linux-kernel - Re: [PATCH-next 2/3] sysctl/sysrq: Remove __sysrq

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Date:   Fri, 10 Jan 2020 17:40:35 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Dmitry Safonov <dima@...sta.com>
Cc:     linux-kernel@...r.kernel.org,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Jiri Slaby <jslaby@...e.com>,
        Vasiliy Khoruzhick <vasilykh@...sta.com>,
        linux-serial@...r.kernel.org, Iurii Zaikin <yzaikin@...gle.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH-next 2/3] sysctl/sysrq: Remove __sysrq_enabled copy

On Thu, Jan 09, 2020 at 09:54:43PM +0000, Dmitry Safonov wrote:
> Many embedded boards have a disconnected TTL level serial which can
> generate some garbage that can lead to spurious false sysrq detects.
> 
> Currently, sysrq can be either completely disabled for serial console
> or always disabled (with CONFIG_MAGIC_SYSRQ_SERIAL), since
> commit 732dbf3a6104 ("serial: do not accept sysrq characters via serial port")
> 
> At Arista, we have such boards that can generate BREAK and random
> garbage. While disabling sysrq for serial console would solve
> the problem with spurious false sysrq triggers, it's also desirable
> to have a way to enable sysrq back.
> 
> Having the way to enable sysrq was beneficial to debug lockups with
> a manual investigation in field and on the other side preventing false
> sysrq detections.
> 
> As a preparation to add sysrq_toggle_support() call into uart,
> remove a private copy of sysrq_enabled from sysctl - it should reflect
> the actual status of sysrq.
> 
> Furthermore, the private copy isn't correct already in case
> sysrq_always_enabled is true. So, remove __sysrq_enabled and use a
> getter-helper for sysrq enabled status.
> 
> Cc: Iurii Zaikin <yzaikin@...gle.com>
> Cc: Jiri Slaby <jslaby@...e.com>
> Cc: Luis Chamberlain <mcgrof@...nel.org>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: linux-fsdevel@...r.kernel.org
> Signed-off-by: Dmitry Safonov <dima@...sta.com>
> ---
>  drivers/tty/sysrq.c   |  7 +++++++
>  include/linux/sysrq.h |  1 +
>  kernel/sysctl.c       | 41 ++++++++++++++++++++++-------------------
>  3 files changed, 30 insertions(+), 19 deletions(-)
> 
> diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
> index f724962a5906..ef3e78967146 100644
> --- a/drivers/tty/sysrq.c
> +++ b/drivers/tty/sysrq.c
> @@ -73,6 +73,13 @@ static bool sysrq_on_mask(int mask)
>  	       (sysrq_enabled & mask);
>  }
>  
> +int sysrq_get_mask(void)
> +{
> +	if (sysrq_always_enabled)
> +		return 1;
> +	return sysrq_enabled;
> +}
> +
>  static int __init sysrq_always_enabled_setup(char *str)
>  {
>  	sysrq_always_enabled = true;
> diff --git a/include/linux/sysrq.h b/include/linux/sysrq.h
> index 8c71874e8485..4a0b351fa2d3 100644
> --- a/include/linux/sysrq.h
> +++ b/include/linux/sysrq.h
> @@ -50,6 +50,7 @@ int unregister_sysrq_key(int key, struct sysrq_key_op *op);
>  struct sysrq_key_op *__sysrq_get_key_op(int key);
>  
>  int sysrq_toggle_support(int enable_mask);
> +int sysrq_get_mask(void);
>  
>  #else
>  
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index d396aaaf19a3..6ddb4d7df0e1 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -229,25 +229,8 @@ static int proc_dopipe_max_size(struct ctl_table *table, int write,
>  		void __user *buffer, size_t *lenp, loff_t *ppos);
>  
>  #ifdef CONFIG_MAGIC_SYSRQ
> -/* Note: sysrq code uses its own private copy */
> -static int __sysrq_enabled = CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE;
> -
>  static int sysrq_sysctl_handler(struct ctl_table *table, int write,
> -				void __user *buffer, size_t *lenp,
> -				loff_t *ppos)
> -{
> -	int error;
> -
> -	error = proc_dointvec(table, write, buffer, lenp, ppos);
> -	if (error)
> -		return error;
> -
> -	if (write)
> -		sysrq_toggle_support(__sysrq_enabled);
> -
> -	return 0;
> -}
> -
> +			void __user *buffer, size_t *lenp, loff_t *ppos);
>  #endif
>  
>  static struct ctl_table kern_table[];
> @@ -747,7 +730,7 @@ static struct ctl_table kern_table[] = {
>  #ifdef CONFIG_MAGIC_SYSRQ
>  	{
>  		.procname	= "sysrq",
> -		.data		= &__sysrq_enabled,
> +		.data		= NULL,
>  		.maxlen		= sizeof (int),
>  		.mode		= 0644,
>  		.proc_handler	= sysrq_sysctl_handler,
> @@ -2844,6 +2827,26 @@ static int proc_dostring_coredump(struct ctl_table *table, int write,
>  }
>  #endif
>  
> +#ifdef CONFIG_MAGIC_SYSRQ
> +static int sysrq_sysctl_handler(struct ctl_table *table, int write,
> +				void __user *buffer, size_t *lenp, loff_t *ppos)
> +{
> +	int tmp, ret;
> +
> +	tmp = sysrq_get_mask();
> +
> +	ret = __do_proc_dointvec(&tmp, table, write, buffer,
> +			       lenp, ppos, NULL, NULL);
> +	if (ret || !write)
> +		return ret;
> +
> +	if (write)
> +		sysrq_toggle_support(tmp);
> +
> +	return 0;
> +}
> +#endif

Why did you move this function down here?  Can't it stay where it is and
you can just fix the logic there?  Now you have two different #ifdef
blocks intead of just one :(

thanks,

greg k-h