[PATCH] trace: turn hist_elt_data field_var_str into a flexible array

[Rosen Penev <rosenp@gmail.com>]

The field_var_str array was allocated separately via kcalloc() with its
length already known at elt_data allocation time.  Convert it to a
flexible array member and fold the two allocations into a single
kzalloc_flex(), reordering hist_trigger_elt_data_alloc() so n_str is
computed and bounds-checked before the struct allocation.

hist_elt_data is only reached through tracing_map_elt::private_data
(a void *), never embedded, so adding a FAM imposes no tail-position
constraint on any enclosing struct.

Added __counted_by for extra runtime analysis.

Assisted-by: Claude:Opus-4.7
Signed-off-by: Rosen Penev <rosenp@gmail.com>
---
 kernel/trace/trace_events_hist.c | 31 +++++++++++--------------------
 1 file changed, 11 insertions(+), 20 deletions(-)

diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
index a64c76f6905f..1fd0899c606c 100644
--- a/kernel/trace/trace_events_hist.c
+++ b/kernel/trace/trace_events_hist.c
@@ -683,8 +683,8 @@ struct track_data {
 struct hist_elt_data {
 	char *comm;
 	u64 *var_ref_vals;
-	char **field_var_str;
 	int n_field_var_str;
+	char *field_var_str[] __counted_by(n_field_var_str);
 };
 
 struct snapshot_context {
@@ -1629,8 +1629,6 @@ static void hist_elt_data_free(struct hist_elt_data *elt_data)
 	for (i = 0; i < elt_data->n_field_var_str; i++)
 		kfree(elt_data->field_var_str[i]);
 
-	kfree(elt_data->field_var_str);
-
 	kfree(elt_data->comm);
 	kfree(elt_data);
 }
@@ -1650,10 +1648,19 @@ static int hist_trigger_elt_data_alloc(struct tracing_map_elt *elt)
 	struct hist_field *hist_field;
 	unsigned int i, n_str;
 
-	elt_data = kzalloc_obj(*elt_data);
+	BUILD_BUG_ON(STR_VAR_LEN_MAX & (sizeof(u64) - 1));
+
+	n_str = hist_data->n_field_var_str + hist_data->n_save_var_str +
+		hist_data->n_var_str;
+	if (n_str > SYNTH_FIELDS_MAX)
+		return -EINVAL;
+
+	elt_data = kzalloc_flex(*elt_data, field_var_str, n_str);
 	if (!elt_data)
 		return -ENOMEM;
 
+	elt_data->n_field_var_str = n_str;
+
 	for_each_hist_field(i, hist_data) {
 		hist_field = hist_data->fields[i];
 
@@ -1667,24 +1674,8 @@ static int hist_trigger_elt_data_alloc(struct tracing_map_elt *elt)
 		}
 	}
 
-	n_str = hist_data->n_field_var_str + hist_data->n_save_var_str +
-		hist_data->n_var_str;
-	if (n_str > SYNTH_FIELDS_MAX) {
-		hist_elt_data_free(elt_data);
-		return -EINVAL;
-	}
-
-	BUILD_BUG_ON(STR_VAR_LEN_MAX & (sizeof(u64) - 1));
-
 	size = STR_VAR_LEN_MAX;
 
-	elt_data->field_var_str = kcalloc(n_str, sizeof(char *), GFP_KERNEL);
-	if (!elt_data->field_var_str) {
-		hist_elt_data_free(elt_data);
-		return -EINVAL;
-	}
-	elt_data->n_field_var_str = n_str;
-
 	for (i = 0; i < n_str; i++) {
 		elt_data->field_var_str[i] = kzalloc(size, GFP_KERNEL);
 		if (!elt_data->field_var_str[i]) {
-- 
2.54.0