xt_log.h

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#define S_SIZE (1024 - (sizeof(unsigned int) + 1))

struct sbuff {
    unsigned int    count;
    char        buf[S_SIZE + 1];
};
static struct sbuff emergency, *emergency_ptr = &emergency;

static __printf(2, 3) int sb_add(struct sbuff *m, const char *f, ...)
{
    va_list args;
    int len;

    if (likely(m->count < S_SIZE)) {
        va_start(args, f);
        len = vsnprintf(m->buf + m->count, S_SIZE - m->count, f, args);
        va_end(args);
        if (likely(m->count + len < S_SIZE)) {
            m->count += len;
            return 0;
        }
    }
    m->count = S_SIZE;
    printk_once(KERN_ERR KBUILD_MODNAME " please increase S_SIZE\n");
    return -1;
}

static struct sbuff *sb_open(void)
{
    struct sbuff *m = kmalloc(sizeof(*m), GFP_ATOMIC);

    if (unlikely(!m)) {
        local_bh_disable();
        do {
            m = xchg(&emergency_ptr, NULL);
        } while (!m);
    }
    m->count = 0;
    return m;
}

static void sb_close(struct sbuff *m)
{
    m->buf[m->count] = 0;
    printk("%s\n", m->buf);

    if (likely(m != &emergency))
        kfree(m);
    else {
        emergency_ptr = m;
        local_bh_enable();
    }
}