proc: prevent accessing /proc/<PID>/environ until it's ready

[deliverable/linux.git] / drivers / tty / tty_buffer.c

/*
 * Tty buffer allocation management
 */

#include <linux/types.h>
#include <linux/errno.h>
#include <linux/tty.h>
#include <linux/tty_driver.h>
#include <linux/tty_flip.h>
#include <linux/timer.h>
#include <linux/string.h>
#include <linux/slab.h>
#include <linux/sched.h>
#include <linux/wait.h>
#include <linux/bitops.h>
#include <linux/delay.h>
#include <linux/module.h>
#include <linux/ratelimit.h>


#define MIN_TTYB_SIZE   256
#define TTYB_ALIGN_MASK 255

/*
 * Byte threshold to limit memory consumption for flip buffers.
 * The actual memory limit is > 2x this amount.
 */
#define TTYB_DEFAULT_MEM_LIMIT  65536

/*
 * We default to dicing tty buffer allocations to this many characters
 * in order to avoid multiple page allocations. We know the size of
 * tty_buffer itself but it must also be taken into account that the
 * the buffer is 256 byte aligned. See tty_buffer_find for the allocation
 * logic this must match
 */

#define TTY_BUFFER_PAGE (((PAGE_SIZE - sizeof(struct tty_buffer)) / 2) & ~0xFF)

/*
 * If all tty flip buffers have been processed by flush_to_ldisc() or
 * dropped by tty_buffer_flush(), check if the linked pty has been closed.
 * If so, wake the reader/poll to process
 */
static inline void check_other_closed(struct tty_struct *tty)
{
        unsigned long flags, old;

        /* transition from TTY_OTHER_CLOSED => TTY_OTHER_DONE must be atomic */
        for (flags = ACCESS_ONCE(tty->flags);
             test_bit(TTY_OTHER_CLOSED, &flags);
             ) {
                old = flags;
                __set_bit(TTY_OTHER_DONE, &flags);
                flags = cmpxchg(&tty->flags, old, flags);
                if (old == flags) {
                        wake_up_interruptible(&tty->read_wait);
                        break;
                }
        }
}

/**
 *      tty_buffer_lock_exclusive       -       gain exclusive access to buffer
 *      tty_buffer_unlock_exclusive     -       release exclusive access
 *
 *      @port - tty_port owning the flip buffer
 *
 *      Guarantees safe use of the line discipline's receive_buf() method by
 *      excluding the buffer work and any pending flush from using the flip
 *      buffer. Data can continue to be added concurrently to the flip buffer
 *      from the driver side.
 *
 *      On release, the buffer work is restarted if there is data in the
 *      flip buffer
 */

void tty_buffer_lock_exclusive(struct tty_port *port)
{
        struct tty_bufhead *buf = &port->buf;

        atomic_inc(&buf->priority);
        mutex_lock(&buf->lock);
}
EXPORT_SYMBOL_GPL(tty_buffer_lock_exclusive);

void tty_buffer_unlock_exclusive(struct tty_port *port)
{
        struct tty_bufhead *buf = &port->buf;
        int restart;

        restart = buf->head->commit != buf->head->read;

        atomic_dec(&buf->priority);
        mutex_unlock(&buf->lock);
        if (restart)
                queue_work(system_unbound_wq, &buf->work);
}
EXPORT_SYMBOL_GPL(tty_buffer_unlock_exclusive);

/**
 *      tty_buffer_space_avail  -       return unused buffer space
 *      @port - tty_port owning the flip buffer
 *
 *      Returns the # of bytes which can be written by the driver without
 *      reaching the buffer limit.
 *
 *      Note: this does not guarantee that memory is available to write
 *      the returned # of bytes (use tty_prepare_flip_string_xxx() to
 *      pre-allocate if memory guarantee is required).
 */

int tty_buffer_space_avail(struct tty_port *port)
{
        int space = port->buf.mem_limit - atomic_read(&port->buf.mem_used);
        return max(space, 0);
}
EXPORT_SYMBOL_GPL(tty_buffer_space_avail);

static void tty_buffer_reset(struct tty_buffer *p, size_t size)
{
        p->used = 0;
        p->size = size;
        p->next = NULL;
        p->commit = 0;
        p->read = 0;
        p->flags = 0;
}

/**
 *      tty_buffer_free_all             -       free buffers used by a tty
 *      @tty: tty to free from
 *
 *      Remove all the buffers pending on a tty whether queued with data
 *      or in the free ring. Must be called when the tty is no longer in use
 */

void tty_buffer_free_all(struct tty_port *port)
{
        struct tty_bufhead *buf = &port->buf;
        struct tty_buffer *p, *next;
        struct llist_node *llist;

        while ((p = buf->head) != NULL) {
                buf->head = p->next;
                if (p->size > 0)
                        kfree(p);
        }
        llist = llist_del_all(&buf->free);
        llist_for_each_entry_safe(p, next, llist, free)
                kfree(p);

        tty_buffer_reset(&buf->sentinel, 0);
        buf->head = &buf->sentinel;
        buf->tail = &buf->sentinel;

        atomic_set(&buf->mem_used, 0);
}

/**
 *      tty_buffer_alloc        -       allocate a tty buffer
 *      @tty: tty device
 *      @size: desired size (characters)
 *
 *      Allocate a new tty buffer to hold the desired number of characters.
 *      We round our buffers off in 256 character chunks to get better
 *      allocation behaviour.
 *      Return NULL if out of memory or the allocation would exceed the
 *      per device queue
 */

static struct tty_buffer *tty_buffer_alloc(struct tty_port *port, size_t size)
{
        struct llist_node *free;
        struct tty_buffer *p;

        /* Round the buffer size out */
        size = __ALIGN_MASK(size, TTYB_ALIGN_MASK);

        if (size <= MIN_TTYB_SIZE) {
                free = llist_del_first(&port->buf.free);
                if (free) {
                        p = llist_entry(free, struct tty_buffer, free);
                        goto found;
                }
        }

        /* Should possibly check if this fails for the largest buffer we
           have queued and recycle that ? */
        if (atomic_read(&port->buf.mem_used) > port->buf.mem_limit)
                return NULL;
        p = kmalloc(sizeof(struct tty_buffer) + 2 * size, GFP_ATOMIC);
        if (p == NULL)
                return NULL;

found:
        tty_buffer_reset(p, size);
        atomic_add(size, &port->buf.mem_used);
        return p;
}

/**
 *      tty_buffer_free         -       free a tty buffer
 *      @tty: tty owning the buffer
 *      @b: the buffer to free
 *
 *      Free a tty buffer, or add it to the free list according to our
 *      internal strategy
 */

static void tty_buffer_free(struct tty_port *port, struct tty_buffer *b)
{
        struct tty_bufhead *buf = &port->buf;

        /* Dumb strategy for now - should keep some stats */
        WARN_ON(atomic_sub_return(b->size, &buf->mem_used) < 0);

        if (b->size > MIN_TTYB_SIZE)
                kfree(b);
        else if (b->size > 0)
                llist_add(&b->free, &buf->free);
}

/**
 *      tty_buffer_flush                -       flush full tty buffers
 *      @tty: tty to flush
 *      @ld:  optional ldisc ptr (must be referenced)
 *
 *      flush all the buffers containing receive data. If ld != NULL,
 *      flush the ldisc input buffer.
 *
 *      Locking: takes buffer lock to ensure single-threaded flip buffer
 *               'consumer'
 */

void tty_buffer_flush(struct tty_struct *tty, struct tty_ldisc *ld)
{
        struct tty_port *port = tty->port;
        struct tty_bufhead *buf = &port->buf;
        struct tty_buffer *next;

        atomic_inc(&buf->priority);

        mutex_lock(&buf->lock);
        /* paired w/ release in __tty_buffer_request_room; ensures there are
         * no pending memory accesses to the freed buffer
         */
        while ((next = smp_load_acquire(&buf->head->next)) != NULL) {
                tty_buffer_free(port, buf->head);
                buf->head = next;
        }
        buf->head->read = buf->head->commit;

        if (ld && ld->ops->flush_buffer)
                ld->ops->flush_buffer(tty);

        check_other_closed(tty);

        atomic_dec(&buf->priority);
        mutex_unlock(&buf->lock);
}

/**
 *      tty_buffer_request_room         -       grow tty buffer if needed
 *      @tty: tty structure
 *      @size: size desired
 *      @flags: buffer flags if new buffer allocated (default = 0)
 *
 *      Make at least size bytes of linear space available for the tty
 *      buffer. If we fail return the size we managed to find.
 *
 *      Will change over to a new buffer if the current buffer is encoded as
 *      TTY_NORMAL (so has no flags buffer) and the new buffer requires
 *      a flags buffer.
 */
static int __tty_buffer_request_room(struct tty_port *port, size_t size,
                                     int flags)
{
        struct tty_bufhead *buf = &port->buf;
        struct tty_buffer *b, *n;
        int left, change;

        b = buf->tail;
        if (b->flags & TTYB_NORMAL)
                left = 2 * b->size - b->used;
        else
                left = b->size - b->used;

        change = (b->flags & TTYB_NORMAL) && (~flags & TTYB_NORMAL);
        if (change || left < size) {
                /* This is the slow path - looking for new buffers to use */
                n = tty_buffer_alloc(port, size);
                if (n != NULL) {
                        n->flags = flags;
                        buf->tail = n;
                        /* paired w/ acquire in flush_to_ldisc(); ensures
                         * flush_to_ldisc() sees buffer data.
                         */
                        smp_store_release(&b->commit, b->used);
                        /* paired w/ acquire in flush_to_ldisc(); ensures the
                         * latest commit value can be read before the head is
                         * advanced to the next buffer
                         */
                        smp_store_release(&b->next, n);
                } else if (change)
                        size = 0;
                else
                        size = left;
        }
        return size;
}

int tty_buffer_request_room(struct tty_port *port, size_t size)
{
        return __tty_buffer_request_room(port, size, 0);
}
EXPORT_SYMBOL_GPL(tty_buffer_request_room);

/**
 *      tty_insert_flip_string_fixed_flag - Add characters to the tty buffer
 *      @port: tty port
 *      @chars: characters
 *      @flag: flag value for each character
 *      @size: size
 *
 *      Queue a series of bytes to the tty buffering. All the characters
 *      passed are marked with the supplied flag. Returns the number added.
 */

int tty_insert_flip_string_fixed_flag(struct tty_port *port,
                const unsigned char *chars, char flag, size_t size)
{
        int copied = 0;
        do {
                int goal = min_t(size_t, size - copied, TTY_BUFFER_PAGE);
                int flags = (flag == TTY_NORMAL) ? TTYB_NORMAL : 0;
                int space = __tty_buffer_request_room(port, goal, flags);
                struct tty_buffer *tb = port->buf.tail;
                if (unlikely(space == 0))
                        break;
                memcpy(char_buf_ptr(tb, tb->used), chars, space);
                if (~tb->flags & TTYB_NORMAL)
                        memset(flag_buf_ptr(tb, tb->used), flag, space);
                tb->used += space;
                copied += space;
                chars += space;
                /* There is a small chance that we need to split the data over
                   several buffers. If this is the case we must loop */
        } while (unlikely(size > copied));
        return copied;
}
EXPORT_SYMBOL(tty_insert_flip_string_fixed_flag);

/**
 *      tty_insert_flip_string_flags    -       Add characters to the tty buffer
 *      @port: tty port
 *      @chars: characters
 *      @flags: flag bytes
 *      @size: size
 *
 *      Queue a series of bytes to the tty buffering. For each character
 *      the flags array indicates the status of the character. Returns the
 *      number added.
 */

int tty_insert_flip_string_flags(struct tty_port *port,
                const unsigned char *chars, const char *flags, size_t size)
{
        int copied = 0;
        do {
                int goal = min_t(size_t, size - copied, TTY_BUFFER_PAGE);
                int space = tty_buffer_request_room(port, goal);
                struct tty_buffer *tb = port->buf.tail;
                if (unlikely(space == 0))
                        break;
                memcpy(char_buf_ptr(tb, tb->used), chars, space);
                memcpy(flag_buf_ptr(tb, tb->used), flags, space);
                tb->used += space;
                copied += space;
                chars += space;
                flags += space;
                /* There is a small chance that we need to split the data over
                   several buffers. If this is the case we must loop */
        } while (unlikely(size > copied));
        return copied;
}
EXPORT_SYMBOL(tty_insert_flip_string_flags);

/**
 *      tty_schedule_flip       -       push characters to ldisc
 *      @port: tty port to push from
 *
 *      Takes any pending buffers and transfers their ownership to the
 *      ldisc side of the queue. It then schedules those characters for
 *      processing by the line discipline.
 */

void tty_schedule_flip(struct tty_port *port)
{
        struct tty_bufhead *buf = &port->buf;

        /* paired w/ acquire in flush_to_ldisc(); ensures
         * flush_to_ldisc() sees buffer data.
         */
        smp_store_release(&buf->tail->commit, buf->tail->used);
        queue_work(system_unbound_wq, &buf->work);
}
EXPORT_SYMBOL(tty_schedule_flip);

/**
 *      tty_prepare_flip_string         -       make room for characters
 *      @port: tty port
 *      @chars: return pointer for character write area
 *      @size: desired size
 *
 *      Prepare a block of space in the buffer for data. Returns the length
 *      available and buffer pointer to the space which is now allocated and
 *      accounted for as ready for normal characters. This is used for drivers
 *      that need their own block copy routines into the buffer. There is no
 *      guarantee the buffer is a DMA target!
 */

int tty_prepare_flip_string(struct tty_port *port, unsigned char **chars,
                size_t size)
{
        int space = __tty_buffer_request_room(port, size, TTYB_NORMAL);
        if (likely(space)) {
                struct tty_buffer *tb = port->buf.tail;
                *chars = char_buf_ptr(tb, tb->used);
                if (~tb->flags & TTYB_NORMAL)
                        memset(flag_buf_ptr(tb, tb->used), TTY_NORMAL, space);
                tb->used += space;
        }
        return space;
}
EXPORT_SYMBOL_GPL(tty_prepare_flip_string);

/**
 *      tty_ldisc_receive_buf           -       forward data to line discipline
 *      @ld:    line discipline to process input
 *      @p:     char buffer
 *      @f:     TTY_* flags buffer
 *      @count: number of bytes to process
 *
 *      Callers other than flush_to_ldisc() need to exclude the kworker
 *      from concurrent use of the line discipline, see paste_selection().
 *
 *      Returns the number of bytes not processed
 */
int tty_ldisc_receive_buf(struct tty_ldisc *ld, unsigned char *p,
                          char *f, int count)
{
        if (ld->ops->receive_buf2)
                count = ld->ops->receive_buf2(ld->tty, p, f, count);
        else {
                count = min_t(int, count, ld->tty->receive_room);
                if (count && ld->ops->receive_buf)
                        ld->ops->receive_buf(ld->tty, p, f, count);
        }
        return count;
}
EXPORT_SYMBOL_GPL(tty_ldisc_receive_buf);

static int
receive_buf(struct tty_ldisc *ld, struct tty_buffer *head, int count)
{
        unsigned char *p = char_buf_ptr(head, head->read);
        char          *f = NULL;

        if (~head->flags & TTYB_NORMAL)
                f = flag_buf_ptr(head, head->read);

        return tty_ldisc_receive_buf(ld, p, f, count);
}

/**
 *      flush_to_ldisc
 *      @work: tty structure passed from work queue.
 *
 *      This routine is called out of the software interrupt to flush data
 *      from the buffer chain to the line discipline.
 *
 *      The receive_buf method is single threaded for each tty instance.
 *
 *      Locking: takes buffer lock to ensure single-threaded flip buffer
 *               'consumer'
 */

static void flush_to_ldisc(struct work_struct *work)
{
        struct tty_port *port = container_of(work, struct tty_port, buf.work);
        struct tty_bufhead *buf = &port->buf;
        struct tty_struct *tty;
        struct tty_ldisc *disc;

        tty = READ_ONCE(port->itty);
        if (tty == NULL)
                return;

        disc = tty_ldisc_ref(tty);
        if (disc == NULL)
                return;

        mutex_lock(&buf->lock);

        while (1) {
                struct tty_buffer *head = buf->head;
                struct tty_buffer *next;
                int count;

                /* Ldisc or user is trying to gain exclusive access */
                if (atomic_read(&buf->priority))
                        break;

                /* paired w/ release in __tty_buffer_request_room();
                 * ensures commit value read is not stale if the head
                 * is advancing to the next buffer
                 */
                next = smp_load_acquire(&head->next);
                /* paired w/ release in __tty_buffer_request_room() or in
                 * tty_buffer_flush(); ensures we see the committed buffer data
                 */
                count = smp_load_acquire(&head->commit) - head->read;
                if (!count) {
                        if (next == NULL) {
                                check_other_closed(tty);
                                break;
                        }
                        buf->head = next;
                        tty_buffer_free(port, head);
                        continue;
                }

                count = receive_buf(disc, head, count);
                if (!count)
                        break;
                head->read += count;
        }

        mutex_unlock(&buf->lock);

        tty_ldisc_deref(disc);
}

/**
 *      tty_flip_buffer_push    -       terminal
 *      @port: tty port to push
 *
 *      Queue a push of the terminal flip buffers to the line discipline.
 *      Can be called from IRQ/atomic context.
 *
 *      In the event of the queue being busy for flipping the work will be
 *      held off and retried later.
 */

void tty_flip_buffer_push(struct tty_port *port)
{
        tty_schedule_flip(port);
}
EXPORT_SYMBOL(tty_flip_buffer_push);

/**
 *      tty_buffer_init         -       prepare a tty buffer structure
 *      @tty: tty to initialise
 *
 *      Set up the initial state of the buffer management for a tty device.
 *      Must be called before the other tty buffer functions are used.
 */

void tty_buffer_init(struct tty_port *port)
{
        struct tty_bufhead *buf = &port->buf;

        mutex_init(&buf->lock);
        tty_buffer_reset(&buf->sentinel, 0);
        buf->head = &buf->sentinel;
        buf->tail = &buf->sentinel;
        init_llist_head(&buf->free);
        atomic_set(&buf->mem_used, 0);
        atomic_set(&buf->priority, 0);
        INIT_WORK(&buf->work, flush_to_ldisc);
        buf->mem_limit = TTYB_DEFAULT_MEM_LIMIT;
}

/**
 *      tty_buffer_set_limit    -       change the tty buffer memory limit
 *      @port: tty port to change
 *
 *      Change the tty buffer memory limit.
 *      Must be called before the other tty buffer functions are used.
 */

int tty_buffer_set_limit(struct tty_port *port, int limit)
{
        if (limit < MIN_TTYB_SIZE)
                return -EINVAL;
        port->buf.mem_limit = limit;
        return 0;
}
EXPORT_SYMBOL_GPL(tty_buffer_set_limit);

/* slave ptys can claim nested buffer lock when handling BRK and INTR */
void tty_buffer_set_lock_subclass(struct tty_port *port)
{
        lockdep_set_subclass(&port->buf.lock, TTY_LOCK_SLAVE);
}

bool tty_buffer_restart_work(struct tty_port *port)
{
        return queue_work(system_unbound_wq, &port->buf.work);
}

bool tty_buffer_cancel_work(struct tty_port *port)
{
        return cancel_work_sync(&port->buf.work);
}