[RFC PATCH v2 06/14] kcov: clean up dataflow state on task exit

If a task exits without calling KCOV_DF_DISABLE, the kcov_df_enabled
flag and area pointer remain set on the freed task_struct. If that
memory is reallocated, subsequent writes could corrupt arbitrary memory.

Add kcov_dataflow_task_exit() which clears the dataflow fields, called
from kernel/exit.c alongside kcov_task_exit(). This matches how
kcov_task_exit() cleans up the legacy kcov state.

Reported-by: sashiko-bot <[email protected]>
Closes: https://sashiko.dev/#/patchset/20260603-kcov-dataflow-next-20260603-v2-0-fee0939de2c4%40est.tech
Signed-off-by: Yunseong Kim <[email protected]>
---
 include/linux/kcov.h   |  2 ++
 kernel/exit.c          |  1 +
 kernel/kcov_dataflow.c | 11 +++++++++++
 3 files changed, 14 insertions(+)

diff --git a/include/linux/kcov.h b/include/linux/kcov.h
index e9822b02982b..07d7823e5d6f 100644
--- a/include/linux/kcov.h
+++ b/include/linux/kcov.h
@@ -30,8 +30,10 @@ void kcov_task_exit(struct task_struct *t);
 
 #if defined(CONFIG_KCOV_DATAFLOW_ARGS) || defined(CONFIG_KCOV_DATAFLOW_RET)
 void kcov_dataflow_task_init(struct task_struct *t);
+void kcov_dataflow_task_exit(struct task_struct *t);
 #else
 static inline void kcov_dataflow_task_init(struct task_struct *t) {}
+static inline void kcov_dataflow_task_exit(struct task_struct *t) {}
 #endif
 
 #define kcov_prepare_switch(t)			\
diff --git a/kernel/exit.c b/kernel/exit.c
index 1056422bc101..af2314500791 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -935,6 +935,7 @@ void __noreturn do_exit(long code)
 		kthread_do_exit(kthread, code);
 
 	kcov_task_exit(tsk);
+	kcov_dataflow_task_exit(tsk);
 	kmsan_task_exit(tsk);
 
 	synchronize_group_exit(tsk, code);
diff --git a/kernel/kcov_dataflow.c b/kernel/kcov_dataflow.c
index 7cfe2495275a..df037b7e90eb 100644
--- a/kernel/kcov_dataflow.c
+++ b/kernel/kcov_dataflow.c
@@ -196,6 +196,17 @@ void kcov_dataflow_task_init(struct task_struct *t)
 	t->kcov_df_enabled = false;
 }
 
+/* Called from kernel/exit.c to clear state on task exit. */
+void kcov_dataflow_task_exit(struct task_struct *t)
+{
+	if (t->kcov_df_enabled) {
+		t->kcov_df_enabled = false;
+		barrier();
+		t->kcov_df_area = NULL;
+		t->kcov_df_size = 0;
+	}
+}
+
 /* File operations for /sys/kernel/debug/kcov_dataflow */
 
 static int kcov_df_open(struct inode *inode, struct file *filep)

-- 
2.43.0