[PATCH 5.15.y 2/2] tracing: Free histogram var refs regardless of how often they are referenced

[Sasha Levin <sashal@kernel.org>]

From: Donggeun Yoo <donggeunyoo.kernel@gmail.com>

[ Upstream commit 4bddcb346a6cf4615ca77f69a589623b877ca267 ]

Using the same variable three or more times in one hist trigger leaks the
variable reference and its strings when the trigger is removed.

commit 656fe2ba85e8 ("tracing: Use hist trigger's var_ref array to destroy
var_refs") made a trigger's var_refs[] array the only owner of a var ref:
destroy_hist_field() returns early for HIST_FIELD_FL_VAR_REF, so the field
expressions never destroy one. One entry, freed once, no count needed.

commit 8bcebc77e85f ("tracing: Fix histogram code when expression has same
var as value") then made repeated references share one object and added a
count of them. Only the increment side exists, since those expressions
still return early and never drop a reference, so __destroy_hist_field()
sees how many references were created rather than how many are left. It
frees when the decremented count is 0 or 1, so two references work and
three or more leak.

Sharing kept one array entry per object, and create_var_ref() searches and
appends within a single trigger, so nothing outside it holds the object.
Removing a trigger whose variables are still referenced is already refused
by check_var_refs() with -EBUSY. Drop the count and free unconditionally.

Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260906124025.3550596-1-donggeunyoo.kernel@gmail.com
Fixes: 8bcebc77e85f ("tracing: Fix histogram code when expression has same var as value")
Signed-off-by: Donggeun Yoo <donggeunyoo.kernel@gmail.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 kernel/trace/trace_events_hist.c | 16 +---------------
 1 file changed, 1 insertion(+), 15 deletions(-)

diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
index 9596bb513ad3a..b9641fecb8c5b 100644
--- a/kernel/trace/trace_events_hist.c
+++ b/kernel/trace/trace_events_hist.c
@@ -146,7 +146,6 @@ struct hist_field {
 	struct hist_field		*operands[HIST_FIELD_OPERANDS_MAX];
 	struct hist_trigger_data	*hist_data;
 	enum hist_field_fn		fn_num;
-	unsigned int			ref;
 	unsigned int			size;
 	unsigned int			offset;
 	unsigned int                    is_signed;
@@ -1629,16 +1628,8 @@ static int contains_operator(char *str)
 	return field_op;
 }
 
-static void get_hist_field(struct hist_field *hist_field)
-{
-	hist_field->ref++;
-}
-
 static void __destroy_hist_field(struct hist_field *hist_field)
 {
-	if (--hist_field->ref > 1)
-		return;
-
 	kfree(hist_field->var.name);
 	kfree(hist_field->name);
 
@@ -1685,8 +1676,6 @@ static struct hist_field *create_hist_field(struct hist_trigger_data *hist_data,
 	if (!hist_field)
 		return NULL;
 
-	hist_field->ref = 1;
-
 	hist_field->hist_data = hist_data;
 
 	if (flags & HIST_FIELD_FL_EXPR || flags & HIST_FIELD_FL_ALIAS)
@@ -1907,10 +1896,8 @@ static struct hist_field *create_var_ref(struct hist_trigger_data *hist_data,
 	for (i = 0; i < hist_data->n_var_refs; i++) {
 		ref_field = hist_data->var_refs[i];
 		if (ref_field->var.idx == var_field->var.idx &&
-		    ref_field->var.hist_data == var_field->hist_data) {
-			get_hist_field(ref_field);
+		    ref_field->var.hist_data == var_field->hist_data)
 			return ref_field;
-		}
 	}
 	/* Sanity check to avoid out-of-bound write on 'hist_data->var_refs' */
 	if (hist_data->n_var_refs >= TRACING_MAP_VARS_MAX)
@@ -2771,7 +2758,6 @@ static struct hist_field *create_var(struct hist_trigger_data *hist_data,
 		goto out;
 	}
 
-	var->ref = 1;
 	var->flags = HIST_FIELD_FL_VAR;
 	var->var.idx = idx;
 	var->var.hist_data = var->hist_data = hist_data;
-- 
2.53.0