[PATCH v5] tty: sysrq: clear the old reset sequence when reset_seq changes

[Zhan Xusheng <zhanxusheng1024@gmail.com>]

From: Zhan Xusheng <zhanxusheng@xiaomi.com>

Changing reset_seq at runtime does not work on a keyboard that already
used the old value, because each keyboard adds the new keys to its own
copy instead of replacing it:

  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

F1+F2 then does nothing; only all five keys together reset.

Clear the old keys when the copy is rebuilt.  If sysrq_downtime_ms is
set, also stop a reset the old keys already started, since releasing
them no longer cancels it.

A key pressed while reset_seq is being written can see a partial
sequence.  With this change that can happen on any keyboard, not only
on ones that never used a sequence, but the window is a few
microseconds and not worth closing.

Fixes: 154b7a489a5b ("Input: sysrq - allow specifying alternate reset sequence")
Cc: stable@vger.kernel.org
Assisted-by: Kiro:claude-opus-5.5
Signed-off-by: Zhan Xusheng <zhanxusheng@xiaomi.com>
---
v5: Dropped v4 1/2 and rewrote the changelog.  Tested in QEMU with a
uinput keyboard: the new sequence resets, while the old one and a reset
armed before the rewrite do not.

v4: https://lore.kernel.org/all/20261030045854.153556-1-zhanxusheng1024@gmail.com/

 drivers/tty/sysrq.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
index f4571d05b496..a90e97a6d7aa 100644
--- a/drivers/tty/sysrq.c
+++ b/drivers/tty/sysrq.c
@@ -689,6 +689,11 @@ static void sysrq_parse_reset_sequence(struct sysrq_state *state)
 	int i;
 	unsigned short key;
 
+	/* A reset armed by the old sequence must not outlive it */
+	timer_delete(&state->keyreset_timer);
+
+	bitmap_zero(state->reset_keybit, KEY_CNT);
+	state->reset_seq_len = 0;
 	state->reset_seq_cnt = 0;
 
 	for (i = 0; i < sysrq_reset_seq_len; i++) {

base-commit: c339f4010dea8b0d754f84ace70cd1b089878ef8
-- 
2.43.0