rxrpc.c

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/* Maintain an RxRPC server socket to do AFS communications through
 *
 * Copyright (C) 2007 Red Hat, Inc. All Rights Reserved.
 * Written by David Howells ([email protected])
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation; either version
 * 2 of the License, or (at your option) any later version.
 */

#include <linux/slab.h>
#include <net/sock.h>
#include <net/af_rxrpc.h>
#include <rxrpc/packet.h>
#include "internal.h"
#include "afs_cm.h"

static struct socket *afs_socket; /* my RxRPC socket */
static struct workqueue_struct *afs_async_calls;
static atomic_t afs_outstanding_calls;
static atomic_t afs_outstanding_skbs;

static void afs_wake_up_call_waiter(struct afs_call *);
static int afs_wait_for_call_to_complete(struct afs_call *);
static void afs_wake_up_async_call(struct afs_call *);
static int afs_dont_wait_for_call_to_complete(struct afs_call *);
static void afs_process_async_call(struct work_struct *);
static void afs_rx_interceptor(struct sock *, unsigned long, struct sk_buff *);
static int afs_deliver_cm_op_id(struct afs_call *, struct sk_buff *, bool);

/* synchronous call management */
const struct afs_wait_mode afs_sync_call = {
    .rx_wakeup  = afs_wake_up_call_waiter,
    .wait       = afs_wait_for_call_to_complete,
};

/* asynchronous call management */
const struct afs_wait_mode afs_async_call = {
    .rx_wakeup  = afs_wake_up_async_call,
    .wait       = afs_dont_wait_for_call_to_complete,
};

/* asynchronous incoming call management */
static const struct afs_wait_mode afs_async_incoming_call = {
    .rx_wakeup  = afs_wake_up_async_call,
};

/* asynchronous incoming call initial processing */
static const struct afs_call_type afs_RXCMxxxx = {
    .name       = "CB.xxxx",
    .deliver    = afs_deliver_cm_op_id,
    .abort_to_error = afs_abort_to_error,
};

static void afs_collect_incoming_call(struct work_struct *);

static struct sk_buff_head afs_incoming_calls;
static DECLARE_WORK(afs_collect_incoming_call_work, afs_collect_incoming_call);

/*
 * open an RxRPC socket and bind it to be a server for callback notifications
 * - the socket is left in blocking mode and non-blocking ops use MSG_DONTWAIT
 */
int afs_open_socket(void)
{
    struct sockaddr_rxrpc srx;
    struct socket *socket;
    int ret;

    _enter("");

    skb_queue_head_init(&afs_incoming_calls);

    afs_async_calls = create_singlethread_workqueue("kafsd");
    if (!afs_async_calls) {
        _leave(" = -ENOMEM [wq]");
        return -ENOMEM;
    }

    ret = sock_create_kern(AF_RXRPC, SOCK_DGRAM, PF_INET, &socket);
    if (ret < 0) {
        destroy_workqueue(afs_async_calls);
        _leave(" = %d [socket]", ret);
        return ret;
    }

    socket->sk->sk_allocation = GFP_NOFS;

    /* bind the callback manager's address to make this a server socket */
    srx.srx_family          = AF_RXRPC;
    srx.srx_service         = CM_SERVICE;
    srx.transport_type      = SOCK_DGRAM;
    srx.transport_len       = sizeof(srx.transport.sin);
    srx.transport.sin.sin_family    = AF_INET;
    srx.transport.sin.sin_port  = htons(AFS_CM_PORT);
    memset(&srx.transport.sin.sin_addr, 0,
           sizeof(srx.transport.sin.sin_addr));

    ret = kernel_bind(socket, (struct sockaddr *) &srx, sizeof(srx));
    if (ret < 0) {
        sock_release(socket);
        destroy_workqueue(afs_async_calls);
        _leave(" = %d [bind]", ret);
        return ret;
    }

    rxrpc_kernel_intercept_rx_messages(socket, afs_rx_interceptor);

    afs_socket = socket;
    _leave(" = 0");
    return 0;
}

/*
 * close the RxRPC socket AFS was using
 */
void afs_close_socket(void)
{
    _enter("");

    sock_release(afs_socket);

    _debug("dework");
    destroy_workqueue(afs_async_calls);

    ASSERTCMP(atomic_read(&afs_outstanding_skbs), ==, 0);
    ASSERTCMP(atomic_read(&afs_outstanding_calls), ==, 0);
    _leave("");
}

/*
 * note that the data in a socket buffer is now delivered and that the buffer
 * should be freed
 */
static void afs_data_delivered(struct sk_buff *skb)
{
    if (!skb) {
        _debug("DLVR NULL [%d]", atomic_read(&afs_outstanding_skbs));
        dump_stack();
    } else {
        _debug("DLVR %p{%u} [%d]",
               skb, skb->mark, atomic_read(&afs_outstanding_skbs));
        if (atomic_dec_return(&afs_outstanding_skbs) == -1)
            BUG();
        rxrpc_kernel_data_delivered(skb);
    }
}

/*
 * free a socket buffer
 */
static void afs_free_skb(struct sk_buff *skb)
{
    if (!skb) {
        _debug("FREE NULL [%d]", atomic_read(&afs_outstanding_skbs));
        dump_stack();
    } else {
        _debug("FREE %p{%u} [%d]",
               skb, skb->mark, atomic_read(&afs_outstanding_skbs));
        if (atomic_dec_return(&afs_outstanding_skbs) == -1)
            BUG();
        rxrpc_kernel_free_skb(skb);
    }
}

/*
 * free a call
 */
static void afs_free_call(struct afs_call *call)
{
    _debug("DONE %p{%s} [%d]",
           call, call->type->name, atomic_read(&afs_outstanding_calls));
    if (atomic_dec_return(&afs_outstanding_calls) == -1)
        BUG();

    ASSERTCMP(call->rxcall, ==, NULL);
    ASSERT(!work_pending(&call->async_work));
    ASSERT(skb_queue_empty(&call->rx_queue));
    ASSERT(call->type->name != NULL);

    kfree(call->request);
    kfree(call);
}

/*
 * allocate a call with flat request and reply buffers
 */
struct afs_call *afs_alloc_flat_call(const struct afs_call_type *type,
                     size_t request_size, size_t reply_size)
{
    struct afs_call *call;

    call = kzalloc(sizeof(*call), GFP_NOFS);
    if (!call)
        goto nomem_call;

    _debug("CALL %p{%s} [%d]",
           call, type->name, atomic_read(&afs_outstanding_calls));
    atomic_inc(&afs_outstanding_calls);

    call->type = type;
    call->request_size = request_size;
    call->reply_max = reply_size;

    if (request_size) {
        call->request = kmalloc(request_size, GFP_NOFS);
        if (!call->request)
            goto nomem_free;
    }

    if (reply_size) {
        call->buffer = kmalloc(reply_size, GFP_NOFS);
        if (!call->buffer)
            goto nomem_free;
    }

    init_waitqueue_head(&call->waitq);
    skb_queue_head_init(&call->rx_queue);
    return call;

nomem_free:
    afs_free_call(call);
nomem_call:
    return NULL;
}

/*
 * clean up a call with flat buffer
 */
void afs_flat_call_destructor(struct afs_call *call)
{
    _enter("");

    kfree(call->request);
    call->request = NULL;
    kfree(call->buffer);
    call->buffer = NULL;
}

/*
 * attach the data from a bunch of pages on an inode to a call
 */
static int afs_send_pages(struct afs_call *call, struct msghdr *msg,
              struct kvec *iov)
{
    struct page *pages[8];
    unsigned count, n, loop, offset, to;
    pgoff_t first = call->first, last = call->last;
    int ret;

    _enter("");

    offset = call->first_offset;
    call->first_offset = 0;

    do {
        _debug("attach %lx-%lx", first, last);

        count = last - first + 1;
        if (count > ARRAY_SIZE(pages))
            count = ARRAY_SIZE(pages);
        n = find_get_pages_contig(call->mapping, first, count, pages);
        ASSERTCMP(n, ==, count);

        loop = 0;
        do {
            msg->msg_flags = 0;
            to = PAGE_SIZE;
            if (first + loop >= last)
                to = call->last_to;
            else
                msg->msg_flags = MSG_MORE;
            iov->iov_base = kmap(pages[loop]) + offset;
            iov->iov_len = to - offset;
            offset = 0;

            _debug("- range %u-%u%s",
                   offset, to, msg->msg_flags ? " [more]" : "");
            msg->msg_iov = (struct iovec *) iov;
            msg->msg_iovlen = 1;

            /* have to change the state *before* sending the last
             * packet as RxRPC might give us the reply before it
             * returns from sending the request */
            if (first + loop >= last)
                call->state = AFS_CALL_AWAIT_REPLY;
            ret = rxrpc_kernel_send_data(call->rxcall, msg,
                             to - offset);
            kunmap(pages[loop]);
            if (ret < 0)
                break;
        } while (++loop < count);
        first += count;

        for (loop = 0; loop < count; loop++)
            put_page(pages[loop]);
        if (ret < 0)
            break;
    } while (first <= last);

    _leave(" = %d", ret);
    return ret;
}

/*
 * initiate a call
 */
int afs_make_call(struct in_addr *addr, struct afs_call *call, gfp_t gfp,
          const struct afs_wait_mode *wait_mode)
{
    struct sockaddr_rxrpc srx;
    struct rxrpc_call *rxcall;
    struct msghdr msg;
    struct kvec iov[1];
    int ret;
    struct sk_buff *skb;

    _enter("%x,{%d},", addr->s_addr, ntohs(call->port));

    ASSERT(call->type != NULL);
    ASSERT(call->type->name != NULL);

    _debug("____MAKE %p{%s,%x} [%d]____",
           call, call->type->name, key_serial(call->key),
           atomic_read(&afs_outstanding_calls));

    call->wait_mode = wait_mode;
    INIT_WORK(&call->async_work, afs_process_async_call);

    memset(&srx, 0, sizeof(srx));
    srx.srx_family = AF_RXRPC;
    srx.srx_service = call->service_id;
    srx.transport_type = SOCK_DGRAM;
    srx.transport_len = sizeof(srx.transport.sin);
    srx.transport.sin.sin_family = AF_INET;
    srx.transport.sin.sin_port = call->port;
    memcpy(&srx.transport.sin.sin_addr, addr, 4);

    /* create a call */
    rxcall = rxrpc_kernel_begin_call(afs_socket, &srx, call->key,
                     (unsigned long) call, gfp);
    call->key = NULL;
    if (IS_ERR(rxcall)) {
        ret = PTR_ERR(rxcall);
        goto error_kill_call;
    }

    call->rxcall = rxcall;

    /* send the request */
    iov[0].iov_base = call->request;
    iov[0].iov_len  = call->request_size;

    msg.msg_name        = NULL;
    msg.msg_namelen     = 0;
    msg.msg_iov     = (struct iovec *) iov;
    msg.msg_iovlen      = 1;
    msg.msg_control     = NULL;
    msg.msg_controllen  = 0;
    msg.msg_flags       = (call->send_pages ? MSG_MORE : 0);

    /* have to change the state *before* sending the last packet as RxRPC
     * might give us the reply before it returns from sending the
     * request */
    if (!call->send_pages)
        call->state = AFS_CALL_AWAIT_REPLY;
    ret = rxrpc_kernel_send_data(rxcall, &msg, call->request_size);
    if (ret < 0)
        goto error_do_abort;

    if (call->send_pages) {
        ret = afs_send_pages(call, &msg, iov);
        if (ret < 0)
            goto error_do_abort;
    }

    /* at this point, an async call may no longer exist as it may have
     * already completed */
    return wait_mode->wait(call);

error_do_abort:
    rxrpc_kernel_abort_call(rxcall, RX_USER_ABORT);
    while ((skb = skb_dequeue(&call->rx_queue)))
        afs_free_skb(skb);
    rxrpc_kernel_end_call(rxcall);
    call->rxcall = NULL;
error_kill_call:
    call->type->destructor(call);
    afs_free_call(call);
    _leave(" = %d", ret);
    return ret;
}

/*
 * handles intercepted messages that were arriving in the socket's Rx queue
 * - called with the socket receive queue lock held to ensure message ordering
 * - called with softirqs disabled
 */
static void afs_rx_interceptor(struct sock *sk, unsigned long user_call_ID,
                   struct sk_buff *skb)
{
    struct afs_call *call = (struct afs_call *) user_call_ID;

    _enter("%p,,%u", call, skb->mark);

    _debug("ICPT %p{%u} [%d]",
           skb, skb->mark, atomic_read(&afs_outstanding_skbs));

    ASSERTCMP(sk, ==, afs_socket->sk);
    atomic_inc(&afs_outstanding_skbs);

    if (!call) {
        /* its an incoming call for our callback service */
        skb_queue_tail(&afs_incoming_calls, skb);
        queue_work(afs_wq, &afs_collect_incoming_call_work);
    } else {
        /* route the messages directly to the appropriate call */
        skb_queue_tail(&call->rx_queue, skb);
        call->wait_mode->rx_wakeup(call);
    }

    _leave("");
}

/*
 * deliver messages to a call
 */
static void afs_deliver_to_call(struct afs_call *call)
{
    struct sk_buff *skb;
    bool last;
    u32 abort_code;
    int ret;

    _enter("");

    while ((call->state == AFS_CALL_AWAIT_REPLY ||
        call->state == AFS_CALL_AWAIT_OP_ID ||
        call->state == AFS_CALL_AWAIT_REQUEST ||
        call->state == AFS_CALL_AWAIT_ACK) &&
           (skb = skb_dequeue(&call->rx_queue))) {
        switch (skb->mark) {
        case RXRPC_SKB_MARK_DATA:
            _debug("Rcv DATA");
            last = rxrpc_kernel_is_data_last(skb);
            ret = call->type->deliver(call, skb, last);
            switch (ret) {
            case 0:
                if (last &&
                    call->state == AFS_CALL_AWAIT_REPLY)
                    call->state = AFS_CALL_COMPLETE;
                break;
            case -ENOTCONN:
                abort_code = RX_CALL_DEAD;
                goto do_abort;
            case -ENOTSUPP:
                abort_code = RX_INVALID_OPERATION;
                goto do_abort;
            default:
                abort_code = RXGEN_CC_UNMARSHAL;
                if (call->state != AFS_CALL_AWAIT_REPLY)
                    abort_code = RXGEN_SS_UNMARSHAL;
            do_abort:
                rxrpc_kernel_abort_call(call->rxcall,
                            abort_code);
                call->error = ret;
                call->state = AFS_CALL_ERROR;
                break;
            }
            afs_data_delivered(skb);
            skb = NULL;
            continue;
        case RXRPC_SKB_MARK_FINAL_ACK:
            _debug("Rcv ACK");
            call->state = AFS_CALL_COMPLETE;
            break;
        case RXRPC_SKB_MARK_BUSY:
            _debug("Rcv BUSY");
            call->error = -EBUSY;
            call->state = AFS_CALL_BUSY;
            break;
        case RXRPC_SKB_MARK_REMOTE_ABORT:
            abort_code = rxrpc_kernel_get_abort_code(skb);
            call->error = call->type->abort_to_error(abort_code);
            call->state = AFS_CALL_ABORTED;
            _debug("Rcv ABORT %u -> %d", abort_code, call->error);
            break;
        case RXRPC_SKB_MARK_NET_ERROR:
            call->error = -rxrpc_kernel_get_error_number(skb);
            call->state = AFS_CALL_ERROR;
            _debug("Rcv NET ERROR %d", call->error);
            break;
        case RXRPC_SKB_MARK_LOCAL_ERROR:
            call->error = -rxrpc_kernel_get_error_number(skb);
            call->state = AFS_CALL_ERROR;
            _debug("Rcv LOCAL ERROR %d", call->error);
            break;
        default:
            BUG();
            break;
        }

        afs_free_skb(skb);
    }

    /* make sure the queue is empty if the call is done with (we might have
     * aborted the call early because of an unmarshalling error) */
    if (call->state >= AFS_CALL_COMPLETE) {
        while ((skb = skb_dequeue(&call->rx_queue)))
            afs_free_skb(skb);
        if (call->incoming) {
            rxrpc_kernel_end_call(call->rxcall);
            call->rxcall = NULL;
            call->type->destructor(call);
            afs_free_call(call);
        }
    }

    _leave("");
}

/*
 * wait synchronously for a call to complete
 */
static int afs_wait_for_call_to_complete(struct afs_call *call)
{
    struct sk_buff *skb;
    int ret;

    DECLARE_WAITQUEUE(myself, current);

    _enter("");

    add_wait_queue(&call->waitq, &myself);
    for (;;) {
        set_current_state(TASK_INTERRUPTIBLE);

        /* deliver any messages that are in the queue */
        if (!skb_queue_empty(&call->rx_queue)) {
            __set_current_state(TASK_RUNNING);
            afs_deliver_to_call(call);
            continue;
        }

        ret = call->error;
        if (call->state >= AFS_CALL_COMPLETE)
            break;
        ret = -EINTR;
        if (signal_pending(current))
            break;
        schedule();
    }

    remove_wait_queue(&call->waitq, &myself);
    __set_current_state(TASK_RUNNING);

    /* kill the call */
    if (call->state < AFS_CALL_COMPLETE) {
        _debug("call incomplete");
        rxrpc_kernel_abort_call(call->rxcall, RX_CALL_DEAD);
        while ((skb = skb_dequeue(&call->rx_queue)))
            afs_free_skb(skb);
    }

    _debug("call complete");
    rxrpc_kernel_end_call(call->rxcall);
    call->rxcall = NULL;
    call->type->destructor(call);
    afs_free_call(call);
    _leave(" = %d", ret);
    return ret;
}

/*
 * wake up a waiting call
 */
static void afs_wake_up_call_waiter(struct afs_call *call)
{
    wake_up(&call->waitq);
}

/*
 * wake up an asynchronous call
 */
static void afs_wake_up_async_call(struct afs_call *call)
{
    _enter("");
    queue_work(afs_async_calls, &call->async_work);
}

/*
 * put a call into asynchronous mode
 * - mustn't touch the call descriptor as the call my have completed by the
 *   time we get here
 */
static int afs_dont_wait_for_call_to_complete(struct afs_call *call)
{
    _enter("");
    return -EINPROGRESS;
}

/*
 * delete an asynchronous call
 */
static void afs_delete_async_call(struct work_struct *work)
{
    struct afs_call *call =
        container_of(work, struct afs_call, async_work);

    _enter("");

    afs_free_call(call);

    _leave("");
}

/*
 * perform processing on an asynchronous call
 * - on a multiple-thread workqueue this work item may try to run on several
 *   CPUs at the same time
 */
static void afs_process_async_call(struct work_struct *work)
{
    struct afs_call *call =
        container_of(work, struct afs_call, async_work);

    _enter("");

    if (!skb_queue_empty(&call->rx_queue))
        afs_deliver_to_call(call);

    if (call->state >= AFS_CALL_COMPLETE && call->wait_mode) {
        if (call->wait_mode->async_complete)
            call->wait_mode->async_complete(call->reply,
                            call->error);
        call->reply = NULL;

        /* kill the call */
        rxrpc_kernel_end_call(call->rxcall);
        call->rxcall = NULL;
        if (call->type->destructor)
            call->type->destructor(call);

        /* we can't just delete the call because the work item may be
         * queued */
        PREPARE_WORK(&call->async_work, afs_delete_async_call);
        queue_work(afs_async_calls, &call->async_work);
    }

    _leave("");
}

/*
 * empty a socket buffer into a flat reply buffer
 */
void afs_transfer_reply(struct afs_call *call, struct sk_buff *skb)
{
    size_t len = skb->len;

    if (skb_copy_bits(skb, 0, call->buffer + call->reply_size, len) < 0)
        BUG();
    call->reply_size += len;
}

/*
 * accept the backlog of incoming calls
 */
static void afs_collect_incoming_call(struct work_struct *work)
{
    struct rxrpc_call *rxcall;
    struct afs_call *call = NULL;
    struct sk_buff *skb;

    while ((skb = skb_dequeue(&afs_incoming_calls))) {
        _debug("new call");

        /* don't need the notification */
        afs_free_skb(skb);

        if (!call) {
            call = kzalloc(sizeof(struct afs_call), GFP_KERNEL);
            if (!call) {
                rxrpc_kernel_reject_call(afs_socket);
                return;
            }

            INIT_WORK(&call->async_work, afs_process_async_call);
            call->wait_mode = &afs_async_incoming_call;
            call->type = &afs_RXCMxxxx;
            init_waitqueue_head(&call->waitq);
            skb_queue_head_init(&call->rx_queue);
            call->state = AFS_CALL_AWAIT_OP_ID;

            _debug("CALL %p{%s} [%d]",
                   call, call->type->name,
                   atomic_read(&afs_outstanding_calls));
            atomic_inc(&afs_outstanding_calls);
        }

        rxcall = rxrpc_kernel_accept_call(afs_socket,
                          (unsigned long) call);
        if (!IS_ERR(rxcall)) {
            call->rxcall = rxcall;
            call = NULL;
        }
    }

    if (call)
        afs_free_call(call);
}

/*
 * grab the operation ID from an incoming cache manager call
 */
static int afs_deliver_cm_op_id(struct afs_call *call, struct sk_buff *skb,
                bool last)
{
    size_t len = skb->len;
    void *oibuf = (void *) &call->operation_ID;

    _enter("{%u},{%zu},%d", call->offset, len, last);

    ASSERTCMP(call->offset, <, 4);

    /* the operation ID forms the first four bytes of the request data */
    len = min_t(size_t, len, 4 - call->offset);
    if (skb_copy_bits(skb, 0, oibuf + call->offset, len) < 0)
        BUG();
    if (!pskb_pull(skb, len))
        BUG();
    call->offset += len;

    if (call->offset < 4) {
        if (last) {
            _leave(" = -EBADMSG [op ID short]");
            return -EBADMSG;
        }
        _leave(" = 0 [incomplete]");
        return 0;
    }

    call->state = AFS_CALL_AWAIT_REQUEST;

    /* ask the cache manager to route the call (it'll change the call type
     * if successful) */
    if (!afs_cm_incoming_call(call))
        return -ENOTSUPP;

    /* pass responsibility for the remainer of this message off to the
     * cache manager op */
    return call->type->deliver(call, skb, last);
}

/*
 * send an empty reply
 */
void afs_send_empty_reply(struct afs_call *call)
{
    struct msghdr msg;
    struct iovec iov[1];

    _enter("");

    iov[0].iov_base     = NULL;
    iov[0].iov_len      = 0;
    msg.msg_name        = NULL;
    msg.msg_namelen     = 0;
    msg.msg_iov     = iov;
    msg.msg_iovlen      = 0;
    msg.msg_control     = NULL;
    msg.msg_controllen  = 0;
    msg.msg_flags       = 0;

    call->state = AFS_CALL_AWAIT_ACK;
    switch (rxrpc_kernel_send_data(call->rxcall, &msg, 0)) {
    case 0:
        _leave(" [replied]");
        return;

    case -ENOMEM:
        _debug("oom");
        rxrpc_kernel_abort_call(call->rxcall, RX_USER_ABORT);
    default:
        rxrpc_kernel_end_call(call->rxcall);
        call->rxcall = NULL;
        call->type->destructor(call);
        afs_free_call(call);
        _leave(" [error]");
        return;
    }
}

/*
 * send a simple reply
 */
void afs_send_simple_reply(struct afs_call *call, const void *buf, size_t len)
{
    struct msghdr msg;
    struct iovec iov[1];
    int n;

    _enter("");

    iov[0].iov_base     = (void *) buf;
    iov[0].iov_len      = len;
    msg.msg_name        = NULL;
    msg.msg_namelen     = 0;
    msg.msg_iov     = iov;
    msg.msg_iovlen      = 1;
    msg.msg_control     = NULL;
    msg.msg_controllen  = 0;
    msg.msg_flags       = 0;

    call->state = AFS_CALL_AWAIT_ACK;
    n = rxrpc_kernel_send_data(call->rxcall, &msg, len);
    if (n >= 0) {
        _leave(" [replied]");
        return;
    }
    if (n == -ENOMEM) {
        _debug("oom");
        rxrpc_kernel_abort_call(call->rxcall, RX_USER_ABORT);
    }
    rxrpc_kernel_end_call(call->rxcall);
    call->rxcall = NULL;
    call->type->destructor(call);
    afs_free_call(call);
    _leave(" [error]");
}

/*
 * extract a piece of data from the received data socket buffers
 */
int afs_extract_data(struct afs_call *call, struct sk_buff *skb,
             bool last, void *buf, size_t count)
{
    size_t len = skb->len;

    _enter("{%u},{%zu},%d,,%zu", call->offset, len, last, count);

    ASSERTCMP(call->offset, <, count);

    len = min_t(size_t, len, count - call->offset);
    if (skb_copy_bits(skb, 0, buf + call->offset, len) < 0 ||
        !pskb_pull(skb, len))
        BUG();
    call->offset += len;

    if (call->offset < count) {
        if (last) {
            _leave(" = -EBADMSG [%d < %zu]", call->offset, count);
            return -EBADMSG;
        }
        _leave(" = -EAGAIN");
        return -EAGAIN;
    }
    return 0;
}