Re: [PATCH v4 1/2] tty: sysrq: don't expose a partially written reset_seq

[Greg KH <gregkh@linuxfoundation.org>]

On Fri, Oct 30, 2026 at 03:58:20PM +0800, Zhan Xusheng wrote:
> Hi Greg,
> 
> On Fri, Oct 30, 2026 at 08:02:30AM +0100, Greg KH wrote:
> > I still don't understand what the root problem is here, sorry.
> 
> Each keyboard keeps its own copy of reset_seq.  It rebuilds the copy on
> the first key press after the parameter changes, and it rebuilds it
> wrong in two ways.
> 
> 1) It adds the new keys to the old copy instead of replacing it (2/2):
> 
>   echo 56,29,111 > /sys/module/sysrq/parameters/reset_seq # Alt+Ctrl+Del
>   (press any key)
>   echo 59,60 > /sys/module/sysrq/parameters/reset_seq     # F1+F2
> 
> Now F1+F2 does nothing.  Only holding all five keys resets.  So setting
> reset_seq once works, but changing it later does not.

So this feature has never worked?  Or it is just "one change only"?  So
your change is an addition to the feature, not a bugfix?

> 2) It can rebuild the copy while the write is half done (1/2).  The
> write stores the keys one at a time, and the keyboard may read the list
> in between:
> 
>   echo 59,60 > reset_seq
>     store 59
>                       <- F1 pressed: keyboard reads "59" as the sequence
>     store 60
> 
> On a keyboard that has not used a reset sequence before, or on any
> keyboard once 2/2 is applied, that F1 press completes the one-key
> sequence and reboots the machine.  The window is microseconds; I only
> hit it with an added delay.

So this isn't a "real" issue?  If you are modifying the key sequence,
how can you press the key at the same time?  Doing "major" rework for
that might just not be worth it.

thanks,

greg k-h