Re: [PATCH 1/3] printk: Fix kdb_trap_printk placement

[Petr Mladek]

On Tue 2016-10-18 19:08:31, Peter Zijlstra wrote:
> Some people figured vprintk_emit() makes for a nice API and exported
> it, bypassing the kdb trap.

I think that nobody saw this problem because kdb_trap_printk was
used only for a limited number of printk's. It is just a trick
how to use generic functions to print messages in the kdb context,
e.g. for getting backtraces.

But the patch makes sense.


> This still leaves vprintk_nmi() outside of the kbd reach, should that
> be fixed too?

It is perfectly fine. Messages from NMI context are not meant for
kdb anyway.

> Cc: Jason Wessel <jason.wessel@xxxxxxxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> ---
>  kernel/printk/printk.c |   19 ++++++++-----------
>  1 file changed, 8 insertions(+), 11 deletions(-)
> 
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -1750,6 +1750,13 @@ asmlinkage int vprintk_emit(int facility
>  	/* cpu currently holding logbuf_lock in this function */
>  	static unsigned int logbuf_cpu = UINT_MAX;
>  
> +#ifdef CONFIG_KGDB_KDB
> +	if (unlikely(kdb_trap_printk)) {
> +		r = vkdb_printf(KDB_MSGSRC_PRINTK, fmt, args);
> +		return r;

r is not defined. It is fixed in the next patch but it breaks
bisectability.

Please, find below an updated patch that also includes
my Reviewed-by.