Disallow local apps tracing for setuid binaries

[lttng-ust.git] / libust / buffers.c

/*
 * buffers.c
 * LTTng userspace tracer buffering system
 *
 * Copyright (C) 2009 - Pierre-Marc Fournier (pierre-marc dot fournier at polymtl dot ca)
 * Copyright (C) 2008-2011 - Mathieu Desnoyers (mathieu.desnoyers@polymtl.ca)
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; version 2.1 of
 * the License.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301 USA
 */

/*
 * Note: this code does not support the ref/noref flag and reader-owned
 * subbuffer scheme. Therefore, flight recorder mode uses a mechanism
 * where the reader can read corrupted data (and detect this), thus
 * returning -EIO.
 */

#include <unistd.h>
#include <sys/mman.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <fcntl.h>
#include <stdlib.h>

#include <ust/clock.h>

#include "buffers.h"
#include "channels.h"
#include "tracer.h"
#include "tracercore.h"
#include <ust/usterr-signal-safe.h>

struct ltt_reserve_switch_offsets {
        long begin, end, old;
        long begin_switch, end_switch_current, end_switch_old;
        size_t before_hdr_pad, size;
};


static DEFINE_MUTEX(ust_buffers_channels_mutex);
static CDS_LIST_HEAD(ust_buffers_channels);

static void ltt_force_switch(struct ust_buffer *buf,
                enum force_switch_mode mode);

/**
 * _ust_buffers_strncpy_fixup - Fix an incomplete string in a ltt_relay buffer.
 * @buf : buffer
 * @offset : offset within the buffer
 * @len : length to write
 * @copied: string actually copied
 * @terminated: does string end with \0
 *
 * Fills string with "X" if incomplete.
 */
void _ust_buffers_strncpy_fixup(struct ust_buffer *buf, size_t offset,
                                size_t len, size_t copied, int terminated)
{
        size_t buf_offset, cpy;

        if (copied == len) {
                /*
                 * Deal with non-terminated string.
                 */
                assert(!terminated);
                offset += copied - 1;
                buf_offset = BUFFER_OFFSET(offset, buf->chan);
                /*
                 * Underlying layer should never ask for writes across
                 * subbuffers.
                 */
                assert(buf_offset
                       < buf->chan->subbuf_size*buf->chan->subbuf_cnt);
                ust_buffers_do_memset(buf->buf_data + buf_offset, '\0', 1);
                return;
        }

        /*
         * Deal with incomplete string.
         * Overwrite string's \0 with X too.
         */
        cpy = copied - 1;
        assert(terminated);
        len -= cpy;
        offset += cpy;
        buf_offset = BUFFER_OFFSET(offset, buf->chan);

        /*
         * Underlying layer should never ask for writes across subbuffers.
         */
        assert(buf_offset
               < buf->chan->subbuf_size*buf->chan->subbuf_cnt);

        ust_buffers_do_memset(buf->buf_data + buf_offset,
                              'X', len);

        /*
         * Overwrite last 'X' with '\0'.
         */
        offset += len - 1;
        buf_offset = BUFFER_OFFSET(offset, buf->chan);
        /*
         * Underlying layer should never ask for writes across subbuffers.
         */
        assert(buf_offset
               < buf->chan->subbuf_size*buf->chan->subbuf_cnt);
        ust_buffers_do_memset(buf->buf_data + buf_offset, '\0', 1);
}

static void ltt_buffer_begin(struct ust_buffer *buf,
                             u64 tsc, unsigned int subbuf_idx)
{
        struct ust_channel *channel = buf->chan;
        struct ltt_subbuffer_header *header =
                (struct ltt_subbuffer_header *)
                        ust_buffers_offset_address(buf,
                                subbuf_idx * buf->chan->subbuf_size);

        header->cycle_count_begin = tsc;
        header->data_size = 0xFFFFFFFF; /* for recognizing crashed buffers */
        header->sb_size = 0xFFFFFFFF; /* for recognizing crashed buffers */
        /*
         * No memory barrier needed to order data_data/sb_size vs commit count
         * update, because commit count update contains a compiler barrier that
         * ensures the order of the writes are OK from a program POV. It only
         * matters for crash dump recovery which is not executed concurrently,
         * so memory write order does not matter.
         */
        ltt_write_trace_header(channel->trace, header);
}

static int map_buf_data(struct ust_buffer *buf, size_t *size)
{
        void *ptr;
        int result;

        *size = PAGE_ALIGN(*size);

        result = buf->shmid = shmget(getpid(), *size, IPC_CREAT | IPC_EXCL | 0700);
        if (result < 0 && errno == EINVAL) {
                ERR("shmget() returned EINVAL; maybe /proc/sys/kernel/shmmax should be increased.");
                return -1;
        } else if (result < 0) {
                PERROR("shmget");
                return -1;
        }

        ptr = shmat(buf->shmid, NULL, 0);
        if (ptr == (void *) -1) {
                perror("shmat");
                goto destroy_shmem;
        }

        /* Already mark the shared memory for destruction. This will occur only
         * when all users have detached.
         */
        result = shmctl(buf->shmid, IPC_RMID, NULL);
        if(result == -1) {
                perror("shmctl");
                return -1;
        }

        buf->buf_data = ptr;
        buf->buf_size = *size;

        return 0;

destroy_shmem:
        result = shmctl(buf->shmid, IPC_RMID, NULL);
        if(result == -1) {
                perror("shmctl");
        }

        return -1;
}

static int open_buf(struct ust_channel *chan, int cpu)
{
        int result, fds[2];
        unsigned int j;
        struct ust_trace *trace = chan->trace;
        struct ust_buffer *buf = chan->buf[cpu];
        unsigned int n_subbufs = chan->subbuf_cnt;


        result = map_buf_data(buf, &chan->alloc_size);
        if (result < 0)
                return -1;

        buf->commit_count =
                zmalloc(sizeof(*buf->commit_count) * n_subbufs);
        if (!buf->commit_count)
                goto unmap_buf;

        result = pipe(fds);
        if (result < 0) {
                PERROR("pipe");
                goto free_commit_count;
        }
        buf->data_ready_fd_read = fds[0];
        buf->data_ready_fd_write = fds[1];

        buf->cpu = cpu;
        buf->chan = chan;

        uatomic_set(&buf->offset, ltt_subbuffer_header_size());
        uatomic_set(&buf->consumed, 0);
        uatomic_set(&buf->active_readers, 0);
        for (j = 0; j < n_subbufs; j++) {
                uatomic_set(&buf->commit_count[j].cc, 0);
                uatomic_set(&buf->commit_count[j].cc_sb, 0);
        }

        ltt_buffer_begin(buf, trace->start_tsc, 0);

        uatomic_add(&buf->commit_count[0].cc, ltt_subbuffer_header_size());

        uatomic_set(&buf->events_lost, 0);
        uatomic_set(&buf->corrupted_subbuffers, 0);

        memset(buf->commit_seq, 0, sizeof(buf->commit_seq[0]) * n_subbufs);

        return 0;

free_commit_count:
        free(buf->commit_count);

unmap_buf:
        if (shmdt(buf->buf_data) < 0) {
                PERROR("shmdt failed");
        }

        return -1;
}

static void close_buf(struct ust_buffer *buf)
{
        int result;

        result = shmdt(buf->buf_data);
        if (result < 0) {
                PERROR("shmdt");
        }

        result = close(buf->data_ready_fd_read);
        if (result < 0) {
                PERROR("close");
        }

        result = close(buf->data_ready_fd_write);
        if (result < 0 && errno != EBADF) {
                PERROR("close");
        }
}


static int open_channel(struct ust_channel *chan, size_t subbuf_size,
                        size_t subbuf_cnt)
{
        int i;
        int result;

        if(subbuf_size == 0 || subbuf_cnt == 0)
                return -1;

        /* Check that the subbuffer size is larger than a page. */
        WARN_ON_ONCE(subbuf_size < PAGE_SIZE);

        /*
         * Make sure the number of subbuffers and subbuffer size are power of 2.
         */
        WARN_ON_ONCE(hweight32(subbuf_size) != 1);
        WARN_ON(hweight32(subbuf_cnt) != 1);

        chan->version = UST_CHANNEL_VERSION;
        chan->subbuf_cnt = subbuf_cnt;
        chan->subbuf_size = subbuf_size;
        chan->subbuf_size_order = get_count_order(subbuf_size);
        chan->alloc_size = subbuf_size * subbuf_cnt;

        pthread_mutex_lock(&ust_buffers_channels_mutex);
        for (i=0; i < chan->n_cpus; i++) {
                result = open_buf(chan, i);
                if (result == -1)
                        goto error;
        }
        cds_list_add(&chan->list, &ust_buffers_channels);
        pthread_mutex_unlock(&ust_buffers_channels_mutex);

        return 0;

        /* Error handling */
error:
        for(i--; i >= 0; i--)
                close_buf(chan->buf[i]);

        pthread_mutex_unlock(&ust_buffers_channels_mutex);
        return -1;
}

static void close_channel(struct ust_channel *chan)
{
        int i;
        if(!chan)
                return;

        pthread_mutex_lock(&ust_buffers_channels_mutex);
        /*
         * checking for chan->buf[i] being NULL or not is useless in
         * practice because we allocate buffers for all possible cpus.
         * However, should we decide to change this and only allocate
         * for online cpus, this check becomes useful.
         */
        for (i=0; i<chan->n_cpus; i++) {
                if (chan->buf[i])
                        close_buf(chan->buf[i]);
        }

        cds_list_del(&chan->list);

        pthread_mutex_unlock(&ust_buffers_channels_mutex);
}

/*
 * offset is assumed to never be 0 here : never deliver a completely empty
 * subbuffer. The lost size is between 0 and subbuf_size-1.
 */
static notrace void ltt_buffer_end(struct ust_buffer *buf,
                u64 tsc, unsigned int offset, unsigned int subbuf_idx)
{
        struct ltt_subbuffer_header *header =
                (struct ltt_subbuffer_header *)
                        ust_buffers_offset_address(buf,
                                subbuf_idx * buf->chan->subbuf_size);
        u32 data_size = SUBBUF_OFFSET(offset - 1, buf->chan) + 1;

        header->sb_size = PAGE_ALIGN(data_size);
        header->cycle_count_end = tsc;
        header->events_lost = uatomic_read(&buf->events_lost);
        header->subbuf_corrupt = uatomic_read(&buf->corrupted_subbuffers);
        if(unlikely(header->events_lost > 0)) {
                DBG("Some events (%d) were lost in %s_%d", header->events_lost, buf->chan->channel_name, buf->cpu);
        }
        /*
         * Makes sure data_size write happens after write of the rest of the
         * buffer end data, because data_size is used to identify a completely
         * written subbuffer in a crash dump.
         */
        cmm_barrier();
        header->data_size = data_size;
}

/*
 * This function should not be called from NMI interrupt context
 */
static notrace void ltt_buf_unfull(struct ust_buffer *buf,
                unsigned int subbuf_idx,
                long offset)
{
}

/*
 * Promote compiler cmm_barrier to a smp_mb().
 * For the specific LTTng case, this IPI call should be removed if the
 * architecture does not reorder writes.  This should eventually be provided by
 * a separate architecture-specific infrastructure.
 */
//ust// static void remote_mb(void *info)
//ust// {
//ust//         smp_mb();
//ust// }

int ust_buffers_get_subbuf(struct ust_buffer *buf, long *consumed)
{
        struct ust_channel *channel = buf->chan;
        long consumed_old, consumed_idx, commit_count, write_offset;
//ust// int retval;

        consumed_old = uatomic_read(&buf->consumed);
        consumed_idx = SUBBUF_INDEX(consumed_old, buf->chan);
        commit_count = uatomic_read(&buf->commit_count[consumed_idx].cc_sb);
        /*
         * Make sure we read the commit count before reading the buffer
         * data and the write offset. Correct consumed offset ordering
         * wrt commit count is insured by the use of cmpxchg to update
         * the consumed offset.
         */

        /*
         * Local rmb to match the remote wmb to read the commit count before the
         * buffer data and the write offset.
         */
        cmm_smp_rmb();

        write_offset = uatomic_read(&buf->offset);
        /*
         * Check that the subbuffer we are trying to consume has been
         * already fully committed.
         */
        if (((commit_count - buf->chan->subbuf_size)
             & channel->commit_count_mask)
            - (BUFFER_TRUNC(consumed_old, buf->chan)
               >> channel->n_subbufs_order)
            != 0) {
                return -EAGAIN;
        }
        /*
         * Check that we are not about to read the same subbuffer in
         * which the writer head is.
         */
        if ((SUBBUF_TRUNC(write_offset, buf->chan)
           - SUBBUF_TRUNC(consumed_old, buf->chan))
           == 0) {
                return -EAGAIN;
        }
        *consumed = consumed_old;

        return 0;
}

int ust_buffers_put_subbuf(struct ust_buffer *buf, unsigned long uconsumed_old)
{
        long consumed_new, consumed_old;

        consumed_old = uatomic_read(&buf->consumed);
        consumed_old = consumed_old & (~0xFFFFFFFFL);
        consumed_old = consumed_old | uconsumed_old;
        consumed_new = SUBBUF_ALIGN(consumed_old, buf->chan);

        if (uatomic_cmpxchg(&buf->consumed, consumed_old,
                                consumed_new)
            != consumed_old) {
                /* We have been pushed by the writer : the last
                 * buffer read _is_ corrupted! It can also
                 * happen if this is a buffer we never got. */
                return -EIO;
        } else {
                /* tell the client that buffer is now unfull */
                int index;
                long data;
                index = SUBBUF_INDEX(consumed_old, buf->chan);
                data = BUFFER_OFFSET(consumed_old, buf->chan);
                ltt_buf_unfull(buf, index, data);
        }
        return 0;
}

static int map_buf_structs(struct ust_channel *chan)
{
        void *ptr;
        int result;
        size_t size;
        int i;

        size = PAGE_ALIGN(1);

        for(i=0; i<chan->n_cpus; i++) {

                result = chan->buf_struct_shmids[i] = shmget(getpid(), size, IPC_CREAT | IPC_EXCL | 0700);
                if(result == -1) {
                        PERROR("shmget");
                        goto destroy_previous;
                }

                ptr = shmat(chan->buf_struct_shmids[i], NULL, 0);
                if(ptr == (void *) -1) {
                        perror("shmat");
                        goto destroy_shm;
                }

                /* Already mark the shared memory for destruction. This will occur only
                 * when all users have detached.
                 */
                result = shmctl(chan->buf_struct_shmids[i], IPC_RMID, NULL);
                if(result == -1) {
                        perror("shmctl");
                        goto destroy_previous;
                }

                chan->buf[i] = ptr;
        }

        return 0;

        /* Jumping inside this loop occurs from within the other loop above with i as
         * counter, so it unallocates the structures for the cpu = current_i down to
         * zero. */
        for(; i>=0; i--) {
                destroy_shm:
                result = shmctl(chan->buf_struct_shmids[i], IPC_RMID, NULL);
                if(result == -1) {
                        perror("shmctl");
                }

                destroy_previous:
                continue;
        }

        return -1;
}

static int unmap_buf_structs(struct ust_channel *chan)
{
        int i;

        for (i=0; i < chan->n_cpus; i++) {
                if (shmdt(chan->buf[i]) < 0) {
                        PERROR("shmdt");
                }
        }
        return 0;
}

/*
 * Create channel.
 */
static int create_channel(const char *trace_name, struct ust_trace *trace,
        const char *channel_name, struct ust_channel *chan,
        unsigned int subbuf_size, unsigned int n_subbufs, int overwrite)
{
        int i, result;

        chan->trace = trace;
        chan->overwrite = overwrite;
        chan->n_subbufs_order = get_count_order(n_subbufs);
        chan->commit_count_mask = (~0UL >> chan->n_subbufs_order);
        chan->n_cpus = get_n_cpus();

        /* These mappings should ideall be per-cpu, if somebody can do that
         * from userspace, that would be cool!
         */
        chan->buf = (void *) zmalloc(chan->n_cpus * sizeof(void *));
        if(chan->buf == NULL) {
                goto error;
        }
        chan->buf_struct_shmids = (int *) zmalloc(chan->n_cpus * sizeof(int));
        if(chan->buf_struct_shmids == NULL)
                goto free_buf;

        result = map_buf_structs(chan);
        if(result != 0) {
                goto free_buf_struct_shmids;
        }

        result = open_channel(chan, subbuf_size, n_subbufs);
        if (result != 0) {
                ERR("Cannot open channel for trace %s", trace_name);
                goto unmap_buf_structs;
        }

        return 0;

unmap_buf_structs:
        for (i=0; i < chan->n_cpus; i++) {
                if (shmdt(chan->buf[i]) < 0) {
                        PERROR("shmdt bufstruct");
                }
        }

free_buf_struct_shmids:
        free(chan->buf_struct_shmids);

free_buf:
        free(chan->buf);

error:
        return -1;
}


static void remove_channel(struct ust_channel *chan)
{
        close_channel(chan);

        unmap_buf_structs(chan);

        free(chan->buf_struct_shmids);

        free(chan->buf);
}

static void ltt_relay_async_wakeup_chan(struct ust_channel *ltt_channel)
{
}

static void ltt_relay_finish_buffer(struct ust_channel *channel, unsigned int cpu)
{
        if (channel->buf[cpu]) {
                struct ust_buffer *buf = channel->buf[cpu];
                ltt_force_switch(buf, FORCE_FLUSH);

                /* closing the pipe tells the consumer the buffer is finished */
                close(buf->data_ready_fd_write);
        }
}


static void finish_channel(struct ust_channel *channel)
{
        unsigned int i;

        for (i=0; i<channel->n_cpus; i++) {
                ltt_relay_finish_buffer(channel, i);
        }
}


/*
 * ltt_reserve_switch_old_subbuf: switch old subbuffer
 *
 * Concurrency safe because we are the last and only thread to alter this
 * sub-buffer. As long as it is not delivered and read, no other thread can
 * alter the offset, alter the reserve_count or call the
 * client_buffer_end_callback on this sub-buffer.
 *
 * The only remaining threads could be the ones with pending commits. They will
 * have to do the deliver themselves.  Not concurrency safe in overwrite mode.
 * We detect corrupted subbuffers with commit and reserve counts. We keep a
 * corrupted sub-buffers count and push the readers across these sub-buffers.
 *
 * Not concurrency safe if a writer is stalled in a subbuffer and another writer
 * switches in, finding out it's corrupted.  The result will be than the old
 * (uncommited) subbuffer will be declared corrupted, and that the new subbuffer
 * will be declared corrupted too because of the commit count adjustment.
 *
 * Note : offset_old should never be 0 here.
 */
static void ltt_reserve_switch_old_subbuf(
                struct ust_channel *chan, struct ust_buffer *buf,
                struct ltt_reserve_switch_offsets *offsets, u64 *tsc)
{
        long oldidx = SUBBUF_INDEX(offsets->old - 1, chan);
        long commit_count, padding_size;

        padding_size = chan->subbuf_size
                        - (SUBBUF_OFFSET(offsets->old - 1, chan) + 1);
        ltt_buffer_end(buf, *tsc, offsets->old, oldidx);

        /*
         * Must write slot data before incrementing commit count.
         * This compiler barrier is upgraded into a cmm_smp_wmb() by the IPI
         * sent by get_subbuf() when it does its cmm_smp_rmb().
         */
        cmm_smp_wmb();
        uatomic_add(&buf->commit_count[oldidx].cc, padding_size);
        commit_count = uatomic_read(&buf->commit_count[oldidx].cc);
        ltt_check_deliver(chan, buf, offsets->old - 1, commit_count, oldidx);
        ltt_write_commit_counter(chan, buf, oldidx,
                offsets->old, commit_count, padding_size);
}

/*
 * ltt_reserve_switch_new_subbuf: Populate new subbuffer.
 *
 * This code can be executed unordered : writers may already have written to the
 * sub-buffer before this code gets executed, caution.  The commit makes sure
 * that this code is executed before the deliver of this sub-buffer.
 */
static void ltt_reserve_switch_new_subbuf(
                struct ust_channel *chan, struct ust_buffer *buf,
                struct ltt_reserve_switch_offsets *offsets, u64 *tsc)
{
        long beginidx = SUBBUF_INDEX(offsets->begin, chan);
        long commit_count;

        ltt_buffer_begin(buf, *tsc, beginidx);

        /*
         * Must write slot data before incrementing commit count.
         * This compiler barrier is upgraded into a cmm_smp_wmb() by the IPI
         * sent by get_subbuf() when it does its cmm_smp_rmb().
         */
        cmm_smp_wmb();
        uatomic_add(&buf->commit_count[beginidx].cc, ltt_subbuffer_header_size());
        commit_count = uatomic_read(&buf->commit_count[beginidx].cc);
        /* Check if the written buffer has to be delivered */
        ltt_check_deliver(chan, buf, offsets->begin, commit_count, beginidx);
        ltt_write_commit_counter(chan, buf, beginidx,
                offsets->begin, commit_count, ltt_subbuffer_header_size());
}

/*
 * ltt_reserve_end_switch_current: finish switching current subbuffer
 *
 * Concurrency safe because we are the last and only thread to alter this
 * sub-buffer. As long as it is not delivered and read, no other thread can
 * alter the offset, alter the reserve_count or call the
 * client_buffer_end_callback on this sub-buffer.
 *
 * The only remaining threads could be the ones with pending commits. They will
 * have to do the deliver themselves.  Not concurrency safe in overwrite mode.
 * We detect corrupted subbuffers with commit and reserve counts. We keep a
 * corrupted sub-buffers count and push the readers across these sub-buffers.
 *
 * Not concurrency safe if a writer is stalled in a subbuffer and another writer
 * switches in, finding out it's corrupted.  The result will be than the old
 * (uncommited) subbuffer will be declared corrupted, and that the new subbuffer
 * will be declared corrupted too because of the commit count adjustment.
 */
static void ltt_reserve_end_switch_current(
                struct ust_channel *chan,
                struct ust_buffer *buf,
                struct ltt_reserve_switch_offsets *offsets, u64 *tsc)
{
        long endidx = SUBBUF_INDEX(offsets->end - 1, chan);
        long commit_count, padding_size;

        padding_size = chan->subbuf_size
                        - (SUBBUF_OFFSET(offsets->end - 1, chan) + 1);

        ltt_buffer_end(buf, *tsc, offsets->end, endidx);

        /*
         * Must write slot data before incrementing commit count.
         * This compiler barrier is upgraded into a cmm_smp_wmb() by the IPI
         * sent by get_subbuf() when it does its cmm_smp_rmb().
         */
        cmm_smp_wmb();
        uatomic_add(&buf->commit_count[endidx].cc, padding_size);
        commit_count = uatomic_read(&buf->commit_count[endidx].cc);
        ltt_check_deliver(chan, buf,
                offsets->end - 1, commit_count, endidx);
        ltt_write_commit_counter(chan, buf, endidx,
                offsets->end, commit_count, padding_size);
}

/*
 * Returns :
 * 0 if ok
 * !0 if execution must be aborted.
 */
static int ltt_relay_try_switch_slow(
                enum force_switch_mode mode,
                struct ust_channel *chan,
                struct ust_buffer *buf,
                struct ltt_reserve_switch_offsets *offsets,
                u64 *tsc)
{
        long subbuf_index;
        long reserve_commit_diff;

        offsets->begin = uatomic_read(&buf->offset);
        offsets->old = offsets->begin;
        offsets->begin_switch = 0;
        offsets->end_switch_old = 0;

        *tsc = trace_clock_read64();

        if (SUBBUF_OFFSET(offsets->begin, buf->chan) != 0) {
                offsets->begin = SUBBUF_ALIGN(offsets->begin, buf->chan);
                offsets->end_switch_old = 1;
        } else {
                /* we do not have to switch : buffer is empty */
                return -1;
        }
        if (mode == FORCE_ACTIVE)
                offsets->begin += ltt_subbuffer_header_size();
        /*
         * Always begin_switch in FORCE_ACTIVE mode.
         * Test new buffer integrity
         */
        subbuf_index = SUBBUF_INDEX(offsets->begin, buf->chan);
        reserve_commit_diff =
                (BUFFER_TRUNC(offsets->begin, buf->chan)
                 >> chan->n_subbufs_order)
                - (uatomic_read(&buf->commit_count[subbuf_index].cc_sb)
                        & chan->commit_count_mask);
        if (reserve_commit_diff == 0) {
                /* Next buffer not corrupted. */
                if (mode == FORCE_ACTIVE
                    && !chan->overwrite
                    && offsets->begin - uatomic_read(&buf->consumed)
                       >= chan->alloc_size) {
                        /*
                         * We do not overwrite non consumed buffers and we are
                         * full : ignore switch while tracing is active.
                         */
                        return -1;
                }
        } else {
                /*
                 * Next subbuffer corrupted. Force pushing reader even in normal
                 * mode
                 */
        }
        offsets->end = offsets->begin;
        return 0;
}

/*
 * Force a sub-buffer switch for a per-cpu buffer. This operation is
 * completely reentrant : can be called while tracing is active with
 * absolutely no lock held.
 */
void ltt_force_switch_lockless_slow(struct ust_buffer *buf,
                enum force_switch_mode mode)
{
        struct ust_channel *chan = buf->chan;
        struct ltt_reserve_switch_offsets offsets;
        u64 tsc;

        offsets.size = 0;

        DBG("Switching (forced) %s_%d", chan->channel_name, buf->cpu);
        /*
         * Perform retryable operations.
         */
        do {
                if (ltt_relay_try_switch_slow(mode, chan, buf,
                                &offsets, &tsc))
                        return;
        } while (uatomic_cmpxchg(&buf->offset, offsets.old,
                        offsets.end) != offsets.old);

        /*
         * Atomically update last_tsc. This update races against concurrent
         * atomic updates, but the race will always cause supplementary full TSC
         * events, never the opposite (missing a full TSC event when it would be
         * needed).
         */
        save_last_tsc(buf, tsc);

        /*
         * Push the reader if necessary
         */
        if (mode == FORCE_ACTIVE) {
                ltt_reserve_push_reader(chan, buf, offsets.end - 1);
        }

        /*
         * Switch old subbuffer if needed.
         */
        if (offsets.end_switch_old) {
                ltt_reserve_switch_old_subbuf(chan, buf, &offsets, &tsc);
        }

        /*
         * Populate new subbuffer.
         */
        if (mode == FORCE_ACTIVE)
                ltt_reserve_switch_new_subbuf(chan, buf, &offsets, &tsc);
}

/*
 * Returns :
 * 0 if ok
 * !0 if execution must be aborted.
 */
static int ltt_relay_try_reserve_slow(struct ust_channel *chan, struct ust_buffer *buf,
                struct ltt_reserve_switch_offsets *offsets, size_t data_size,
                u64 *tsc, unsigned int *rflags, int largest_align)
{
        long reserve_commit_diff;

        offsets->begin = uatomic_read(&buf->offset);
        offsets->old = offsets->begin;
        offsets->begin_switch = 0;
        offsets->end_switch_current = 0;
        offsets->end_switch_old = 0;

        *tsc = trace_clock_read64();
        if (last_tsc_overflow(buf, *tsc))
                *rflags = LTT_RFLAG_ID_SIZE_TSC;

        if (unlikely(SUBBUF_OFFSET(offsets->begin, buf->chan) == 0)) {
                offsets->begin_switch = 1;              /* For offsets->begin */
        } else {
                offsets->size = ust_get_header_size(chan,
                                        offsets->begin, data_size,
                                        &offsets->before_hdr_pad, *rflags);
                offsets->size += ltt_align(offsets->begin + offsets->size,
                                           largest_align)
                                 + data_size;
                if (unlikely((SUBBUF_OFFSET(offsets->begin, buf->chan) +
                             offsets->size) > buf->chan->subbuf_size)) {
                        offsets->end_switch_old = 1;    /* For offsets->old */
                        offsets->begin_switch = 1;      /* For offsets->begin */
                }
        }
        if (unlikely(offsets->begin_switch)) {
                long subbuf_index;

                /*
                 * We are typically not filling the previous buffer completely.
                 */
                if (likely(offsets->end_switch_old))
                        offsets->begin = SUBBUF_ALIGN(offsets->begin,
                                                      buf->chan);
                offsets->begin = offsets->begin + ltt_subbuffer_header_size();
                /* Test new buffer integrity */
                subbuf_index = SUBBUF_INDEX(offsets->begin, buf->chan);
                reserve_commit_diff =
                  (BUFFER_TRUNC(offsets->begin, buf->chan)
                   >> chan->n_subbufs_order)
                  - (uatomic_read(&buf->commit_count[subbuf_index].cc_sb)
                                & chan->commit_count_mask);
                if (likely(reserve_commit_diff == 0)) {
                        /* Next buffer not corrupted. */
                        if (unlikely(!chan->overwrite &&
                                (SUBBUF_TRUNC(offsets->begin, buf->chan)
                                 - SUBBUF_TRUNC(uatomic_read(
                                                        &buf->consumed),
                                                buf->chan))
                                >= chan->alloc_size)) {
                                /*
                                 * We do not overwrite non consumed buffers
                                 * and we are full : event is lost.
                                 */
                                uatomic_inc(&buf->events_lost);
                                return -1;
                        } else {
                                /*
                                 * next buffer not corrupted, we are either in
                                 * overwrite mode or the buffer is not full.
                                 * It's safe to write in this new subbuffer.
                                 */
                        }
                } else {
                        /*
                         * Next subbuffer corrupted. Drop event in normal and
                         * overwrite mode. Caused by either a writer OOPS or
                         * too many nested writes over a reserve/commit pair.
                         */
                        uatomic_inc(&buf->events_lost);
                        return -1;
                }
                offsets->size = ust_get_header_size(chan,
                                        offsets->begin, data_size,
                                        &offsets->before_hdr_pad, *rflags);
                offsets->size += ltt_align(offsets->begin + offsets->size,
                                           largest_align)
                                 + data_size;
                if (unlikely((SUBBUF_OFFSET(offsets->begin, buf->chan)
                             + offsets->size) > buf->chan->subbuf_size)) {
                        /*
                         * Event too big for subbuffers, report error, don't
                         * complete the sub-buffer switch.
                         */
                        uatomic_inc(&buf->events_lost);
                        return -1;
                } else {
                        /*
                         * We just made a successful buffer switch and the event
                         * fits in the new subbuffer. Let's write.
                         */
                }
        } else {
                /*
                 * Event fits in the current buffer and we are not on a switch
                 * boundary. It's safe to write.
                 */
        }
        offsets->end = offsets->begin + offsets->size;

        if (unlikely((SUBBUF_OFFSET(offsets->end, buf->chan)) == 0)) {
                /*
                 * The offset_end will fall at the very beginning of the next
                 * subbuffer.
                 */
                offsets->end_switch_current = 1;        /* For offsets->begin */
        }
        return 0;
}

/**
 * ltt_relay_reserve_slot_lockless_slow - Atomic slot reservation in a buffer.
 * @trace: the trace structure to log to.
 * @ltt_channel: channel structure
 * @transport_data: data structure specific to ltt relay
 * @data_size: size of the variable length data to log.
 * @slot_size: pointer to total size of the slot (out)
 * @buf_offset : pointer to reserved buffer offset (out)
 * @tsc: pointer to the tsc at the slot reservation (out)
 * @cpu: cpuid
 *
 * Return : -ENOSPC if not enough space, else returns 0.
 * It will take care of sub-buffer switching.
 */
int ltt_reserve_slot_lockless_slow(struct ust_channel *chan,
                struct ust_trace *trace, size_t data_size,
                int largest_align, int cpu,
                struct ust_buffer **ret_buf,
                size_t *slot_size, long *buf_offset,
                u64 *tsc, unsigned int *rflags)
{
        struct ust_buffer *buf = *ret_buf = chan->buf[cpu];
        struct ltt_reserve_switch_offsets offsets;

        offsets.size = 0;

        do {
                if (unlikely(ltt_relay_try_reserve_slow(chan, buf, &offsets,
                                data_size, tsc, rflags, largest_align)))
                        return -ENOSPC;
        } while (unlikely(uatomic_cmpxchg(&buf->offset, offsets.old,
                        offsets.end) != offsets.old));

        /*
         * Atomically update last_tsc. This update races against concurrent
         * atomic updates, but the race will always cause supplementary full TSC
         * events, never the opposite (missing a full TSC event when it would be
         * needed).
         */
        save_last_tsc(buf, *tsc);

        /*
         * Push the reader if necessary
         */
        ltt_reserve_push_reader(chan, buf, offsets.end - 1);

        /*
         * Switch old subbuffer if needed.
         */
        if (unlikely(offsets.end_switch_old)) {
                ltt_reserve_switch_old_subbuf(chan, buf, &offsets, tsc);
                DBG("Switching %s_%d", chan->channel_name, cpu);
        }

        /*
         * Populate new subbuffer.
         */
        if (unlikely(offsets.begin_switch))
                ltt_reserve_switch_new_subbuf(chan, buf, &offsets, tsc);

        if (unlikely(offsets.end_switch_current))
                ltt_reserve_end_switch_current(chan, buf, &offsets, tsc);

        *slot_size = offsets.size;
        *buf_offset = offsets.begin + offsets.before_hdr_pad;
        return 0;
}

static struct ltt_transport ust_relay_transport = {
        .name = "ustrelay",
        .ops = {
                .create_channel = create_channel,
                .finish_channel = finish_channel,
                .remove_channel = remove_channel,
                .wakeup_channel = ltt_relay_async_wakeup_chan,
        },
};

static char initialized = 0;

void __attribute__((constructor)) init_ustrelay_transport(void)
{
        init_usterr();
        if(!initialized) {
                ltt_transport_register(&ust_relay_transport);
                initialized = 1;
        }
}

static void __attribute__((destructor)) ust_buffers_exit(void)
{
        ltt_transport_unregister(&ust_relay_transport);
}

size_t ltt_write_event_header_slow(struct ust_channel *channel,
                struct ust_buffer *buf, long buf_offset,
                u16 eID, u32 event_size,
                u64 tsc, unsigned int rflags)
{
        struct ltt_event_header header;
        u16 small_size;

        switch (rflags) {
        case LTT_RFLAG_ID_SIZE_TSC:
                header.id_time = 29 << LTT_TSC_BITS;
                break;
        case LTT_RFLAG_ID_SIZE:
                header.id_time = 30 << LTT_TSC_BITS;
                break;
        case LTT_RFLAG_ID:
                header.id_time = 31 << LTT_TSC_BITS;
                break;
        default:
                WARN_ON_ONCE(1);
                header.id_time = 0;
                break;
        }

        header.id_time |= (u32)tsc & LTT_TSC_MASK;
        ust_buffers_write(buf, buf_offset, &header, sizeof(header));
        buf_offset += sizeof(header);

        switch (rflags) {
        case LTT_RFLAG_ID_SIZE_TSC:
                small_size = (u16)min_t(u32, event_size, LTT_MAX_SMALL_SIZE);
                ust_buffers_write(buf, buf_offset,
                        &eID, sizeof(u16));
                buf_offset += sizeof(u16);
                ust_buffers_write(buf, buf_offset,
                        &small_size, sizeof(u16));
                buf_offset += sizeof(u16);
                if (small_size == LTT_MAX_SMALL_SIZE) {
                        ust_buffers_write(buf, buf_offset,
                                &event_size, sizeof(u32));
                        buf_offset += sizeof(u32);
                }
                buf_offset += ltt_align(buf_offset, sizeof(u64));
                ust_buffers_write(buf, buf_offset,
                        &tsc, sizeof(u64));
                buf_offset += sizeof(u64);
                break;
        case LTT_RFLAG_ID_SIZE:
                small_size = (u16)min_t(u32, event_size, LTT_MAX_SMALL_SIZE);
                ust_buffers_write(buf, buf_offset,
                        &eID, sizeof(u16));
                buf_offset += sizeof(u16);
                ust_buffers_write(buf, buf_offset,
                        &small_size, sizeof(u16));
                buf_offset += sizeof(u16);
                if (small_size == LTT_MAX_SMALL_SIZE) {
                        ust_buffers_write(buf, buf_offset,
                                &event_size, sizeof(u32));
                        buf_offset += sizeof(u32);
                }
                break;
        case LTT_RFLAG_ID:
                ust_buffers_write(buf, buf_offset,
                        &eID, sizeof(u16));
                buf_offset += sizeof(u16);
                break;
        }

        return buf_offset;
}