cpuset: restore sanity to cpuset_cpus_allowed_fallback()

[linux/fpc-iii.git] / fs / file.c

// SPDX-License-Identifier: GPL-2.0
/*
 *  linux/fs/file.c
 *
 *  Copyright (C) 1998-1999, Stephen Tweedie and Bill Hawes
 *
 *  Manage the dynamic fd arrays in the process files_struct.
 */

#include <linux/syscalls.h>
#include <linux/export.h>
#include <linux/fs.h>
#include <linux/mm.h>
#include <linux/sched/signal.h>
#include <linux/slab.h>
#include <linux/file.h>
#include <linux/fdtable.h>
#include <linux/bitops.h>
#include <linux/spinlock.h>
#include <linux/rcupdate.h>

unsigned int sysctl_nr_open __read_mostly = 1024*1024;
unsigned int sysctl_nr_open_min = BITS_PER_LONG;
/* our min() is unusable in constant expressions ;-/ */
#define __const_min(x, y) ((x) < (y) ? (x) : (y))
unsigned int sysctl_nr_open_max =
        __const_min(INT_MAX, ~(size_t)0/sizeof(void *)) & -BITS_PER_LONG;

static void __free_fdtable(struct fdtable *fdt)
{
        kvfree(fdt->fd);
        kvfree(fdt->open_fds);
        kfree(fdt);
}

static void free_fdtable_rcu(struct rcu_head *rcu)
{
        __free_fdtable(container_of(rcu, struct fdtable, rcu));
}

#define BITBIT_NR(nr)   BITS_TO_LONGS(BITS_TO_LONGS(nr))
#define BITBIT_SIZE(nr) (BITBIT_NR(nr) * sizeof(long))

/*
 * Copy 'count' fd bits from the old table to the new table and clear the extra
 * space if any.  This does not copy the file pointers.  Called with the files
 * spinlock held for write.
 */
static void copy_fd_bitmaps(struct fdtable *nfdt, struct fdtable *ofdt,
                            unsigned int count)
{
        unsigned int cpy, set;

        cpy = count / BITS_PER_BYTE;
        set = (nfdt->max_fds - count) / BITS_PER_BYTE;
        memcpy(nfdt->open_fds, ofdt->open_fds, cpy);
        memset((char *)nfdt->open_fds + cpy, 0, set);
        memcpy(nfdt->close_on_exec, ofdt->close_on_exec, cpy);
        memset((char *)nfdt->close_on_exec + cpy, 0, set);

        cpy = BITBIT_SIZE(count);
        set = BITBIT_SIZE(nfdt->max_fds) - cpy;
        memcpy(nfdt->full_fds_bits, ofdt->full_fds_bits, cpy);
        memset((char *)nfdt->full_fds_bits + cpy, 0, set);
}

/*
 * Copy all file descriptors from the old table to the new, expanded table and
 * clear the extra space.  Called with the files spinlock held for write.
 */
static void copy_fdtable(struct fdtable *nfdt, struct fdtable *ofdt)
{
        unsigned int cpy, set;

        BUG_ON(nfdt->max_fds < ofdt->max_fds);

        cpy = ofdt->max_fds * sizeof(struct file *);
        set = (nfdt->max_fds - ofdt->max_fds) * sizeof(struct file *);
        memcpy(nfdt->fd, ofdt->fd, cpy);
        memset((char *)nfdt->fd + cpy, 0, set);

        copy_fd_bitmaps(nfdt, ofdt, ofdt->max_fds);
}

static struct fdtable * alloc_fdtable(unsigned int nr)
{
        struct fdtable *fdt;
        void *data;

        /*
         * Figure out how many fds we actually want to support in this fdtable.
         * Allocation steps are keyed to the size of the fdarray, since it
         * grows far faster than any of the other dynamic data. We try to fit
         * the fdarray into comfortable page-tuned chunks: starting at 1024B
         * and growing in powers of two from there on.
         */
        nr /= (1024 / sizeof(struct file *));
        nr = roundup_pow_of_two(nr + 1);
        nr *= (1024 / sizeof(struct file *));
        /*
         * Note that this can drive nr *below* what we had passed if sysctl_nr_open
         * had been set lower between the check in expand_files() and here.  Deal
         * with that in caller, it's cheaper that way.
         *
         * We make sure that nr remains a multiple of BITS_PER_LONG - otherwise
         * bitmaps handling below becomes unpleasant, to put it mildly...
         */
        if (unlikely(nr > sysctl_nr_open))
                nr = ((sysctl_nr_open - 1) | (BITS_PER_LONG - 1)) + 1;

        fdt = kmalloc(sizeof(struct fdtable), GFP_KERNEL_ACCOUNT);
        if (!fdt)
                goto out;
        fdt->max_fds = nr;
        data = kvmalloc_array(nr, sizeof(struct file *), GFP_KERNEL_ACCOUNT);
        if (!data)
                goto out_fdt;
        fdt->fd = data;

        data = kvmalloc(max_t(size_t,
                                 2 * nr / BITS_PER_BYTE + BITBIT_SIZE(nr), L1_CACHE_BYTES),
                                 GFP_KERNEL_ACCOUNT);
        if (!data)
                goto out_arr;
        fdt->open_fds = data;
        data += nr / BITS_PER_BYTE;
        fdt->close_on_exec = data;
        data += nr / BITS_PER_BYTE;
        fdt->full_fds_bits = data;

        return fdt;

out_arr:
        kvfree(fdt->fd);
out_fdt:
        kfree(fdt);
out:
        return NULL;
}

/*
 * Expand the file descriptor table.
 * This function will allocate a new fdtable and both fd array and fdset, of
 * the given size.
 * Return <0 error code on error; 1 on successful completion.
 * The files->file_lock should be held on entry, and will be held on exit.
 */
static int expand_fdtable(struct files_struct *files, unsigned int nr)
        __releases(files->file_lock)
        __acquires(files->file_lock)
{
        struct fdtable *new_fdt, *cur_fdt;

        spin_unlock(&files->file_lock);
        new_fdt = alloc_fdtable(nr);

        /* make sure all __fd_install() have seen resize_in_progress
         * or have finished their rcu_read_lock_sched() section.
         */
        if (atomic_read(&files->count) > 1)
                synchronize_sched();

        spin_lock(&files->file_lock);
        if (!new_fdt)
                return -ENOMEM;
        /*
         * extremely unlikely race - sysctl_nr_open decreased between the check in
         * caller and alloc_fdtable().  Cheaper to catch it here...
         */
        if (unlikely(new_fdt->max_fds <= nr)) {
                __free_fdtable(new_fdt);
                return -EMFILE;
        }
        cur_fdt = files_fdtable(files);
        BUG_ON(nr < cur_fdt->max_fds);
        copy_fdtable(new_fdt, cur_fdt);
        rcu_assign_pointer(files->fdt, new_fdt);
        if (cur_fdt != &files->fdtab)
                call_rcu(&cur_fdt->rcu, free_fdtable_rcu);
        /* coupled with smp_rmb() in __fd_install() */
        smp_wmb();
        return 1;
}

/*
 * Expand files.
 * This function will expand the file structures, if the requested size exceeds
 * the current capacity and there is room for expansion.
 * Return <0 error code on error; 0 when nothing done; 1 when files were
 * expanded and execution may have blocked.
 * The files->file_lock should be held on entry, and will be held on exit.
 */
static int expand_files(struct files_struct *files, unsigned int nr)
        __releases(files->file_lock)
        __acquires(files->file_lock)
{
        struct fdtable *fdt;
        int expanded = 0;

repeat:
        fdt = files_fdtable(files);

        /* Do we need to expand? */
        if (nr < fdt->max_fds)
                return expanded;

        /* Can we expand? */
        if (nr >= sysctl_nr_open)
                return -EMFILE;

        if (unlikely(files->resize_in_progress)) {
                spin_unlock(&files->file_lock);
                expanded = 1;
                wait_event(files->resize_wait, !files->resize_in_progress);
                spin_lock(&files->file_lock);
                goto repeat;
        }

        /* All good, so we try */
        files->resize_in_progress = true;
        expanded = expand_fdtable(files, nr);
        files->resize_in_progress = false;

        wake_up_all(&files->resize_wait);
        return expanded;
}

static inline void __set_close_on_exec(unsigned int fd, struct fdtable *fdt)
{
        __set_bit(fd, fdt->close_on_exec);
}

static inline void __clear_close_on_exec(unsigned int fd, struct fdtable *fdt)
{
        if (test_bit(fd, fdt->close_on_exec))
                __clear_bit(fd, fdt->close_on_exec);
}

static inline void __set_open_fd(unsigned int fd, struct fdtable *fdt)
{
        __set_bit(fd, fdt->open_fds);
        fd /= BITS_PER_LONG;
        if (!~fdt->open_fds[fd])
                __set_bit(fd, fdt->full_fds_bits);
}

static inline void __clear_open_fd(unsigned int fd, struct fdtable *fdt)
{
        __clear_bit(fd, fdt->open_fds);
        __clear_bit(fd / BITS_PER_LONG, fdt->full_fds_bits);
}

static unsigned int count_open_files(struct fdtable *fdt)
{
        unsigned int size = fdt->max_fds;
        unsigned int i;

        /* Find the last open fd */
        for (i = size / BITS_PER_LONG; i > 0; ) {
                if (fdt->open_fds[--i])
                        break;
        }
        i = (i + 1) * BITS_PER_LONG;
        return i;
}

/*
 * Allocate a new files structure and copy contents from the
 * passed in files structure.
 * errorp will be valid only when the returned files_struct is NULL.
 */
struct files_struct *dup_fd(struct files_struct *oldf, int *errorp)
{
        struct files_struct *newf;
        struct file **old_fds, **new_fds;
        unsigned int open_files, i;
        struct fdtable *old_fdt, *new_fdt;

        *errorp = -ENOMEM;
        newf = kmem_cache_alloc(files_cachep, GFP_KERNEL);
        if (!newf)
                goto out;

        atomic_set(&newf->count, 1);

        spin_lock_init(&newf->file_lock);
        newf->resize_in_progress = false;
        init_waitqueue_head(&newf->resize_wait);
        newf->next_fd = 0;
        new_fdt = &newf->fdtab;
        new_fdt->max_fds = NR_OPEN_DEFAULT;
        new_fdt->close_on_exec = newf->close_on_exec_init;
        new_fdt->open_fds = newf->open_fds_init;
        new_fdt->full_fds_bits = newf->full_fds_bits_init;
        new_fdt->fd = &newf->fd_array[0];

        spin_lock(&oldf->file_lock);
        old_fdt = files_fdtable(oldf);
        open_files = count_open_files(old_fdt);

        /*
         * Check whether we need to allocate a larger fd array and fd set.
         */
        while (unlikely(open_files > new_fdt->max_fds)) {
                spin_unlock(&oldf->file_lock);

                if (new_fdt != &newf->fdtab)
                        __free_fdtable(new_fdt);

                new_fdt = alloc_fdtable(open_files - 1);
                if (!new_fdt) {
                        *errorp = -ENOMEM;
                        goto out_release;
                }

                /* beyond sysctl_nr_open; nothing to do */
                if (unlikely(new_fdt->max_fds < open_files)) {
                        __free_fdtable(new_fdt);
                        *errorp = -EMFILE;
                        goto out_release;
                }

                /*
                 * Reacquire the oldf lock and a pointer to its fd table
                 * who knows it may have a new bigger fd table. We need
                 * the latest pointer.
                 */
                spin_lock(&oldf->file_lock);
                old_fdt = files_fdtable(oldf);
                open_files = count_open_files(old_fdt);
        }

        copy_fd_bitmaps(new_fdt, old_fdt, open_files);

        old_fds = old_fdt->fd;
        new_fds = new_fdt->fd;

        for (i = open_files; i != 0; i--) {
                struct file *f = *old_fds++;
                if (f) {
                        get_file(f);
                } else {
                        /*
                         * The fd may be claimed in the fd bitmap but not yet
                         * instantiated in the files array if a sibling thread
                         * is partway through open().  So make sure that this
                         * fd is available to the new process.
                         */
                        __clear_open_fd(open_files - i, new_fdt);
                }
                rcu_assign_pointer(*new_fds++, f);
        }
        spin_unlock(&oldf->file_lock);

        /* clear the remainder */
        memset(new_fds, 0, (new_fdt->max_fds - open_files) * sizeof(struct file *));

        rcu_assign_pointer(newf->fdt, new_fdt);

        return newf;

out_release:
        kmem_cache_free(files_cachep, newf);
out:
        return NULL;
}

static struct fdtable *close_files(struct files_struct * files)
{
        /*
         * It is safe to dereference the fd table without RCU or
         * ->file_lock because this is the last reference to the
         * files structure.
         */
        struct fdtable *fdt = rcu_dereference_raw(files->fdt);
        unsigned int i, j = 0;

        for (;;) {
                unsigned long set;
                i = j * BITS_PER_LONG;
                if (i >= fdt->max_fds)
                        break;
                set = fdt->open_fds[j++];
                while (set) {
                        if (set & 1) {
                                struct file * file = xchg(&fdt->fd[i], NULL);
                                if (file) {
                                        filp_close(file, files);
                                        cond_resched();
                                }
                        }
                        i++;
                        set >>= 1;
                }
        }

        return fdt;
}

struct files_struct *get_files_struct(struct task_struct *task)
{
        struct files_struct *files;

        task_lock(task);
        files = task->files;
        if (files)
                atomic_inc(&files->count);
        task_unlock(task);

        return files;
}

void put_files_struct(struct files_struct *files)
{
        if (atomic_dec_and_test(&files->count)) {
                struct fdtable *fdt = close_files(files);

                /* free the arrays if they are not embedded */
                if (fdt != &files->fdtab)
                        __free_fdtable(fdt);
                kmem_cache_free(files_cachep, files);
        }
}

void reset_files_struct(struct files_struct *files)
{
        struct task_struct *tsk = current;
        struct files_struct *old;

        old = tsk->files;
        task_lock(tsk);
        tsk->files = files;
        task_unlock(tsk);
        put_files_struct(old);
}

void exit_files(struct task_struct *tsk)
{
        struct files_struct * files = tsk->files;

        if (files) {
                task_lock(tsk);
                tsk->files = NULL;
                task_unlock(tsk);
                put_files_struct(files);
        }
}

struct files_struct init_files = {
        .count          = ATOMIC_INIT(1),
        .fdt            = &init_files.fdtab,
        .fdtab          = {
                .max_fds        = NR_OPEN_DEFAULT,
                .fd             = &init_files.fd_array[0],
                .close_on_exec  = init_files.close_on_exec_init,
                .open_fds       = init_files.open_fds_init,
                .full_fds_bits  = init_files.full_fds_bits_init,
        },
        .file_lock      = __SPIN_LOCK_UNLOCKED(init_files.file_lock),
        .resize_wait    = __WAIT_QUEUE_HEAD_INITIALIZER(init_files.resize_wait),
};

static unsigned int find_next_fd(struct fdtable *fdt, unsigned int start)
{
        unsigned int maxfd = fdt->max_fds;
        unsigned int maxbit = maxfd / BITS_PER_LONG;
        unsigned int bitbit = start / BITS_PER_LONG;

        bitbit = find_next_zero_bit(fdt->full_fds_bits, maxbit, bitbit) * BITS_PER_LONG;
        if (bitbit > maxfd)
                return maxfd;
        if (bitbit > start)
                start = bitbit;
        return find_next_zero_bit(fdt->open_fds, maxfd, start);
}

/*
 * allocate a file descriptor, mark it busy.
 */
int __alloc_fd(struct files_struct *files,
               unsigned start, unsigned end, unsigned flags)
{
        unsigned int fd;
        int error;
        struct fdtable *fdt;

        spin_lock(&files->file_lock);
repeat:
        fdt = files_fdtable(files);
        fd = start;
        if (fd < files->next_fd)
                fd = files->next_fd;

        if (fd < fdt->max_fds)
                fd = find_next_fd(fdt, fd);

        /*
         * N.B. For clone tasks sharing a files structure, this test
         * will limit the total number of files that can be opened.
         */
        error = -EMFILE;
        if (fd >= end)
                goto out;

        error = expand_files(files, fd);
        if (error < 0)
                goto out;

        /*
         * If we needed to expand the fs array we
         * might have blocked - try again.
         */
        if (error)
                goto repeat;

        if (start <= files->next_fd)
                files->next_fd = fd + 1;

        __set_open_fd(fd, fdt);
        if (flags & O_CLOEXEC)
                __set_close_on_exec(fd, fdt);
        else
                __clear_close_on_exec(fd, fdt);
        error = fd;
#if 1
        /* Sanity check */
        if (rcu_access_pointer(fdt->fd[fd]) != NULL) {
                printk(KERN_WARNING "alloc_fd: slot %d not NULL!\n", fd);
                rcu_assign_pointer(fdt->fd[fd], NULL);
        }
#endif

out:
        spin_unlock(&files->file_lock);
        return error;
}

static int alloc_fd(unsigned start, unsigned flags)
{
        return __alloc_fd(current->files, start, rlimit(RLIMIT_NOFILE), flags);
}

int get_unused_fd_flags(unsigned flags)
{
        return __alloc_fd(current->files, 0, rlimit(RLIMIT_NOFILE), flags);
}
EXPORT_SYMBOL(get_unused_fd_flags);

static void __put_unused_fd(struct files_struct *files, unsigned int fd)
{
        struct fdtable *fdt = files_fdtable(files);
        __clear_open_fd(fd, fdt);
        if (fd < files->next_fd)
                files->next_fd = fd;
}

void put_unused_fd(unsigned int fd)
{
        struct files_struct *files = current->files;
        spin_lock(&files->file_lock);
        __put_unused_fd(files, fd);
        spin_unlock(&files->file_lock);
}

EXPORT_SYMBOL(put_unused_fd);

/*
 * Install a file pointer in the fd array.
 *
 * The VFS is full of places where we drop the files lock between
 * setting the open_fds bitmap and installing the file in the file
 * array.  At any such point, we are vulnerable to a dup2() race
 * installing a file in the array before us.  We need to detect this and
 * fput() the struct file we are about to overwrite in this case.
 *
 * It should never happen - if we allow dup2() do it, _really_ bad things
 * will follow.
 *
 * NOTE: __fd_install() variant is really, really low-level; don't
 * use it unless you are forced to by truly lousy API shoved down
 * your throat.  'files' *MUST* be either current->files or obtained
 * by get_files_struct(current) done by whoever had given it to you,
 * or really bad things will happen.  Normally you want to use
 * fd_install() instead.
 */

void __fd_install(struct files_struct *files, unsigned int fd,
                struct file *file)
{
        struct fdtable *fdt;

        rcu_read_lock_sched();

        if (unlikely(files->resize_in_progress)) {
                rcu_read_unlock_sched();
                spin_lock(&files->file_lock);
                fdt = files_fdtable(files);
                BUG_ON(fdt->fd[fd] != NULL);
                rcu_assign_pointer(fdt->fd[fd], file);
                spin_unlock(&files->file_lock);
                return;
        }
        /* coupled with smp_wmb() in expand_fdtable() */
        smp_rmb();
        fdt = rcu_dereference_sched(files->fdt);
        BUG_ON(fdt->fd[fd] != NULL);
        rcu_assign_pointer(fdt->fd[fd], file);
        rcu_read_unlock_sched();
}

void fd_install(unsigned int fd, struct file *file)
{
        __fd_install(current->files, fd, file);
}

EXPORT_SYMBOL(fd_install);

/*
 * The same warnings as for __alloc_fd()/__fd_install() apply here...
 */
int __close_fd(struct files_struct *files, unsigned fd)
{
        struct file *file;
        struct fdtable *fdt;

        spin_lock(&files->file_lock);
        fdt = files_fdtable(files);
        if (fd >= fdt->max_fds)
                goto out_unlock;
        file = fdt->fd[fd];
        if (!file)
                goto out_unlock;
        rcu_assign_pointer(fdt->fd[fd], NULL);
        __put_unused_fd(files, fd);
        spin_unlock(&files->file_lock);
        return filp_close(file, files);

out_unlock:
        spin_unlock(&files->file_lock);
        return -EBADF;
}
EXPORT_SYMBOL(__close_fd); /* for ksys_close() */

void do_close_on_exec(struct files_struct *files)
{
        unsigned i;
        struct fdtable *fdt;

        /* exec unshares first */
        spin_lock(&files->file_lock);
        for (i = 0; ; i++) {
                unsigned long set;
                unsigned fd = i * BITS_PER_LONG;
                fdt = files_fdtable(files);
                if (fd >= fdt->max_fds)
                        break;
                set = fdt->close_on_exec[i];
                if (!set)
                        continue;
                fdt->close_on_exec[i] = 0;
                for ( ; set ; fd++, set >>= 1) {
                        struct file *file;
                        if (!(set & 1))
                                continue;
                        file = fdt->fd[fd];
                        if (!file)
                                continue;
                        rcu_assign_pointer(fdt->fd[fd], NULL);
                        __put_unused_fd(files, fd);
                        spin_unlock(&files->file_lock);
                        filp_close(file, files);
                        cond_resched();
                        spin_lock(&files->file_lock);
                }

        }
        spin_unlock(&files->file_lock);
}

static struct file *__fget(unsigned int fd, fmode_t mask)
{
        struct files_struct *files = current->files;
        struct file *file;

        rcu_read_lock();
loop:
        file = fcheck_files(files, fd);
        if (file) {
                /* File object ref couldn't be taken.
                 * dup2() atomicity guarantee is the reason
                 * we loop to catch the new file (or NULL pointer)
                 */
                if (file->f_mode & mask)
                        file = NULL;
                else if (!get_file_rcu(file))
                        goto loop;
        }
        rcu_read_unlock();

        return file;
}

struct file *fget(unsigned int fd)
{
        return __fget(fd, FMODE_PATH);
}
EXPORT_SYMBOL(fget);

struct file *fget_raw(unsigned int fd)
{
        return __fget(fd, 0);
}
EXPORT_SYMBOL(fget_raw);

/*
 * Lightweight file lookup - no refcnt increment if fd table isn't shared.
 *
 * You can use this instead of fget if you satisfy all of the following
 * conditions:
 * 1) You must call fput_light before exiting the syscall and returning control
 *    to userspace (i.e. you cannot remember the returned struct file * after
 *    returning to userspace).
 * 2) You must not call filp_close on the returned struct file * in between
 *    calls to fget_light and fput_light.
 * 3) You must not clone the current task in between the calls to fget_light
 *    and fput_light.
 *
 * The fput_needed flag returned by fget_light should be passed to the
 * corresponding fput_light.
 */
static unsigned long __fget_light(unsigned int fd, fmode_t mask)
{
        struct files_struct *files = current->files;
        struct file *file;

        if (atomic_read(&files->count) == 1) {
                file = __fcheck_files(files, fd);
                if (!file || unlikely(file->f_mode & mask))
                        return 0;
                return (unsigned long)file;
        } else {
                file = __fget(fd, mask);
                if (!file)
                        return 0;
                return FDPUT_FPUT | (unsigned long)file;
        }
}
unsigned long __fdget(unsigned int fd)
{
        return __fget_light(fd, FMODE_PATH);
}
EXPORT_SYMBOL(__fdget);

unsigned long __fdget_raw(unsigned int fd)
{
        return __fget_light(fd, 0);
}

unsigned long __fdget_pos(unsigned int fd)
{
        unsigned long v = __fdget(fd);
        struct file *file = (struct file *)(v & ~3);

        if (file && (file->f_mode & FMODE_ATOMIC_POS)) {
                if (file_count(file) > 1) {
                        v |= FDPUT_POS_UNLOCK;
                        mutex_lock(&file->f_pos_lock);
                }
        }
        return v;
}

void __f_unlock_pos(struct file *f)
{
        mutex_unlock(&f->f_pos_lock);
}

/*
 * We only lock f_pos if we have threads or if the file might be
 * shared with another process. In both cases we'll have an elevated
 * file count (done either by fdget() or by fork()).
 */

void set_close_on_exec(unsigned int fd, int flag)
{
        struct files_struct *files = current->files;
        struct fdtable *fdt;
        spin_lock(&files->file_lock);
        fdt = files_fdtable(files);
        if (flag)
                __set_close_on_exec(fd, fdt);
        else
                __clear_close_on_exec(fd, fdt);
        spin_unlock(&files->file_lock);
}

bool get_close_on_exec(unsigned int fd)
{
        struct files_struct *files = current->files;
        struct fdtable *fdt;
        bool res;
        rcu_read_lock();
        fdt = files_fdtable(files);
        res = close_on_exec(fd, fdt);
        rcu_read_unlock();
        return res;
}

static int do_dup2(struct files_struct *files,
        struct file *file, unsigned fd, unsigned flags)
__releases(&files->file_lock)
{
        struct file *tofree;
        struct fdtable *fdt;

        /*
         * We need to detect attempts to do dup2() over allocated but still
         * not finished descriptor.  NB: OpenBSD avoids that at the price of
         * extra work in their equivalent of fget() - they insert struct
         * file immediately after grabbing descriptor, mark it larval if
         * more work (e.g. actual opening) is needed and make sure that
         * fget() treats larval files as absent.  Potentially interesting,
         * but while extra work in fget() is trivial, locking implications
         * and amount of surgery on open()-related paths in VFS are not.
         * FreeBSD fails with -EBADF in the same situation, NetBSD "solution"
         * deadlocks in rather amusing ways, AFAICS.  All of that is out of
         * scope of POSIX or SUS, since neither considers shared descriptor
         * tables and this condition does not arise without those.
         */
        fdt = files_fdtable(files);
        tofree = fdt->fd[fd];
        if (!tofree && fd_is_open(fd, fdt))
                goto Ebusy;
        get_file(file);
        rcu_assign_pointer(fdt->fd[fd], file);
        __set_open_fd(fd, fdt);
        if (flags & O_CLOEXEC)
                __set_close_on_exec(fd, fdt);
        else
                __clear_close_on_exec(fd, fdt);
        spin_unlock(&files->file_lock);

        if (tofree)
                filp_close(tofree, files);

        return fd;

Ebusy:
        spin_unlock(&files->file_lock);
        return -EBUSY;
}

int replace_fd(unsigned fd, struct file *file, unsigned flags)
{
        int err;
        struct files_struct *files = current->files;

        if (!file)
                return __close_fd(files, fd);

        if (fd >= rlimit(RLIMIT_NOFILE))
                return -EBADF;

        spin_lock(&files->file_lock);
        err = expand_files(files, fd);
        if (unlikely(err < 0))
                goto out_unlock;
        return do_dup2(files, file, fd, flags);

out_unlock:
        spin_unlock(&files->file_lock);
        return err;
}

static int ksys_dup3(unsigned int oldfd, unsigned int newfd, int flags)
{
        int err = -EBADF;
        struct file *file;
        struct files_struct *files = current->files;

        if ((flags & ~O_CLOEXEC) != 0)
                return -EINVAL;

        if (unlikely(oldfd == newfd))
                return -EINVAL;

        if (newfd >= rlimit(RLIMIT_NOFILE))
                return -EBADF;

        spin_lock(&files->file_lock);
        err = expand_files(files, newfd);
        file = fcheck(oldfd);
        if (unlikely(!file))
                goto Ebadf;
        if (unlikely(err < 0)) {
                if (err == -EMFILE)
                        goto Ebadf;
                goto out_unlock;
        }
        return do_dup2(files, file, newfd, flags);

Ebadf:
        err = -EBADF;
out_unlock:
        spin_unlock(&files->file_lock);
        return err;
}

SYSCALL_DEFINE3(dup3, unsigned int, oldfd, unsigned int, newfd, int, flags)
{
        return ksys_dup3(oldfd, newfd, flags);
}

SYSCALL_DEFINE2(dup2, unsigned int, oldfd, unsigned int, newfd)
{
        if (unlikely(newfd == oldfd)) { /* corner case */
                struct files_struct *files = current->files;
                int retval = oldfd;

                rcu_read_lock();
                if (!fcheck_files(files, oldfd))
                        retval = -EBADF;
                rcu_read_unlock();
                return retval;
        }
        return ksys_dup3(oldfd, newfd, 0);
}

int ksys_dup(unsigned int fildes)
{
        int ret = -EBADF;
        struct file *file = fget_raw(fildes);

        if (file) {
                ret = get_unused_fd_flags(0);
                if (ret >= 0)
                        fd_install(ret, file);
                else
                        fput(file);
        }
        return ret;
}

SYSCALL_DEFINE1(dup, unsigned int, fildes)
{
        return ksys_dup(fildes);
}

int f_dupfd(unsigned int from, struct file *file, unsigned flags)
{
        int err;
        if (from >= rlimit(RLIMIT_NOFILE))
                return -EINVAL;
        err = alloc_fd(from, flags);
        if (err >= 0) {
                get_file(file);
                fd_install(err, file);
        }
        return err;
}

int iterate_fd(struct files_struct *files, unsigned n,
                int (*f)(const void *, struct file *, unsigned),
                const void *p)
{
        struct fdtable *fdt;
        int res = 0;
        if (!files)
                return 0;
        spin_lock(&files->file_lock);
        for (fdt = files_fdtable(files); n < fdt->max_fds; n++) {
                struct file *file;
                file = rcu_dereference_check_fdtable(files, fdt->fd[n]);
                if (!file)
                        continue;
                res = f(p, file, n);
                if (res)
                        break;
        }
        spin_unlock(&files->file_lock);
        return res;
}
EXPORT_SYMBOL(iterate_fd);