select.c

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/*
 * This file contains the procedures for the handling of select and poll
 *
 * Created for Linux based loosely upon Mathius Lattner's minix
 * patches by Peter MacDonald. Heavily edited by Linus.
 *
 *  4 February 1994
 *     COFF/ELF binary emulation. If the process has the STICKY_TIMEOUTS
 *     flag set in its personality we do *not* modify the given timeout
 *     parameter to reflect time remaining.
 *
 *  24 January 2000
 *     Changed sys_poll()/do_poll() to use PAGE_SIZE chunk-based allocation 
 *     of fds to overcome nfds < 16390 descriptors limit (Tigran Aivazian).
 */

#include <linux/kernel.h>
#include <linux/sched.h>
#include <linux/syscalls.h>
#include <linux/export.h>
#include <linux/slab.h>
#include <linux/poll.h>
#include <linux/personality.h> /* for STICKY_TIMEOUTS */
#include <linux/file.h>
#include <linux/fdtable.h>
#include <linux/fs.h>
#include <linux/rcupdate.h>
#include <linux/hrtimer.h>

#include <asm/uaccess.h>


/*
 * Estimate expected accuracy in ns from a timeval.
 *
 * After quite a bit of churning around, we've settled on
 * a simple thing of taking 0.1% of the timeout as the
 * slack, with a cap of 100 msec.
 * "nice" tasks get a 0.5% slack instead.
 *
 * Consider this comment an open invitation to come up with even
 * better solutions..
 */

#define MAX_SLACK   (100 * NSEC_PER_MSEC)

static long __estimate_accuracy(struct timespec *tv)
{
    long slack;
    int divfactor = 1000;

    if (tv->tv_sec < 0)
        return 0;

    if (task_nice(current) > 0)
        divfactor = divfactor / 5;

    if (tv->tv_sec > MAX_SLACK / (NSEC_PER_SEC/divfactor))
        return MAX_SLACK;

    slack = tv->tv_nsec / divfactor;
    slack += tv->tv_sec * (NSEC_PER_SEC/divfactor);

    if (slack > MAX_SLACK)
        return MAX_SLACK;

    return slack;
}

long select_estimate_accuracy(struct timespec *tv)
{
    unsigned long ret;
    struct timespec now;

    /*
     * Realtime tasks get a slack of 0 for obvious reasons.
     */

    if (rt_task(current))
        return 0;

    ktime_get_ts(&now);
    now = timespec_sub(*tv, now);
    ret = __estimate_accuracy(&now);
    if (ret < current->timer_slack_ns)
        return current->timer_slack_ns;
    return ret;
}



struct poll_table_page {
    struct poll_table_page * next;
    struct poll_table_entry * entry;
    struct poll_table_entry entries[0];
};

#define POLL_TABLE_FULL(table) \
    ((unsigned long)((table)->entry+1) > PAGE_SIZE + (unsigned long)(table))

/*
 * Ok, Peter made a complicated, but straightforward multiple_wait() function.
 * I have rewritten this, taking some shortcuts: This code may not be easy to
 * follow, but it should be free of race-conditions, and it's practical. If you
 * understand what I'm doing here, then you understand how the linux
 * sleep/wakeup mechanism works.
 *
 * Two very simple procedures, poll_wait() and poll_freewait() make all the
 * work.  poll_wait() is an inline-function defined in <linux/poll.h>,
 * as all select/poll functions have to call it to add an entry to the
 * poll table.
 */
static void __pollwait(struct file *filp, wait_queue_head_t *wait_address,
               poll_table *p);

void poll_initwait(struct poll_wqueues *pwq)
{
    init_poll_funcptr(&pwq->pt, __pollwait);
    pwq->polling_task = current;
    pwq->triggered = 0;
    pwq->error = 0;
    pwq->table = NULL;
    pwq->inline_index = 0;
}
EXPORT_SYMBOL(poll_initwait);

static void free_poll_entry(struct poll_table_entry *entry)
{
    remove_wait_queue(entry->wait_address, &entry->wait);
    fput(entry->filp);
}

void poll_freewait(struct poll_wqueues *pwq)
{
    struct poll_table_page * p = pwq->table;
    int i;
    for (i = 0; i < pwq->inline_index; i++)
        free_poll_entry(pwq->inline_entries + i);
    while (p) {
        struct poll_table_entry * entry;
        struct poll_table_page *old;

        entry = p->entry;
        do {
            entry--;
            free_poll_entry(entry);
        } while (entry > p->entries);
        old = p;
        p = p->next;
        free_page((unsigned long) old);
    }
}
EXPORT_SYMBOL(poll_freewait);

static struct poll_table_entry *poll_get_entry(struct poll_wqueues *p)
{
    struct poll_table_page *table = p->table;

    if (p->inline_index < N_INLINE_POLL_ENTRIES)
        return p->inline_entries + p->inline_index++;

    if (!table || POLL_TABLE_FULL(table)) {
        struct poll_table_page *new_table;

        new_table = (struct poll_table_page *) __get_free_page(GFP_KERNEL);
        if (!new_table) {
            p->error = -ENOMEM;
            return NULL;
        }
        new_table->entry = new_table->entries;
        new_table->next = table;
        p->table = new_table;
        table = new_table;
    }

    return table->entry++;
}

static int __pollwake(wait_queue_t *wait, unsigned mode, int sync, void *key)
{
    struct poll_wqueues *pwq = wait->private;
    DECLARE_WAITQUEUE(dummy_wait, pwq->polling_task);

    /*
     * Although this function is called under waitqueue lock, LOCK
     * doesn't imply write barrier and the users expect write
     * barrier semantics on wakeup functions.  The following
     * smp_wmb() is equivalent to smp_wmb() in try_to_wake_up()
     * and is paired with set_mb() in poll_schedule_timeout.
     */
    smp_wmb();
    pwq->triggered = 1;

    /*
     * Perform the default wake up operation using a dummy
     * waitqueue.
     *
     * TODO: This is hacky but there currently is no interface to
     * pass in @sync.  @sync is scheduled to be removed and once
     * that happens, wake_up_process() can be used directly.
     */
    return default_wake_function(&dummy_wait, mode, sync, key);
}

static int pollwake(wait_queue_t *wait, unsigned mode, int sync, void *key)
{
    struct poll_table_entry *entry;

    entry = container_of(wait, struct poll_table_entry, wait);
    if (key && !((unsigned long)key & entry->key))
        return 0;
    return __pollwake(wait, mode, sync, key);
}

/* Add a new entry */
static void __pollwait(struct file *filp, wait_queue_head_t *wait_address,
                poll_table *p)
{
    struct poll_wqueues *pwq = container_of(p, struct poll_wqueues, pt);
    struct poll_table_entry *entry = poll_get_entry(pwq);
    if (!entry)
        return;
    entry->filp = get_file(filp);
    entry->wait_address = wait_address;
    entry->key = p->_key;
    init_waitqueue_func_entry(&entry->wait, pollwake);
    entry->wait.private = pwq;
    add_wait_queue(wait_address, &entry->wait);
}

int poll_schedule_timeout(struct poll_wqueues *pwq, int state,
              ktime_t *expires, unsigned long slack)
{
    int rc = -EINTR;

    set_current_state(state);
    if (!pwq->triggered)
        rc = schedule_hrtimeout_range(expires, slack, HRTIMER_MODE_ABS);
    __set_current_state(TASK_RUNNING);

    /*
     * Prepare for the next iteration.
     *
     * The following set_mb() serves two purposes.  First, it's
     * the counterpart rmb of the wmb in pollwake() such that data
     * written before wake up is always visible after wake up.
     * Second, the full barrier guarantees that triggered clearing
     * doesn't pass event check of the next iteration.  Note that
     * this problem doesn't exist for the first iteration as
     * add_wait_queue() has full barrier semantics.
     */
    set_mb(pwq->triggered, 0);

    return rc;
}
EXPORT_SYMBOL(poll_schedule_timeout);

int poll_select_set_timeout(struct timespec *to, long sec, long nsec)
{
    struct timespec ts = {.tv_sec = sec, .tv_nsec = nsec};

    if (!timespec_valid(&ts))
        return -EINVAL;

    /* Optimize for the zero timeout value here */
    if (!sec && !nsec) {
        to->tv_sec = to->tv_nsec = 0;
    } else {
        ktime_get_ts(to);
        *to = timespec_add_safe(*to, ts);
    }
    return 0;
}

static int poll_select_copy_remaining(struct timespec *end_time, void __user *p,
                      int timeval, int ret)
{
    struct timespec rts;
    struct timeval rtv;

    if (!p)
        return ret;

    if (current->personality & STICKY_TIMEOUTS)
        goto sticky;

    /* No update for zero timeout */
    if (!end_time->tv_sec && !end_time->tv_nsec)
        return ret;

    ktime_get_ts(&rts);
    rts = timespec_sub(*end_time, rts);
    if (rts.tv_sec < 0)
        rts.tv_sec = rts.tv_nsec = 0;

    if (timeval) {
        if (sizeof(rtv) > sizeof(rtv.tv_sec) + sizeof(rtv.tv_usec))
            memset(&rtv, 0, sizeof(rtv));
        rtv.tv_sec = rts.tv_sec;
        rtv.tv_usec = rts.tv_nsec / NSEC_PER_USEC;

        if (!copy_to_user(p, &rtv, sizeof(rtv)))
            return ret;

    } else if (!copy_to_user(p, &rts, sizeof(rts)))
        return ret;

    /*
     * If an application puts its timeval in read-only memory, we
     * don't want the Linux-specific update to the timeval to
     * cause a fault after the select has completed
     * successfully. However, because we're not updating the
     * timeval, we can't restart the system call.
     */

sticky:
    if (ret == -ERESTARTNOHAND)
        ret = -EINTR;
    return ret;
}

#define FDS_IN(fds, n)      (fds->in + n)
#define FDS_OUT(fds, n)     (fds->out + n)
#define FDS_EX(fds, n)      (fds->ex + n)

#define BITS(fds, n)    (*FDS_IN(fds, n)|*FDS_OUT(fds, n)|*FDS_EX(fds, n))

static int max_select_fd(unsigned long n, fd_set_bits *fds)
{
    unsigned long *open_fds;
    unsigned long set;
    int max;
    struct fdtable *fdt;

    /* handle last in-complete long-word first */
    set = ~(~0UL << (n & (BITS_PER_LONG-1)));
    n /= BITS_PER_LONG;
    fdt = files_fdtable(current->files);
    open_fds = fdt->open_fds + n;
    max = 0;
    if (set) {
        set &= BITS(fds, n);
        if (set) {
            if (!(set & ~*open_fds))
                goto get_max;
            return -EBADF;
        }
    }
    while (n) {
        open_fds--;
        n--;
        set = BITS(fds, n);
        if (!set)
            continue;
        if (set & ~*open_fds)
            return -EBADF;
        if (max)
            continue;
get_max:
        do {
            max++;
            set >>= 1;
        } while (set);
        max += n * BITS_PER_LONG;
    }

    return max;
}

#define POLLIN_SET (POLLRDNORM | POLLRDBAND | POLLIN | POLLHUP | POLLERR)
#define POLLOUT_SET (POLLWRBAND | POLLWRNORM | POLLOUT | POLLERR)
#define POLLEX_SET (POLLPRI)

static inline void wait_key_set(poll_table *wait, unsigned long in,
                unsigned long out, unsigned long bit)
{
    wait->_key = POLLEX_SET;
    if (in & bit)
        wait->_key |= POLLIN_SET;
    if (out & bit)
        wait->_key |= POLLOUT_SET;
}

int do_select(int n, fd_set_bits *fds, struct timespec *end_time)
{
    ktime_t expire, *to = NULL;
    struct poll_wqueues table;
    poll_table *wait;
    int retval, i, timed_out = 0;
    unsigned long slack = 0;

    rcu_read_lock();
    retval = max_select_fd(n, fds);
    rcu_read_unlock();

    if (retval < 0)
        return retval;
    n = retval;

    poll_initwait(&table);
    wait = &table.pt;
    if (end_time && !end_time->tv_sec && !end_time->tv_nsec) {
        wait->_qproc = NULL;
        timed_out = 1;
    }

    if (end_time && !timed_out)
        slack = select_estimate_accuracy(end_time);

    retval = 0;
    for (;;) {
        unsigned long *rinp, *routp, *rexp, *inp, *outp, *exp;

        inp = fds->in; outp = fds->out; exp = fds->ex;
        rinp = fds->res_in; routp = fds->res_out; rexp = fds->res_ex;

        for (i = 0; i < n; ++rinp, ++routp, ++rexp) {
            unsigned long in, out, ex, all_bits, bit = 1, mask, j;
            unsigned long res_in = 0, res_out = 0, res_ex = 0;

            in = *inp++; out = *outp++; ex = *exp++;
            all_bits = in | out | ex;
            if (all_bits == 0) {
                i += BITS_PER_LONG;
                continue;
            }

            for (j = 0; j < BITS_PER_LONG; ++j, ++i, bit <<= 1) {
                struct fd f;
                if (i >= n)
                    break;
                if (!(bit & all_bits))
                    continue;
                f = fdget(i);
                if (f.file) {
                    const struct file_operations *f_op;
                    f_op = f.file->f_op;
                    mask = DEFAULT_POLLMASK;
                    if (f_op && f_op->poll) {
                        wait_key_set(wait, in, out, bit);
                        mask = (*f_op->poll)(f.file, wait);
                    }
                    fdput(f);
                    if ((mask & POLLIN_SET) && (in & bit)) {
                        res_in |= bit;
                        retval++;
                        wait->_qproc = NULL;
                    }
                    if ((mask & POLLOUT_SET) && (out & bit)) {
                        res_out |= bit;
                        retval++;
                        wait->_qproc = NULL;
                    }
                    if ((mask & POLLEX_SET) && (ex & bit)) {
                        res_ex |= bit;
                        retval++;
                        wait->_qproc = NULL;
                    }
                }
            }
            if (res_in)
                *rinp = res_in;
            if (res_out)
                *routp = res_out;
            if (res_ex)
                *rexp = res_ex;
            cond_resched();
        }
        wait->_qproc = NULL;
        if (retval || timed_out || signal_pending(current))
            break;
        if (table.error) {
            retval = table.error;
            break;
        }

        /*
         * If this is the first loop and we have a timeout
         * given, then we convert to ktime_t and set the to
         * pointer to the expiry value.
         */
        if (end_time && !to) {
            expire = timespec_to_ktime(*end_time);
            to = &expire;
        }

        if (!poll_schedule_timeout(&table, TASK_INTERRUPTIBLE,
                       to, slack))
            timed_out = 1;
    }

    poll_freewait(&table);

    return retval;
}

/*
 * We can actually return ERESTARTSYS instead of EINTR, but I'd
 * like to be certain this leads to no problems. So I return
 * EINTR just for safety.
 *
 * Update: ERESTARTSYS breaks at least the xview clock binary, so
 * I'm trying ERESTARTNOHAND which restart only when you want to.
 */
int core_sys_select(int n, fd_set __user *inp, fd_set __user *outp,
               fd_set __user *exp, struct timespec *end_time)
{
    fd_set_bits fds;
    void *bits;
    int ret, max_fds;
    unsigned int size;
    struct fdtable *fdt;
    /* Allocate small arguments on the stack to save memory and be faster */
    long stack_fds[SELECT_STACK_ALLOC/sizeof(long)];

    ret = -EINVAL;
    if (n < 0)
        goto out_nofds;

    /* max_fds can increase, so grab it once to avoid race */
    rcu_read_lock();
    fdt = files_fdtable(current->files);
    max_fds = fdt->max_fds;
    rcu_read_unlock();
    if (n > max_fds)
        n = max_fds;

    /*
     * We need 6 bitmaps (in/out/ex for both incoming and outgoing),
     * since we used fdset we need to allocate memory in units of
     * long-words. 
     */
    size = FDS_BYTES(n);
    bits = stack_fds;
    if (size > sizeof(stack_fds) / 6) {
        /* Not enough space in on-stack array; must use kmalloc */
        ret = -ENOMEM;
        bits = kmalloc(6 * size, GFP_KERNEL);
        if (!bits)
            goto out_nofds;
    }
    fds.in      = bits;
    fds.out     = bits +   size;
    fds.ex      = bits + 2*size;
    fds.res_in  = bits + 3*size;
    fds.res_out = bits + 4*size;
    fds.res_ex  = bits + 5*size;

    if ((ret = get_fd_set(n, inp, fds.in)) ||
        (ret = get_fd_set(n, outp, fds.out)) ||
        (ret = get_fd_set(n, exp, fds.ex)))
        goto out;
    zero_fd_set(n, fds.res_in);
    zero_fd_set(n, fds.res_out);
    zero_fd_set(n, fds.res_ex);

    ret = do_select(n, &fds, end_time);

    if (ret < 0)
        goto out;
    if (!ret) {
        ret = -ERESTARTNOHAND;
        if (signal_pending(current))
            goto out;
        ret = 0;
    }

    if (set_fd_set(n, inp, fds.res_in) ||
        set_fd_set(n, outp, fds.res_out) ||
        set_fd_set(n, exp, fds.res_ex))
        ret = -EFAULT;

out:
    if (bits != stack_fds)
        kfree(bits);
out_nofds:
    return ret;
}

SYSCALL_DEFINE5(select, int, n, fd_set __user *, inp, fd_set __user *, outp,
        fd_set __user *, exp, struct timeval __user *, tvp)
{
    struct timespec end_time, *to = NULL;
    struct timeval tv;
    int ret;

    if (tvp) {
        if (copy_from_user(&tv, tvp, sizeof(tv)))
            return -EFAULT;

        to = &end_time;
        if (poll_select_set_timeout(to,
                tv.tv_sec + (tv.tv_usec / USEC_PER_SEC),
                (tv.tv_usec % USEC_PER_SEC) * NSEC_PER_USEC))
            return -EINVAL;
    }

    ret = core_sys_select(n, inp, outp, exp, to);
    ret = poll_select_copy_remaining(&end_time, tvp, 1, ret);

    return ret;
}

static long do_pselect(int n, fd_set __user *inp, fd_set __user *outp,
               fd_set __user *exp, struct timespec __user *tsp,
               const sigset_t __user *sigmask, size_t sigsetsize)
{
    sigset_t ksigmask, sigsaved;
    struct timespec ts, end_time, *to = NULL;
    int ret;

    if (tsp) {
        if (copy_from_user(&ts, tsp, sizeof(ts)))
            return -EFAULT;

        to = &end_time;
        if (poll_select_set_timeout(to, ts.tv_sec, ts.tv_nsec))
            return -EINVAL;
    }

    if (sigmask) {
        /* XXX: Don't preclude handling different sized sigset_t's.  */
        if (sigsetsize != sizeof(sigset_t))
            return -EINVAL;
        if (copy_from_user(&ksigmask, sigmask, sizeof(ksigmask)))
            return -EFAULT;

        sigdelsetmask(&ksigmask, sigmask(SIGKILL)|sigmask(SIGSTOP));
        sigprocmask(SIG_SETMASK, &ksigmask, &sigsaved);
    }

    ret = core_sys_select(n, inp, outp, exp, to);
    ret = poll_select_copy_remaining(&end_time, tsp, 0, ret);

    if (ret == -ERESTARTNOHAND) {
        /*
         * Don't restore the signal mask yet. Let do_signal() deliver
         * the signal on the way back to userspace, before the signal
         * mask is restored.
         */
        if (sigmask) {
            memcpy(&current->saved_sigmask, &sigsaved,
                    sizeof(sigsaved));
            set_restore_sigmask();
        }
    } else if (sigmask)
        sigprocmask(SIG_SETMASK, &sigsaved, NULL);

    return ret;
}

/*
 * Most architectures can't handle 7-argument syscalls. So we provide a
 * 6-argument version where the sixth argument is a pointer to a structure
 * which has a pointer to the sigset_t itself followed by a size_t containing
 * the sigset size.
 */
SYSCALL_DEFINE6(pselect6, int, n, fd_set __user *, inp, fd_set __user *, outp,
        fd_set __user *, exp, struct timespec __user *, tsp,
        void __user *, sig)
{
    size_t sigsetsize = 0;
    sigset_t __user *up = NULL;

    if (sig) {
        if (!access_ok(VERIFY_READ, sig, sizeof(void *)+sizeof(size_t))
            || __get_user(up, (sigset_t __user * __user *)sig)
            || __get_user(sigsetsize,
                (size_t __user *)(sig+sizeof(void *))))
            return -EFAULT;
    }

    return do_pselect(n, inp, outp, exp, tsp, up, sigsetsize);
}

#ifdef __ARCH_WANT_SYS_OLD_SELECT
struct sel_arg_struct {
    unsigned long n;
    fd_set __user *inp, *outp, *exp;
    struct timeval __user *tvp;
};

SYSCALL_DEFINE1(old_select, struct sel_arg_struct __user *, arg)
{
    struct sel_arg_struct a;

    if (copy_from_user(&a, arg, sizeof(a)))
        return -EFAULT;
    return sys_select(a.n, a.inp, a.outp, a.exp, a.tvp);
}
#endif

struct poll_list {
    struct poll_list *next;
    int len;
    struct pollfd entries[0];
};

#define POLLFD_PER_PAGE  ((PAGE_SIZE-sizeof(struct poll_list)) / sizeof(struct pollfd))

/*
 * Fish for pollable events on the pollfd->fd file descriptor. We're only
 * interested in events matching the pollfd->events mask, and the result
 * matching that mask is both recorded in pollfd->revents and returned. The
 * pwait poll_table will be used by the fd-provided poll handler for waiting,
 * if pwait->_qproc is non-NULL.
 */
static inline unsigned int do_pollfd(struct pollfd *pollfd, poll_table *pwait)
{
    unsigned int mask;
    int fd;

    mask = 0;
    fd = pollfd->fd;
    if (fd >= 0) {
        struct fd f = fdget(fd);
        mask = POLLNVAL;
        if (f.file) {
            mask = DEFAULT_POLLMASK;
            if (f.file->f_op && f.file->f_op->poll) {
                pwait->_key = pollfd->events|POLLERR|POLLHUP;
                mask = f.file->f_op->poll(f.file, pwait);
            }
            /* Mask out unneeded events. */
            mask &= pollfd->events | POLLERR | POLLHUP;
            fdput(f);
        }
    }
    pollfd->revents = mask;

    return mask;
}

static int do_poll(unsigned int nfds,  struct poll_list *list,
           struct poll_wqueues *wait, struct timespec *end_time)
{
    poll_table* pt = &wait->pt;
    ktime_t expire, *to = NULL;
    int timed_out = 0, count = 0;
    unsigned long slack = 0;

    /* Optimise the no-wait case */
    if (end_time && !end_time->tv_sec && !end_time->tv_nsec) {
        pt->_qproc = NULL;
        timed_out = 1;
    }

    if (end_time && !timed_out)
        slack = select_estimate_accuracy(end_time);

    for (;;) {
        struct poll_list *walk;

        for (walk = list; walk != NULL; walk = walk->next) {
            struct pollfd * pfd, * pfd_end;

            pfd = walk->entries;
            pfd_end = pfd + walk->len;
            for (; pfd != pfd_end; pfd++) {
                /*
                 * Fish for events. If we found one, record it
                 * and kill poll_table->_qproc, so we don't
                 * needlessly register any other waiters after
                 * this. They'll get immediately deregistered
                 * when we break out and return.
                 */
                if (do_pollfd(pfd, pt)) {
                    count++;
                    pt->_qproc = NULL;
                }
            }
        }
        /*
         * All waiters have already been registered, so don't provide
         * a poll_table->_qproc to them on the next loop iteration.
         */
        pt->_qproc = NULL;
        if (!count) {
            count = wait->error;
            if (signal_pending(current))
                count = -EINTR;
        }
        if (count || timed_out)
            break;

        /*
         * If this is the first loop and we have a timeout
         * given, then we convert to ktime_t and set the to
         * pointer to the expiry value.
         */
        if (end_time && !to) {
            expire = timespec_to_ktime(*end_time);
            to = &expire;
        }

        if (!poll_schedule_timeout(wait, TASK_INTERRUPTIBLE, to, slack))
            timed_out = 1;
    }
    return count;
}

#define N_STACK_PPS ((sizeof(stack_pps) - sizeof(struct poll_list))  / \
            sizeof(struct pollfd))

int do_sys_poll(struct pollfd __user *ufds, unsigned int nfds,
        struct timespec *end_time)
{
    struct poll_wqueues table;
    int err = -EFAULT, fdcount, len, size;
    /* Allocate small arguments on the stack to save memory and be
       faster - use long to make sure the buffer is aligned properly
       on 64 bit archs to avoid unaligned access */
    long stack_pps[POLL_STACK_ALLOC/sizeof(long)];
    struct poll_list *const head = (struct poll_list *)stack_pps;
    struct poll_list *walk = head;
    unsigned long todo = nfds;

    if (nfds > rlimit(RLIMIT_NOFILE))
        return -EINVAL;

    len = min_t(unsigned int, nfds, N_STACK_PPS);
    for (;;) {
        walk->next = NULL;
        walk->len = len;
        if (!len)
            break;

        if (copy_from_user(walk->entries, ufds + nfds-todo,
                    sizeof(struct pollfd) * walk->len))
            goto out_fds;

        todo -= walk->len;
        if (!todo)
            break;

        len = min(todo, POLLFD_PER_PAGE);
        size = sizeof(struct poll_list) + sizeof(struct pollfd) * len;
        walk = walk->next = kmalloc(size, GFP_KERNEL);
        if (!walk) {
            err = -ENOMEM;
            goto out_fds;
        }
    }

    poll_initwait(&table);
    fdcount = do_poll(nfds, head, &table, end_time);
    poll_freewait(&table);

    for (walk = head; walk; walk = walk->next) {
        struct pollfd *fds = walk->entries;
        int j;

        for (j = 0; j < walk->len; j++, ufds++)
            if (__put_user(fds[j].revents, &ufds->revents))
                goto out_fds;
    }

    err = fdcount;
out_fds:
    walk = head->next;
    while (walk) {
        struct poll_list *pos = walk;
        walk = walk->next;
        kfree(pos);
    }

    return err;
}

static long do_restart_poll(struct restart_block *restart_block)
{
    struct pollfd __user *ufds = restart_block->poll.ufds;
    int nfds = restart_block->poll.nfds;
    struct timespec *to = NULL, end_time;
    int ret;

    if (restart_block->poll.has_timeout) {
        end_time.tv_sec = restart_block->poll.tv_sec;
        end_time.tv_nsec = restart_block->poll.tv_nsec;
        to = &end_time;
    }

    ret = do_sys_poll(ufds, nfds, to);

    if (ret == -EINTR) {
        restart_block->fn = do_restart_poll;
        ret = -ERESTART_RESTARTBLOCK;
    }
    return ret;
}

SYSCALL_DEFINE3(poll, struct pollfd __user *, ufds, unsigned int, nfds,
        int, timeout_msecs)
{
    struct timespec end_time, *to = NULL;
    int ret;

    if (timeout_msecs >= 0) {
        to = &end_time;
        poll_select_set_timeout(to, timeout_msecs / MSEC_PER_SEC,
            NSEC_PER_MSEC * (timeout_msecs % MSEC_PER_SEC));
    }

    ret = do_sys_poll(ufds, nfds, to);

    if (ret == -EINTR) {
        struct restart_block *restart_block;

        restart_block = &current_thread_info()->restart_block;
        restart_block->fn = do_restart_poll;
        restart_block->poll.ufds = ufds;
        restart_block->poll.nfds = nfds;

        if (timeout_msecs >= 0) {
            restart_block->poll.tv_sec = end_time.tv_sec;
            restart_block->poll.tv_nsec = end_time.tv_nsec;
            restart_block->poll.has_timeout = 1;
        } else
            restart_block->poll.has_timeout = 0;

        ret = -ERESTART_RESTARTBLOCK;
    }
    return ret;
}

SYSCALL_DEFINE5(ppoll, struct pollfd __user *, ufds, unsigned int, nfds,
        struct timespec __user *, tsp, const sigset_t __user *, sigmask,
        size_t, sigsetsize)
{
    sigset_t ksigmask, sigsaved;
    struct timespec ts, end_time, *to = NULL;
    int ret;

    if (tsp) {
        if (copy_from_user(&ts, tsp, sizeof(ts)))
            return -EFAULT;

        to = &end_time;
        if (poll_select_set_timeout(to, ts.tv_sec, ts.tv_nsec))
            return -EINVAL;
    }

    if (sigmask) {
        /* XXX: Don't preclude handling different sized sigset_t's.  */
        if (sigsetsize != sizeof(sigset_t))
            return -EINVAL;
        if (copy_from_user(&ksigmask, sigmask, sizeof(ksigmask)))
            return -EFAULT;

        sigdelsetmask(&ksigmask, sigmask(SIGKILL)|sigmask(SIGSTOP));
        sigprocmask(SIG_SETMASK, &ksigmask, &sigsaved);
    }

    ret = do_sys_poll(ufds, nfds, to);

    /* We can restart this syscall, usually */
    if (ret == -EINTR) {
        /*
         * Don't restore the signal mask yet. Let do_signal() deliver
         * the signal on the way back to userspace, before the signal
         * mask is restored.
         */
        if (sigmask) {
            memcpy(&current->saved_sigmask, &sigsaved,
                    sizeof(sigsaved));
            set_restore_sigmask();
        }
        ret = -ERESTARTNOHAND;
    } else if (sigmask)
        sigprocmask(SIG_SETMASK, &sigsaved, NULL);

    ret = poll_select_copy_remaining(&end_time, tsp, 0, ret);

    return ret;
}