Re: [PATCH v2] panic: add an option to replay all the printk message in buffer

[Feng Tang <feng.tang@intel.com>]

Hi Andrew,

On Tue, Apr 16, 2019 at 09:19:22PM -0700, Andrew Morton wrote:
> On Wed, 10 Apr 2019 23:37:18 +0800 Feng Tang <feng.tang@intel.com> wrote:
> 
> > Currently on panic, kernel will lower the loglevel and print out
> > new printk msg only with console_flush_on_panic().
> > 
> > Add an option for users to configure the "panic_print" to see
> > all dmesg in buffer, some of which they may have never seen due
> > to the loglevel setting, which will help debugging too.
> > 
> > Thanks to Petr Mladek as somes codes come directly from the sample
> > code in his review comments.
> 
> CONFIG_PRINTK=n:
> 
> kernel/printk/printk.c: In function console_unlock:
> kernel/printk/printk.c:2419:11: warning: __builtin_memcpy writing 27 bytes into a region of size 0 overflows the destination [-Wstringop-overflow=]
>     len += sprintf(text + len,
>            ^~~~~~~~~~~~~~~~~~~
>             "Replaying the entire log:\n");
>             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> because LOG_LINE_MAX=0 and PREFIX_MAX=0.

Thanks for catching this!

> 
> 
> Which is interesting.  The pre-existing
> 
> 			len = sprintf(text,
> 				      "** %llu printk messages dropped **\n",
> 				      log_first_seq - console_seq);
> 
> in console_unlock() has the same issue, but the compiler doesn't seem
> to want to warn.

For this one, I did some check, and it should be related with the
conditional check 
	if (console_seq < log_first_seq) {

Both the console_seq and log_first_seq will not be touched by any code
when CONFIG_PRINTK=n, and compiler will simply skip the whole code block,
as "console_seq < log_first_seq" will never happen.

But code block following "if (console_replay)" will be compiled, that's
why these warning message will be shown.

> 
> (Also, using sprintf() is a bit lame for the new message - could use
> strlcpy()).
> 
> I'll drop the patch for now - we don't want that warning to come out. 
> console_unlock() needs some fixing for the CONFIG_PRINTK=n case.

My instant thought would be put the console_unlcok() and similar funcs
under CONFIG_PRINTK protection, while adding nop functions in the "else"
segment.

But complexer question will be when CONFIG_PRINTK=n, how those console_xxx
functions should consider these to make compiled binary smaller (though it
rarely happens). would wait for Petr/Sergey/Steven's insights.

Thanks,
Feng