[PATCH net v2 1/2] s390/ctcm: Fix timer corruption in fsm_addtimer()

[Nagamani PV <nagamani@linux.ibm.com>]

fsm_addtimer() calls timer_setup() unconditionally before add_timer().
If called on an already-pending timer, timer_setup() re-initializes
the timer's list_head fields while the timer is still enqueued in the
wheel, corrupting the timer list.

The timer is already initialized once by fsm_settimer() which calls
timer_setup() correctly. Multiple callsites invoke fsm_addtimer()
without a preceding fsm_deltimer(), including ctcm_main.c
ctcmpc_send_sweep_req() and ctcm_mpc.c mpc_action_side_xid(), making the
redundant timer_setup() in fsm_addtimer() a real corruption risk.

Remove the redundant timer_setup() calls from fsm_addtimer() and
fsm_modtimer(), and replace add_timer() with mod_timer() which safely
handles both pending and non-pending timers atomically without
corrupting the timer wheel.

Fixes: e99e88a9d2b0 ("treewide: setup_timer() -> timer_setup()")
Cc: stable@vger.kernel.org
Reported-by: Sashiko <sashiko-bot@kernel.org>
Link: https://sashiko.dev/#/patchset/20260803182736.2356374-1-nagamani@linux.ibm.com?part=1
Reviewed-by: Aswin Karuvally <aswin@linux.ibm.com>
Tested-by: Aswin Karuvally <aswin@linux.ibm.com>
Signed-off-by: Nagamani PV <nagamani@linux.ibm.com>
---
Changes in v2:
- fix function name ctcm_send_sweep() -> ctcmpc_send_sweep_req()
  in the commit message (Sashiko netdev-bot)
- call mod_timer() then return 0 explicitly, preserving the
  "Always returns 0" contract documented in fsm.h (Sashiko netdev-bot)
---
 drivers/s390/net/fsm.c | 9 ++-------
 1 file changed, 2 insertions(+), 7 deletions(-)

diff --git a/drivers/s390/net/fsm.c b/drivers/s390/net/fsm.c
index 9b8622aef807..936ed0bc8401 100644
--- a/drivers/s390/net/fsm.c
+++ b/drivers/s390/net/fsm.c
@@ -171,11 +171,9 @@ fsm_addtimer(fsm_timer *this, int millisec, int event, void *arg)
 	       this->fi->name, this, millisec);
 #endif
 
-	timer_setup(&this->tl, fsm_expire_timer, 0);
 	this->expire_event = event;
 	this->event_arg = arg;
-	this->tl.expires = jiffies + (millisec * HZ) / 1000;
-	add_timer(&this->tl);
+	mod_timer(&this->tl, jiffies + msecs_to_jiffies(millisec));
 	return 0;
 }
 
@@ -189,12 +187,9 @@ fsm_modtimer(fsm_timer *this, int millisec, int event, void *arg)
 		this->fi->name, this, millisec);
 #endif
 
-	timer_delete(&this->tl);
-	timer_setup(&this->tl, fsm_expire_timer, 0);
 	this->expire_event = event;
 	this->event_arg = arg;
-	this->tl.expires = jiffies + (millisec * HZ) / 1000;
-	add_timer(&this->tl);
+	mod_timer(&this->tl, jiffies + msecs_to_jiffies(millisec));
 }
 
 EXPORT_SYMBOL(init_fsm);
-- 
2.53.0