[for-next][PATCH 1/5] tracing/histogram: Dont use strlen to find length of stacktrace variables

[Steven Rostedt]

From: Tom Zanussi <zanussi@xxxxxxxxxx>

Because stacktraces are saved in dynamic strings,
trace_event_raw_event_synth() uses strlen to determine the length of
the stack.  Stacktraces may contain 0-bytes, though, in the saved
addresses, so the length found and passed to reserve() will be too
small.

Fix this by using the first unsigned long in the stack variables to
store the actual number of elements in the stack and have
trace_event_raw_event_synth() use that to determine the length of the
stack.

Link: https://lkml.kernel.org/r/1ed6906cd9d6477ef2bd8e63c61de20a9ffe64d7.1676063532.git.zanussi@xxxxxxxxxx

Signed-off-by: Tom Zanussi <zanussi@xxxxxxxxxx>
Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
---
 kernel/trace/trace_events_hist.c  | 12 ++++++++----
 kernel/trace/trace_events_synth.c |  7 ++++++-
 2 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
index 7f3e6ca6f0fa..f21e42ddba69 100644
--- a/kernel/trace/trace_events_hist.c
+++ b/kernel/trace/trace_events_hist.c
@@ -3137,13 +3137,15 @@ static inline void __update_field_vars(struct tracing_map_elt *elt,
 				size = min(val->size, STR_VAR_LEN_MAX);
 				strscpy(str, val_str, size);
 			} else {
+				char *stack_start = str + sizeof(unsigned long);
 				int e;
 
-				e = stack_trace_save((void *)str,
+				e = stack_trace_save((void *)stack_start,
 						     HIST_STACKTRACE_DEPTH,
 						     HIST_STACKTRACE_SKIP);
 				if (e < HIST_STACKTRACE_DEPTH - 1)
-					((unsigned long *)str)[e] = 0;
+					((unsigned long *)stack_start)[e] = 0;
+				*((unsigned long *)str) = e;
 			}
 			var_val = (u64)(uintptr_t)str;
 		}
@@ -5135,13 +5137,15 @@ static void hist_trigger_elt_update(struct hist_trigger_data *hist_data,
 					size = min(hist_field->size, STR_VAR_LEN_MAX);
 					strscpy(str, val_str, size);
 				} else {
+					char *stack_start = str + sizeof(unsigned long);
 					int e;
 
-					e = stack_trace_save((void *)str,
+					e = stack_trace_save((void *)stack_start,
 							     HIST_STACKTRACE_DEPTH,
 							     HIST_STACKTRACE_SKIP);
 					if (e < HIST_STACKTRACE_DEPTH - 1)
-						((unsigned long *)str)[e] = 0;
+						((unsigned long *)stack_start)[e] = 0;
+					*((unsigned long *)str) = e;
 				}
 				hist_val = (u64)(uintptr_t)str;
 			}
diff --git a/kernel/trace/trace_events_synth.c b/kernel/trace/trace_events_synth.c
index 306c89e0ce55..70bddb25d9c0 100644
--- a/kernel/trace/trace_events_synth.c
+++ b/kernel/trace/trace_events_synth.c
@@ -538,7 +538,12 @@ static notrace void trace_event_raw_event_synth(void *__data,
 		val_idx = var_ref_idx[field_pos];
 		str_val = (char *)(long)var_ref_vals[val_idx];
 
-		len = kern_fetch_store_strlen((unsigned long)str_val);
+		if (event->dynamic_fields[i]->is_stack) {
+			len = *((unsigned long *)str_val);
+			len *= sizeof(unsigned long);
+		} else {
+			len = kern_fetch_store_strlen((unsigned long)str_val);
+		}
 
 		fields_size += len;
 	}
-- 
2.39.1