kmsg: export printk records to the /dev/kmsg interface

Support for multiple concurrent readers of /dev/kmsg, with read(),
seek(), poll() support. Output of message sequence numbers, to allow
userspace log consumers to reliably reconnect and reconstruct their
state at any given time. After open("/dev/kmsg"), read() always
returns *all* buffered records. If only future messages should be
read, SEEK_END can be used. In case records get overwritten while
/dev/kmsg is held open, or records get faster overwritten than they
are read, the next read() will return -EPIPE and the current reading
position gets updated to the next available record. The passed
sequence numbers allow the log consumer to calculate the amount of
lost messages.

  [root@mop ~]# cat /dev/kmsg
  5,0,0;Linux version 3.4.0-rc1+ (kay@mop) (gcc version 4.7.0 20120315 ...
  6,159,423091;ACPI: PCI Root Bridge [PCI0] (domain 0000 [bus 00-ff])
  7,160,424069;pci_root PNP0A03:00: host bridge window [io  0x0000-0x0cf7] (ignored)
   SUBSYSTEM=acpi
   DEVICE=+acpi:PNP0A03:00
  6,339,5140900;NET: Registered protocol family 10
  30,340,5690716;udevd[80]: starting version 181
  6,341,6081421;FDC 0 is a S82078B
  6,345,6154686;microcode: CPU0 sig=0x623, pf=0x0, revision=0x0
  7,346,6156968;sr 1:0:0:0: Attached scsi CD-ROM sr0
   SUBSYSTEM=scsi
   DEVICE=+scsi:1:0:0:0
  6,347,6289375;microcode: CPU1 sig=0x623, pf=0x0, revision=0x0

Cc: Karel Zak <kzak@redhat.com>
Tested-by: William Douglas <william.douglas@intel.com>
Signed-off-by: Kay Sievers <kay@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 313 insertions, 0 deletions

diff --git a/kernel/printk.c b/kernel/printk.c
index 74357329550f..1ccc6d986cb3 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -41,6 +41,7 @@
 #include <linux/cpu.h>
 #include <linux/notifier.h>
 #include <linux/rculist.h>
+#include <linux/poll.h>
 
 #include <asm/uaccess.h>
 
@@ -149,6 +150,48 @@ static int console_may_schedule;
  * length of the message text is stored in the header, the stored message
  * is not terminated.
  *
+ * Optionally, a message can carry a dictionary of properties (key/value pairs),
+ * to provide userspace with a machine-readable message context.
+ *
+ * Examples for well-defined, commonly used property names are:
+ *   DEVICE=b12:8               device identifier
+ *                                b12:8         block dev_t
+ *                                c127:3        char dev_t
+ *                                n8            netdev ifindex
+ *                                +sound:card0  subsystem:devname
+ *   SUBSYSTEM=pci              driver-core subsystem name
+ *
+ * Valid characters in property names are [a-zA-Z0-9.-_]. The plain text value
+ * follows directly after a '=' character. Every property is terminated by
+ * a '\0' character. The last property is not terminated.
+ *
+ * Example of a message structure:
+ *   0000  ff 8f 00 00 00 00 00 00      monotonic time in nsec
+ *   0008  34 00                        record is 52 bytes long
+ *   000a        0b 00                  text is 11 bytes long
+ *   000c              1f 00            dictionary is 23 bytes long
+ *   000e                    03 00      LOG_KERN (facility) LOG_ERR (level)
+ *   0010  69 74 27 73 20 61 20 6c      "it's a l"
+ *         69 6e 65                     "ine"
+ *   001b           44 45 56 49 43      "DEVIC"
+ *         45 3d 62 38 3a 32 00 44      "E=b8:2\0D"
+ *         52 49 56 45 52 3d 62 75      "RIVER=bu"
+ *         67                           "g"
+ *   0032     00 00 00                  padding to next message header
+ *
+ * The 'struct log' buffer header must never be directly exported to
+ * userspace, it is a kernel-private implementation detail that might
+ * need to be changed in the future, when the requirements change.
+ *
+ * /dev/kmsg exports the structured data in the following line format:
+ *   "level,sequnum,timestamp;<message text>\n"
+ *
+ * The optional key/value pairs are attached as continuation lines starting
+ * with a space character and terminated by a newline. All possible
+ * non-prinatable characters are escaped in the "\xff" notation.
+ *
+ * Users of the export format should ignore possible additional values
+ * separated by ',', and find the message after the ';' character.
  */
 
 struct log {
@@ -297,6 +340,276 @@ static void log_store(int facility, int level,
         log_next_seq++;
 }
 
+/* /dev/kmsg - userspace message inject/listen interface */
+struct devkmsg_user {
+        u64 seq;
+        u32 idx;
+        struct mutex lock;
+        char buf[8192];
+};
+
+static ssize_t devkmsg_writev(struct kiocb *iocb, const struct iovec *iv,
+                              unsigned long count, loff_t pos)
+{
+        char *buf, *line;
+        int i;
+        int level = default_message_loglevel;
+        int facility = 1;       /* LOG_USER */
+        size_t len = iov_length(iv, count);
+        ssize_t ret = len;
+
+        if (len > LOG_LINE_MAX)
+                return -EINVAL;
+        buf = kmalloc(len+1, GFP_KERNEL);
+        if (buf == NULL)
+                return -ENOMEM;
+
+        line = buf;
+        for (i = 0; i < count; i++) {
+                if (copy_from_user(line, iv[i].iov_base, iv[i].iov_len))
+                        goto out;
+                line += iv[i].iov_len;
+        }
+
+        /*
+         * Extract and skip the syslog prefix <[0-9]*>. Coming from userspace
+         * the decimal value represents 32bit, the lower 3 bit are the log
+         * level, the rest are the log facility.
+         *
+         * If no prefix or no userspace facility is specified, we
+         * enforce LOG_USER, to be able to reliably distinguish
+         * kernel-generated messages from userspace-injected ones.
+         */
+        line = buf;
+        if (line[0] == '<') {
+                char *endp = NULL;
+
+                i = simple_strtoul(line+1, &endp, 10);
+                if (endp && endp[0] == '>') {
+                        level = i & 7;
+                        if (i >> 3)
+                                facility = i >> 3;
+                        endp++;
+                        len -= endp - line;
+                        line = endp;
+                }
+        }
+        line[len] = '\0';
+
+        printk_emit(facility, level, NULL, 0, "%s", line);
+out:
+        kfree(buf);
+        return ret;
+}
+
+static ssize_t devkmsg_read(struct file *file, char __user *buf,
+                            size_t count, loff_t *ppos)
+{
+        struct devkmsg_user *user = file->private_data;
+        struct log *msg;
+        size_t i;
+        size_t len;
+        ssize_t ret;
+
+        if (!user)
+                return -EBADF;
+
+        mutex_lock(&user->lock);
+        raw_spin_lock(&logbuf_lock);
+        while (user->seq == log_next_seq) {
+                if (file->f_flags & O_NONBLOCK) {
+                        ret = -EAGAIN;
+                        raw_spin_unlock(&logbuf_lock);
+                        goto out;
+                }
+
+                raw_spin_unlock(&logbuf_lock);
+                ret = wait_event_interruptible(log_wait,
+                                               user->seq != log_next_seq);
+                if (ret)
+                        goto out;
+                raw_spin_lock(&logbuf_lock);
+        }
+
+        if (user->seq < log_first_seq) {
+                /* our last seen message is gone, return error and reset */
+                user->idx = log_first_idx;
+                user->seq = log_first_seq;
+                ret = -EPIPE;
+                raw_spin_unlock(&logbuf_lock);
+                goto out;
+        }
+
+        msg = log_from_idx(user->idx);
+        len = sprintf(user->buf, "%u,%llu,%llu;",
+                      msg->level, user->seq, msg->ts_nsec / 1000);
+
+        /* escape non-printable characters */
+        for (i = 0; i < msg->text_len; i++) {
+                char c = log_text(msg)[i];
+
+                if (c < ' ' || c >= 128)
+                        len += sprintf(user->buf + len, "\\x%02x", c);
+                else
+                        user->buf[len++] = c;
+        }
+        user->buf[len++] = '\n';
+
+        if (msg->dict_len) {
+                bool line = true;
+
+                for (i = 0; i < msg->dict_len; i++) {
+                        char c = log_dict(msg)[i];
+
+                        if (line) {
+                                user->buf[len++] = ' ';
+                                line = false;
+                        }
+
+                        if (c == '\0') {
+                                user->buf[len++] = '\n';
+                                line = true;
+                                continue;
+                        }
+
+                        if (c < ' ' || c >= 128) {
+                                len += sprintf(user->buf + len, "\\x%02x", c);
+                                continue;
+                        }
+
+                        user->buf[len++] = c;
+                }
+                user->buf[len++] = '\n';
+        }
+
+        user->idx = log_next(user->idx);
+        user->seq++;
+        raw_spin_unlock(&logbuf_lock);
+
+        if (len > count) {
+                ret = -EINVAL;
+                goto out;
+        }
+
+        if (copy_to_user(buf, user->buf, len)) {
+                ret = -EFAULT;
+                goto out;
+        }
+        ret = len;
+out:
+        mutex_unlock(&user->lock);
+        return ret;
+}
+
+static loff_t devkmsg_llseek(struct file *file, loff_t offset, int whence)
+{
+        struct devkmsg_user *user = file->private_data;
+        loff_t ret = 0;
+
+        if (!user)
+                return -EBADF;
+        if (offset)
+                return -ESPIPE;
+
+        raw_spin_lock(&logbuf_lock);
+        switch (whence) {
+        case SEEK_SET:
+                /* the first record */
+                user->idx = log_first_idx;
+                user->seq = log_first_seq;
+                break;
+        case SEEK_DATA:
+                /*
+                 * The first record after the last SYSLOG_ACTION_CLEAR,
+                 * like issued by 'dmesg -c'. Reading /dev/kmsg itself
+                 * changes no global state, and does not clear anything.
+                 */
+                user->idx = clear_idx;
+                user->seq = clear_seq;
+                break;
+        case SEEK_END:
+                /* after the last record */
+                user->idx = log_next_idx;
+                user->seq = log_next_seq;
+                break;
+        default:
+                ret = -EINVAL;
+        }
+        raw_spin_unlock(&logbuf_lock);
+        return ret;
+}
+
+static unsigned int devkmsg_poll(struct file *file, poll_table *wait)
+{
+        struct devkmsg_user *user = file->private_data;
+        int ret = 0;
+
+        if (!user)
+                return POLLERR|POLLNVAL;
+
+        poll_wait(file, &log_wait, wait);
+
+        raw_spin_lock(&logbuf_lock);
+        if (user->seq < log_next_seq) {
+                /* return error when data has vanished underneath us */
+                if (user->seq < log_first_seq)
+                        ret = POLLIN|POLLRDNORM|POLLERR|POLLPRI;
+                ret = POLLIN|POLLRDNORM;
+        }
+        raw_spin_unlock(&logbuf_lock);
+
+        return ret;
+}
+
+static int devkmsg_open(struct inode *inode, struct file *file)
+{
+        struct devkmsg_user *user;
+        int err;
+
+        /* write-only does not need any file context */
+        if ((file->f_flags & O_ACCMODE) == O_WRONLY)
+                return 0;
+
+        err = security_syslog(SYSLOG_ACTION_READ_ALL);
+        if (err)
+                return err;
+
+        user = kmalloc(sizeof(struct devkmsg_user), GFP_KERNEL);
+        if (!user)
+                return -ENOMEM;
+
+        mutex_init(&user->lock);
+
+        raw_spin_lock(&logbuf_lock);
+        user->idx = log_first_idx;
+        user->seq = log_first_seq;
+        raw_spin_unlock(&logbuf_lock);
+
+        file->private_data = user;
+        return 0;
+}
+
+static int devkmsg_release(struct inode *inode, struct file *file)
+{
+        struct devkmsg_user *user = file->private_data;
+
+        if (!user)
+                return 0;
+
+        mutex_destroy(&user->lock);
+        kfree(user);
+        return 0;
+}
+
+const struct file_operations kmsg_fops = {
+        .open = devkmsg_open,
+        .read = devkmsg_read,
+        .aio_write = devkmsg_writev,
+        .llseek = devkmsg_llseek,
+        .poll = devkmsg_poll,
+        .release = devkmsg_release,
+};
+
 #ifdef CONFIG_KEXEC
 /*
  * This appends the listed symbols to /proc/vmcoreinfo