dmesg: fix kmsg usability detection

On old kernels (<3.5) kernel allows to open read-only /dev/kmsg for
root, but read() returns -EINVAL. It means that open() is not enough
to detect /dev/kmsg usability. We have to call read() (or epoll).

Reported-by: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Karel Zak <kzak@redhat.com>

diff --git a/sys-utils/dmesg.c b/sys-utils/dmesg.c
index 1c5de2a5e4a11206422057841ccae7f331f9067d..e92904f64033d2c72390e156451edb34952ce34a 100644 (file)
--- a/sys-utils/dmesg.c
+++ b/sys-utils/dmesg.c
@@ -123,8 +123,12 @@ struct dmesg_control {
 
        int             action;         /* SYSLOG_ACTION_* */
        int             method;         /* DMESG_METHOD_* */
-       size_t          bufsize;        /* size of buffer created by read_buffer() */
+
+       size_t          bufsize;        /* size of syslog buffer */
+
        int             kmsg;           /* /dev/kmsg file descriptor */
+       ssize_t         kmsg_first_read;/* initial read() return code */
+       char            kmsg_buf[BUFSIZ];/* buffer to read kmsg data */
 
        /*
         * For the --file option we mmap whole file. The unnecessary (already
@@ -865,6 +869,22 @@ static int init_kmsg(struct dmesg_control *ctl)
         * ... otherwise SYSLOG_ACTION_CLEAR will have no effect for kmsg.
         */
        lseek(ctl->kmsg, 0, SEEK_DATA);
+
+       /*
+        * Old kernels (<3.5) allow to successfully open /dev/kmsg for
+        * read-only, but read() returns -EINVAL :-(((
+        *
+        * Let's try to read the first record. The record is later processed in
+        * read_kmsg().
+        */
+       ctl->kmsg_first_read = read(ctl->kmsg, ctl->kmsg_buf,
+                                 sizeof(ctl->kmsg_buf) - 1);
+       if (ctl->kmsg_first_read < 0) {
+               close(ctl->kmsg);
+               ctl->kmsg = -1;
+               return -1;
+       }
+
        return 0;
 }
 
@@ -956,25 +976,28 @@ mesg:
  */
 static int read_kmsg(struct dmesg_control *ctl)
 {
-       char buf[BUFSIZ];
        struct dmesg_record rec;
+       ssize_t sz;
 
        if (ctl->method != DMESG_METHOD_KMSG || ctl->kmsg < 0)
                return -1;
 
-       do {
-               ssize_t sz = read(ctl->kmsg, buf, sizeof(buf) - 1);
-
-               if (sz <= 0)
-                       break;
+       /*
+        * The very first read() call is done in kmsg_init() where we test
+        * /dev/kmsg usability. The return code from the initial read() is
+        * stored in ctl->kmsg_first_read;
+        */
+       sz = ctl->kmsg_first_read;
 
-               *(buf + sz) = '\0';     /* for debug messages */
+       while (sz > 0) {
+               *(ctl->kmsg_buf + sz) = '\0';   /* for debug messages */
 
-               if (parse_kmsg_record(ctl, &rec, buf, (size_t) sz) != 0)
-                       continue;
+               if (parse_kmsg_record(ctl, &rec,
+                                     ctl->kmsg_buf, (size_t) sz) == 0)
+                       print_record(ctl, &rec);
 
-               print_record(ctl, &rec);
-       } while (1);
+               sz = read(ctl->kmsg, ctl->kmsg_buf, sizeof(ctl->kmsg_buf) - 1);
+       }
 
        return 0;
 }