tracing: Support __rel_loc relative dynamic data location attribute

Add '__rel_loc' new dynamic data location attribute which encodes
the data location from the next to the field itself.

The '__data_loc' is used for encoding the dynamic data location on
the trace event record. But '__data_loc' is not useful if the writer
doesn't know the event header (e.g. user event), because it records
the dynamic data offset from the entry of the record, not the field
itself.

This new '__rel_loc' attribute encodes the data location relatively
from the next of the field. For example, when there is a record like
below (the number in the parentheses is the size of fields)

 |header(N)|common(M)|fields(K)|__data_loc(4)|fields(L)|data(G)|

In this case, '__data_loc' field will be

 __data_loc = (G << 16) | (N+M+K+4+L)

If '__rel_loc' is used, this will be

 |header(N)|common(M)|fields(K)|__rel_loc(4)|fields(L)|data(G)|

where

 __rel_loc = (G << 16) | (L)

This case shows L bytes after the '__rel_loc' attribute  field,
if there is no fields after the __rel_loc field, L must be 0.

This is relatively easy (and no need to consider the kernel header
change) when the event data fields are composed by user who doesn't
know header and common fields.

Link: https://lkml.kernel.org/r/163757341258.510314.4214431827833229956.stgit@devnote2

Cc: Beau Belgrave <beaub@linux.microsoft.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Tom Zanussi <zanussi@kernel.org>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

1 files changed, 19 insertions, 2 deletions

diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
index 319f9c8ca7e7..9b8da439149c 100644
--- a/kernel/trace/trace_events_hist.c
+++ b/kernel/trace/trace_events_hist.c
@@ -217,6 +217,20 @@ static u64 hist_field_dynstring(struct hist_field *hist_field,
 	return (u64)(unsigned long)addr;
 }
 
+static u64 hist_field_reldynstring(struct hist_field *hist_field,
+				   struct tracing_map_elt *elt,
+				   struct trace_buffer *buffer,
+				   struct ring_buffer_event *rbe,
+				   void *event)
+{
+	u32 *item = event + hist_field->field->offset;
+	u32 str_item = *item;
+	int str_loc = str_item & 0xffff;
+	char *addr = (char *)&item[1] + str_loc;
+
+	return (u64)(unsigned long)addr;
+}
+
 static u64 hist_field_pstring(struct hist_field *hist_field,
 			      struct tracing_map_elt *elt,
 			      struct trace_buffer *buffer,
@@ -1956,8 +1970,10 @@ static struct hist_field *create_hist_field(struct hist_trigger_data *hist_data,
 		if (field->filter_type == FILTER_STATIC_STRING) {
 			hist_field->fn = hist_field_string;
 			hist_field->size = field->size;
-		} else if (field->filter_type == FILTER_DYN_STRING)
+		} else if (field->filter_type == FILTER_DYN_STRING) {
 			hist_field->fn = hist_field_dynstring;
+		} else if (field->filter_type == FILTER_RDYN_STRING)
+			hist_field->fn = hist_field_reldynstring;
 		else
 			hist_field->fn = hist_field_pstring;
 	} else {
@@ -4961,7 +4977,8 @@ static inline void add_to_key(char *compound_key, void *key,
 		struct ftrace_event_field *field;
 
 		field = key_field->field;
-		if (field->filter_type == FILTER_DYN_STRING)
+		if (field->filter_type == FILTER_DYN_STRING ||
+		    field->filter_type == FILTER_RDYN_STRING)
 			size = *(u32 *)(rec + field->offset) >> 16;
 		else if (field->filter_type == FILTER_STATIC_STRING)
 			size = field->size;