put_decimal_ll(m, " ", nsec_to_clock_t(cgtime));

        if (mm && permitted) {
                seq_put_decimal_ull(m, " ", mm->start_data);
                seq_put_decimal_ull(m, " ", mm->end_data);
                seq_put_decimal_ull(m, " ", mm->start_brk);
                seq_put_decimal_ull(m, " ", mm->arg_start);
                seq_put_decimal_ull(m, " ", mm->arg_end);
                seq_put_decimal_ull(m, " ", mm->env_start);
                seq_put_decimal_ull(m, " ", mm->env_end);
        } else
                seq_puts(m, " 0 0 0 0 0 0 0");

        if (permitted)
                seq_put_decimal_ll(m, " ", task->exit_code);
        else
                seq_puts(m, " 0");

        seq_putc(m, '\n');
        if (mm)
                mmput(mm);
        return 0;
}

int proc_tid_stat(struct seq_file *m, struct pid_namespace *ns,
                        struct pid *pid, struct task_struct *task)
{
        return do_task_stat(m, ns, pid, task, 0);
}

int proc_tgid_stat(struct seq_file *m, struct pid_namespace *ns,
                        struct pid *pid, struct task_struct *task)
{
        return do_task_stat(m, ns, pid, task, 1);
}

int proc_pid_statm(struct seq_file *m, struct pid_namespace *ns,
                        struct pid *pid, struct task_struct *task)
{
        unsigned long size = 0, resident = 0, shared = 0, text = 0, data = 0;
        struct mm_struct *mm = get_task_mm(task);

        if (mm) {
                size = task_statm(mm, &shared, &text, &data, &resident);
                mmput(mm);
        }
        /*
         * For quick read, open code by putting numbers directly
         * expected format is
         * seq_printf(m, "%lu %lu %lu %lu 0 %lu 0\n",
         *               size, resident, shared, text, data);
         */
        seq_put_decimal_ull(m, "", size);
        seq_put_decimal_ull(m, " ", resident);
        seq_put_decimal_ull(m, " ", shared);
        seq_put_decimal_ull(m, " ", text);
        seq_put_decimal_ull(m, " ", 0);
        seq_put_decimal_ull(m, " ", data);
        seq_put_decimal_ull(m, " ", 0);
        seq_putc(m, '\n');

        return 0;
}

#ifdef CONFIG_PROC_CHILDREN
static struct pid *
get_children_pid(struct inode *inode, struct pid *pid_prev, loff_t pos)
{
        struct task_struct *start, *task;
        struct pid *pid = NULL;

        read_lock(&tasklist_lock);

        start = pid_task(proc_pid(inode), PIDTYPE_PID);
        if (!start)
                goto out;

        /*
         * Lets try to continue searching first, this gives
         * us significant speedup on children-rich processes.
         */
        if (pid_prev) {
                task = pid_task(pid_prev, PIDTYPE_PID);
                if (task && task->real_parent == start &&
                    !(list_empty(&task->sibling))) {
                        if (list_is_last(&task->sibling, &start->children))
                                goto out;
                        task = list_first_entry(&task->sibling,
                                                struct task_struct, sibling);
                        pid = get_pid(task_pid(task));
                        goto out;
                }
        }

        /*
         * Slow search case.
         *
         * We might miss some children here if children
         * are exited while we were not holding the lock,
         * but it was never promised to be accurate that
         * much.
         *
         * "Just suppose that the parent sleeps, but N children
         *  exit after we printed their tids. Now the slow paths
         *  skips N extra children, we miss N tasks." (c)
         *
         * So one need to stop or freeze the leader and all
         * its children to get a precise result.
         */
        list_for_each_entry(task, &start->children, sibling) {
                if (pos-- == 0) {
                        pid = get_pid(task_pid(task));
                        break;
                }
        }

out:
        read_unlock(&tasklist_lock);
        return pid;
}

static int children_seq_show(struct seq_file *seq, void *v)
{
        struct inode *inode = seq->private;
        pid_t pid;

        pid = pid_nr_ns(v, inode->i_sb->s_fs_info);
        seq_printf(seq, "%d ", pid);

        return 0;
}

static void *children_seq_start(struct seq_file *seq, loff_t *pos)
{
        return get_children_pid(seq->private, NULL, *pos);
}

static void *children_seq_next(struct seq_file *seq, void *v, loff_t *pos)
{
        struct pid *pid;

        pid = get_children_pid(seq->private, v, *pos + 1);
        put_pid(v);

        ++*pos;
        return pid;
}

static void children_seq_stop(struct seq_file *seq, void *v)
{
        put_pid(v);
}

static const struct seq_operations children_seq_ops = {
        .start  = children_seq_start,
        .next   = children_seq_next,
        .stop   = children_seq_stop,
        .show   = children_seq_show,
};

static int children_seq_open(struct inode *inode, struct file *file)
{
        struct seq_file *m;
        int ret;

        ret = seq_open(file, &children_seq_ops);
        if (ret)
                return ret;

        m = file->private_data;
        m->private = inode;

        return ret;
}

int children_seq_release(struct inode *inode, struct file *file)
{
        seq_release(inode, file);
        return 0;
}

const struct file_operations proc_tid_children_operations = {
        .open    = children_seq_open,
        .read    = seq_read,
        .llseek  = seq_lseek,
        .release = children_seq_release,
};
#endif /* CONFIG_PROC_CHILDREN */