linux-kernel - Re: [PATCH v6] /proc/sysrq-trigger: accept multiple keys at once

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Date:   Tue, 14 Nov 2023 19:04:31 +0100
From:   Jiri Slaby <jirislaby@...nel.org>
To:     Tomas Mudrunka <tomas.mudrunka@...il.com>
Cc:     corbet@....net, gregkh@...uxfoundation.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-serial@...r.kernel.org
Subject: Re: [PATCH v6] /proc/sysrq-trigger: accept multiple keys at once

On 14. 11. 23, 16:12, Tomas Mudrunka wrote:
> Just for convenience.
> This way we can do:
> `echo _reisub > /proc/sysrq-trigger`
> Instead of:
> `for i in r e i s u b; do echo "$i" > /proc/sysrq-trigger; done;`
> 
> This can be very useful when trying to execute sysrq combo remotely
> or from userspace. When sending keys in multiple separate writes,
> userspace can be killed before whole combo is completed.
> Therefore putting all keys in single write is more robust approach.
> 
> Signed-off-by: Tomas Mudrunka <tomas.mudrunka@...il.com>
> ---
> V5 -> V6: Documentation now has notice about undefined behavior
> V4 -> V5: Added this list of changes
> V3 -> V4: Bulk is now bool instead of char (and fixed typo)
> V2 -> V3: Fixed code styling (and introduced typo)
> V1 -> V2: Bulk mode only activated by underscore now, added docs
> 
>   Documentation/admin-guide/sysrq.rst | 11 ++++++++++-
>   drivers/tty/sysrq.c                 | 17 ++++++++++++++---
>   2 files changed, 24 insertions(+), 4 deletions(-)
> 
> diff --git a/Documentation/admin-guide/sysrq.rst b/Documentation/admin-guide/sysrq.rst
> index 51906e473..e7a82cba7 100644
> --- a/Documentation/admin-guide/sysrq.rst
> +++ b/Documentation/admin-guide/sysrq.rst
> @@ -75,10 +75,19 @@ On other
>   	submit a patch to be included in this section.
>   
>   On all
> -	Write a character to /proc/sysrq-trigger.  e.g.::
> +	Write single character to /proc/sysrq-trigger.

a single

> +	Only first character is interpreted, rest of string is ignored.

the first; the rest of the string

> +	However it is not reccomended to write any extra characters

However, <- comma
recommended

> +	as the behavior is undefined and might change in the future versions.
> +	e.g.::

Even the original was lowercase. But it should be "E.g.::", right -- 
Greg/Jon?

>   
>   		echo t > /proc/sysrq-trigger
>   
> +	Alternatively write multiple keys combo prepended by underscore.

Alternatively, <- comma
s/keys/characters/
an underscore

> +	All characters are interpreted this way. e.g.::

This way, all characters are interpreted. (IMO this has a different 
meaning, but natives would have to tell us.)

> +
> +		echo _reisub > /proc/sysrq-trigger
> +
>   The :kbd:`<command key>` is case sensitive.
>   
>   What are the 'command' keys?
> diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
> index 6b4a28bcf..ad07bc812 100644
> --- a/drivers/tty/sysrq.c
> +++ b/drivers/tty/sysrq.c
> @@ -1150,16 +1150,27 @@ EXPORT_SYMBOL(unregister_sysrq_key);
>   #ifdef CONFIG_PROC_FS
>   /*
>    * writing 'C' to /proc/sysrq-trigger is like sysrq-C
> + * If first character in write is underscore, all characters are interpreted.

If the first character written is

thanks,
-- 
js
suse labs