linux-kernel - Re: [RFC V2] printk: add warning while drop partial text in msg

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Date:   Wed, 18 Oct 2017 01:07:34 +0800
From:   pierre kuo <vichy.kuo@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        rostedt@...dmis.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC V2] printk: add warning while drop partial text in msg

hi:
> There are several possible solutions:
>
> + We could update vprintk_emit() to detect all newlines and
>   call log_store() for each part. But this would be a waste
>   of the space.
>
> + We could increase the size provided by syslog_printk().
>   But this is ugly.
>
> + We could go back to the original idea and print a warning
>   about shrunken message when the first record is not fully
>   stored by msg_print_text().
>
> I think that the last solution is the best. Note that the
> original patch was wrong because it warned in any
> msg_print_text() and not only the first one.

Would you mind to let me know more about the wrong places you mean
about the patch?
(since I cannot quite understand the "warned in any msg_print_text()
and not only the first one" mean)

In Aug. 11, https://lkml.org/lkml/2017/8/10/707,
Only if below a) and b) are both true, the patch will try to put
warning message at the end of output buffer.
a) sizeof((the next phrase cut by "\n" token ) + prefix composed by
(level | facility | timestamp)) > left length of output buffer
b) (the left length of output buffer) > strlen ("<dropped>");

so If any one of above a) or b) is not satisfied, there will be no
warning message.
Meanwhile, console_seq, clear_seq and syslog_seq will still increase
for next msg_print_text() run.
So the warning message will not always shows.

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index fc47863..fcd1dd4 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -557,6 +557,7 @@ static u32 msg_used_size(u16 text_len, u16
dict_len, u32 *pad_len)
  */
 #define MAX_LOG_TAKE_PART 4
 static const char trunc_msg[] = "<truncated>";
+static const char drop_msg[] = "<dropped>";

 static u32 truncate_msg(u16 *text_len, u16 *trunc_msg_len,
                        u16 *dict_len, u32 *pad_len)
@@ -1264,8 +1265,14 @@ static size_t msg_print_text(const struct
printk_log *msg, bool syslog, char *bu

                if (buf) {
                        if (print_prefix(msg, syslog, NULL) +
-                           text_len + 1 >= size - len)
+                           text_len + 1 >= size - len) {
+                               /* below adding warning message
+                                * related information into output buffer
+                                */
+                               if ((size - len) > strlen(drop_msg))
+                                       memcpy(buf + len, drop_msg,
strlen(drop_msg));
                                break;
+                       }

>Another question is that printk() is used a wrong way here.
>I will comment this in another mail in this thread.
please see my answer in another mail.