Re: [PATCH] pstore: Fix printing of duplicate boot messages to console

[Kees Cook <keescook@chromium.org>]

On Thu, Jan 23, 2020 at 09:30:31PM +0530, Sai Prakash Ranjan wrote:
> Since commit f92b070f2dc8 ("printk: Do not miss new messages
> when replaying the log"), CON_PRINTBUFFER flag causes the
> duplicate boot messages to be printed on the console when
> PSTORE_CONSOLE and earlycon (boot console) is enabled.
> Pstore console registers to boot console when earlycon is
> enabled during pstore_register_console as a part of ramoops
> initialization in postcore_initcall and the printk core
> checks for CON_PRINTBUFFER flag and replays the log buffer
> to registered console (in this case pstore console which
> just registered to boot console) causing duplicate messages
> to be printed. Remove the CON_PRINTBUFFER flag from pstore
> console since pstore is not concerned with the printing of
> buffer to console but with writing of the buffer to the
> backend.

I agree this patch isn't the solution, but I'm trying to understand
where better logic could be added. Is the issue that printk sees both
earlycon and CON_PRINTBUFFER active? Can we add a new CON_* flag that
means "not actually printing anything"? (Or maybe a new flag for
non-printing to replace CON_PRINTBUFFER that lets pstore still work?)

-Kees

> 
> Console log with earlycon and pstore console enabled:
> 
> [    0.008342] Console: colour dummy device 80x25
> [    0.000000] Booting Linux on physical CPU 0x0000000000 [0x51df805e]
> ...
> [    1.244049] hw-breakpoint: found 6 breakpoint and 4 watchpoint registers.
> [    0.000000] Booting Linux on physical CPU 0x0000000000 [0x51df805e]
> 
> Fixes: f92b070f2dc8 ("printk: Do not miss new messages when replaying the log")
> Reported-by: Douglas Anderson <dianders@chromium.org>
> Signed-off-by: Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>
> ---
>  fs/pstore/platform.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/pstore/platform.c b/fs/pstore/platform.c
> index d896457e7c11..271b00db0973 100644
> --- a/fs/pstore/platform.c
> +++ b/fs/pstore/platform.c
> @@ -505,7 +505,7 @@ static void pstore_console_write(struct console *con, const char *s, unsigned c)
>  static struct console pstore_console = {
>  	.name	= "pstore",
>  	.write	= pstore_console_write,
> -	.flags	= CON_PRINTBUFFER | CON_ENABLED | CON_ANYTIME,
> +	.flags	= CON_ENABLED | CON_ANYTIME,
>  	.index	= -1,
>  };
>  
> -- 
> QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member
> of Code Aurora Forum, hosted by The Linux Foundation

-- 
Kees Cook