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[mirror_ubuntu-bionic-kernel.git] / kernel / printk / printk_safe.c

/*
 * printk_safe.c - Safe printk for printk-deadlock-prone contexts
 *
 * 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.
 *
 * This program 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 General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, see <http://www.gnu.org/licenses/>.
 */

#include <linux/preempt.h>
#include <linux/spinlock.h>
#include <linux/debug_locks.h>
#include <linux/smp.h>
#include <linux/cpumask.h>
#include <linux/irq_work.h>
#include <linux/printk.h>

#include "internal.h"

/*
 * printk() could not take logbuf_lock in NMI context. Instead,
 * it uses an alternative implementation that temporary stores
 * the strings into a per-CPU buffer. The content of the buffer
 * is later flushed into the main ring buffer via IRQ work.
 *
 * The alternative implementation is chosen transparently
 * by examinig current printk() context mask stored in @printk_context
 * per-CPU variable.
 *
 * The implementation allows to flush the strings also from another CPU.
 * There are situations when we want to make sure that all buffers
 * were handled or when IRQs are blocked.
 */
static int printk_safe_irq_ready __read_mostly;

#define SAFE_LOG_BUF_LEN ((1 << CONFIG_PRINTK_SAFE_LOG_BUF_SHIFT) -     \
                                sizeof(atomic_t) -                      \
                                sizeof(atomic_t) -                      \
                                sizeof(struct irq_work))

struct printk_safe_seq_buf {
        atomic_t                len;    /* length of written data */
        atomic_t                message_lost;
        struct irq_work         work;   /* IRQ work that flushes the buffer */
        unsigned char           buffer[SAFE_LOG_BUF_LEN];
};

static DEFINE_PER_CPU(struct printk_safe_seq_buf, safe_print_seq);
static DEFINE_PER_CPU(int, printk_context);

#ifdef CONFIG_PRINTK_NMI
static DEFINE_PER_CPU(struct printk_safe_seq_buf, nmi_print_seq);
#endif

/* Get flushed in a more safe context. */
static void queue_flush_work(struct printk_safe_seq_buf *s)
{
        if (printk_safe_irq_ready)
                irq_work_queue(&s->work);
}

/*
 * Add a message to per-CPU context-dependent buffer. NMI and printk-safe
 * have dedicated buffers, because otherwise printk-safe preempted by
 * NMI-printk would have overwritten the NMI messages.
 *
 * The messages are flushed from irq work (or from panic()), possibly,
 * from other CPU, concurrently with printk_safe_log_store(). Should this
 * happen, printk_safe_log_store() will notice the buffer->len mismatch
 * and repeat the write.
 */
static __printf(2, 0) int printk_safe_log_store(struct printk_safe_seq_buf *s,
                                                const char *fmt, va_list args)
{
        int add;
        size_t len;

again:
        len = atomic_read(&s->len);

        /* The trailing '\0' is not counted into len. */
        if (len >= sizeof(s->buffer) - 1) {
                atomic_inc(&s->message_lost);
                queue_flush_work(s);
                return 0;
        }

        /*
         * Make sure that all old data have been read before the buffer
         * was reset. This is not needed when we just append data.
         */
        if (!len)
                smp_rmb();

        add = vscnprintf(s->buffer + len, sizeof(s->buffer) - len, fmt, args);
        if (!add)
                return 0;

        /*
         * Do it once again if the buffer has been flushed in the meantime.
         * Note that atomic_cmpxchg() is an implicit memory barrier that
         * makes sure that the data were written before updating s->len.
         */
        if (atomic_cmpxchg(&s->len, len, len + add) != len)
                goto again;

        queue_flush_work(s);
        return add;
}

static inline void printk_safe_flush_line(const char *text, int len)
{
        /*
         * Avoid any console drivers calls from here, because we may be
         * in NMI or printk_safe context (when in panic). The messages
         * must go only into the ring buffer at this stage.  Consoles will
         * get explicitly called later when a crashdump is not generated.
         */
        printk_deferred("%.*s", len, text);
}

/* printk part of the temporary buffer line by line */
static int printk_safe_flush_buffer(const char *start, size_t len)
{
        const char *c, *end;
        bool header;

        c = start;
        end = start + len;
        header = true;

        /* Print line by line. */
        while (c < end) {
                if (*c == '\n') {
                        printk_safe_flush_line(start, c - start + 1);
                        start = ++c;
                        header = true;
                        continue;
                }

                /* Handle continuous lines or missing new line. */
                if ((c + 1 < end) && printk_get_level(c)) {
                        if (header) {
                                c = printk_skip_level(c);
                                continue;
                        }

                        printk_safe_flush_line(start, c - start);
                        start = c++;
                        header = true;
                        continue;
                }

                header = false;
                c++;
        }

        /* Check if there was a partial line. Ignore pure header. */
        if (start < end && !header) {
                static const char newline[] = KERN_CONT "\n";

                printk_safe_flush_line(start, end - start);
                printk_safe_flush_line(newline, strlen(newline));
        }

        return len;
}

static void report_message_lost(struct printk_safe_seq_buf *s)
{
        int lost = atomic_xchg(&s->message_lost, 0);

        if (lost)
                printk_deferred("Lost %d message(s)!\n", lost);
}

/*
 * Flush data from the associated per-CPU buffer. The function
 * can be called either via IRQ work or independently.
 */
static void __printk_safe_flush(struct irq_work *work)
{
        static raw_spinlock_t read_lock =
                __RAW_SPIN_LOCK_INITIALIZER(read_lock);
        struct printk_safe_seq_buf *s =
                container_of(work, struct printk_safe_seq_buf, work);
        unsigned long flags;
        size_t len;
        int i;

        /*
         * The lock has two functions. First, one reader has to flush all
         * available message to make the lockless synchronization with
         * writers easier. Second, we do not want to mix messages from
         * different CPUs. This is especially important when printing
         * a backtrace.
         */
        raw_spin_lock_irqsave(&read_lock, flags);

        i = 0;
more:
        len = atomic_read(&s->len);

        /*
         * This is just a paranoid check that nobody has manipulated
         * the buffer an unexpected way. If we printed something then
         * @len must only increase. Also it should never overflow the
         * buffer size.
         */
        if ((i && i >= len) || len > sizeof(s->buffer)) {
                const char *msg = "printk_safe_flush: internal error\n";

                printk_safe_flush_line(msg, strlen(msg));
                len = 0;
        }

        if (!len)
                goto out; /* Someone else has already flushed the buffer. */

        /* Make sure that data has been written up to the @len */
        smp_rmb();
        i += printk_safe_flush_buffer(s->buffer + i, len - i);

        /*
         * Check that nothing has got added in the meantime and truncate
         * the buffer. Note that atomic_cmpxchg() is an implicit memory
         * barrier that makes sure that the data were copied before
         * updating s->len.
         */
        if (atomic_cmpxchg(&s->len, len, 0) != len)
                goto more;

out:
        report_message_lost(s);
        raw_spin_unlock_irqrestore(&read_lock, flags);
}

/**
 * printk_safe_flush - flush all per-cpu nmi buffers.
 *
 * The buffers are flushed automatically via IRQ work. This function
 * is useful only when someone wants to be sure that all buffers have
 * been flushed at some point.
 */
void printk_safe_flush(void)
{
        int cpu;

        for_each_possible_cpu(cpu) {
#ifdef CONFIG_PRINTK_NMI
                __printk_safe_flush(&per_cpu(nmi_print_seq, cpu).work);
#endif
                __printk_safe_flush(&per_cpu(safe_print_seq, cpu).work);
        }
}

/**
 * printk_safe_flush_on_panic - flush all per-cpu nmi buffers when the system
 *      goes down.
 *
 * Similar to printk_safe_flush() but it can be called even in NMI context when
 * the system goes down. It does the best effort to get NMI messages into
 * the main ring buffer.
 *
 * Note that it could try harder when there is only one CPU online.
 */
void printk_safe_flush_on_panic(void)
{
        /*
         * Make sure that we could access the main ring buffer.
         * Do not risk a double release when more CPUs are up.
         */
        if (in_nmi() && raw_spin_is_locked(&logbuf_lock)) {
                if (num_online_cpus() > 1)
                        return;

                debug_locks_off();
                raw_spin_lock_init(&logbuf_lock);
        }

        printk_safe_flush();
}

#ifdef CONFIG_PRINTK_NMI
/*
 * Safe printk() for NMI context. It uses a per-CPU buffer to
 * store the message. NMIs are not nested, so there is always only
 * one writer running. But the buffer might get flushed from another
 * CPU, so we need to be careful.
 */
static __printf(1, 0) int vprintk_nmi(const char *fmt, va_list args)
{
        struct printk_safe_seq_buf *s = this_cpu_ptr(&nmi_print_seq);

        return printk_safe_log_store(s, fmt, args);
}

void printk_nmi_enter(void)
{
        /*
         * The size of the extra per-CPU buffer is limited. Use it only when
         * the main one is locked. If this CPU is not in the safe context,
         * the lock must be taken on another CPU and we could wait for it.
         */
        if ((this_cpu_read(printk_context) & PRINTK_SAFE_CONTEXT_MASK) &&
            raw_spin_is_locked(&logbuf_lock)) {
                this_cpu_or(printk_context, PRINTK_NMI_CONTEXT_MASK);
        } else {
                this_cpu_or(printk_context, PRINTK_NMI_DEFERRED_CONTEXT_MASK);
        }
}

void printk_nmi_exit(void)
{
        this_cpu_and(printk_context,
                     ~(PRINTK_NMI_CONTEXT_MASK |
                       PRINTK_NMI_DEFERRED_CONTEXT_MASK));
}

#else

static __printf(1, 0) int vprintk_nmi(const char *fmt, va_list args)
{
        return 0;
}

#endif /* CONFIG_PRINTK_NMI */

/*
 * Lock-less printk(), to avoid deadlocks should the printk() recurse
 * into itself. It uses a per-CPU buffer to store the message, just like
 * NMI.
 */
static __printf(1, 0) int vprintk_safe(const char *fmt, va_list args)
{
        struct printk_safe_seq_buf *s = this_cpu_ptr(&safe_print_seq);

        return printk_safe_log_store(s, fmt, args);
}

/* Can be preempted by NMI. */
void __printk_safe_enter(void)
{
        this_cpu_inc(printk_context);
}

/* Can be preempted by NMI. */
void __printk_safe_exit(void)
{
        this_cpu_dec(printk_context);
}

__printf(1, 0) int vprintk_func(const char *fmt, va_list args)
{
        /* Use extra buffer in NMI when logbuf_lock is taken or in safe mode. */
        if (this_cpu_read(printk_context) & PRINTK_NMI_CONTEXT_MASK)
                return vprintk_nmi(fmt, args);

        /* Use extra buffer to prevent a recursion deadlock in safe mode. */
        if (this_cpu_read(printk_context) & PRINTK_SAFE_CONTEXT_MASK)
                return vprintk_safe(fmt, args);

        /*
         * Use the main logbuf when logbuf_lock is available in NMI.
         * But avoid calling console drivers that might have their own locks.
         */
        if (this_cpu_read(printk_context) & PRINTK_NMI_DEFERRED_CONTEXT_MASK)
                return vprintk_deferred(fmt, args);

        /* No obstacles. */
        return vprintk_default(fmt, args);
}

void __init printk_safe_init(void)
{
        int cpu;

        for_each_possible_cpu(cpu) {
                struct printk_safe_seq_buf *s;

                s = &per_cpu(safe_print_seq, cpu);
                init_irq_work(&s->work, __printk_safe_flush);

#ifdef CONFIG_PRINTK_NMI
                s = &per_cpu(nmi_print_seq, cpu);
                init_irq_work(&s->work, __printk_safe_flush);
#endif
        }

        /*
         * In the highly unlikely event that a NMI were to trigger at
         * this moment. Make sure IRQ work is set up before this
         * variable is set.
         */
        barrier();
        printk_safe_irq_ready = 1;

        /* Flush pending messages that did not have scheduled IRQ works. */
        printk_safe_flush();
}