[PATCH v2 0/2] libtracefs: Make hist variable names unique

[PATCH v2 0/2] libtracefs: Make hist variable names unique

[Steven Rostedt]

From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>

In order to be able to have API that can do onmax() and onchange() as well
as include the .save() and .snapshot() actions, we need to make sure that
the variable names are unique even between different events.

The first adds a "randomness" to the name, and the second patch fixes an
issue where the "end" event fields are saved in the same name as they are
called, then the ".save" action can not be used on them.

Changes since v1:

 - Use open coded "rand()" as rand() is not reentrant, and it we use our own
   seed each time.

 - Use gettimeofday() instead of time() as Yordan pointed out that time()
   has a resolution of 1 second, and a thread could call this for different
   synthetic events.


Steven Rostedt (VMware) (2):
  libtracefs: Add random number to keep synthetic variables unique
  libtracefs: Have end event variables not be the end event field name

 src/tracefs-hist.c | 45 +++++++++++++++++++++++++++++++++++----------
 1 file changed, 35 insertions(+), 10 deletions(-)

-- 
2.30.2

[PATCH v2 1/2] libtracefs: Add random number to keep synthetic variables unique

[Steven Rostedt]

From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>

The 'hist' triggers expect that all variables are unique. If two synthetic
events are created, it is possible that they will use the same variable
names, and this can break the logic for synthetic events. Add a random
number to the argument names that will help prevent that from happening.
There's no guarantee that there wont be collisions, but the chances of
that happening is 1 in 32768. If this is a problem, we could possibly look
for variables that are already in use.

Link: https://lore.kernel.org/linux-trace-devel/20210812005546.910833-2-rostedt@goodmis.org/

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
 src/tracefs-hist.c | 23 ++++++++++++++++++++++-
 1 file changed, 22 insertions(+), 1 deletion(-)

diff --git a/src/tracefs-hist.c b/src/tracefs-hist.c
index 7f9cf3820611..9de70579a871 100644
--- a/src/tracefs-hist.c
+++ b/src/tracefs-hist.c
@@ -13,6 +13,8 @@
 #include <errno.h>
 #include <fcntl.h>
 #include <limits.h>
+#include <sys/time.h>
+#include <sys/types.h>
 
 #include "tracefs.h"
 #include "tracefs-local.h"
@@ -558,6 +560,7 @@ struct tracefs_synth {
 	unsigned int		end_parens;
 	unsigned int		end_state;
 	int			*start_type;
+	char			arg_name[16];
 	int			arg_cnt;
 };
 
@@ -951,13 +954,31 @@ int tracefs_synth_add_match_field(struct tracefs_synth *synth,
 	return -1;
 }
 
+static unsigned int make_rand(void)
+{
+	struct timeval tv;
+	unsigned long seed;
+
+	gettimeofday(&tv, NULL);
+	seed = (tv.tv_sec + tv.tv_usec) + gettid();
+
+	/* taken from the rand(3) man page */
+	seed = seed * 1103515245 + 12345;
+	return((unsigned)(seed/65536) % 32768);
+}
+
 static char *new_arg(struct tracefs_synth *synth)
 {
 	int cnt = synth->arg_cnt + 1;
 	char *arg;
 	int ret;
 
-	ret = asprintf(&arg, "__arg__%d", cnt);
+	/* Create a unique argument name */
+	if (!synth->arg_name[0]) {
+		/* make_rand() returns at most 32768 (total 13 bytes in use) */
+		sprintf(synth->arg_name, "__arg_%u_", make_rand());
+	}
+	ret = asprintf(&arg, "%s%d", synth->arg_name, cnt);
 	if (ret < 0)
 		return NULL;
 
-- 
2.30.2

[PATCH v2 2/2] libtracefs: Have end event variables not be the end event field name

[Steven Rostedt]

From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>

Currently we have:

 # sqlhist -n wakeup 'select end.next_pid, (end.TIMESTAMP - start.TIMESTAMP) as lat
   from sched_waking as start join sched_switch as end on start.pid = end.next_pid'

produces:

 echo 'wakeup s32 next_pid; u64 lat;' > /sys/kernel/tracing/synthetic_events
 echo 'hist:keys=pid:__arg_18871_1=common_timestamp' > /sys/kernel/tracing/events/sched/sched_waking/trigger
 echo 'hist:keys=next_pid:next_pid=next_pid,lat=common_timestamp-$__arg_18871_1:onmatch(sched.sched_waking).trace(wakeup,$next_pid,$lat)' > /sys/kernel/tracing/events/sched/sched_switch/trigger

The issue is that we have "next_pid=next_pid" where if we want to change
the above to use the "save" action:

 hist:keys=next_pid:next_pid=next_pid,lat=common_timestamp-$__arg_18871_1:onmax($lat).save(next_pid)

It fails with:

   hist:sched:sched_switch: error: Couldn't find field
    Command: hist:keys=next_pid:next_pid=next_pid,lat=common_timestamp-$__arg_18871_1:onmax($lat).save(next_pid)
                                                                       ^

But by having the end vars be unique, then the above "save" works.

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
 src/tracefs-hist.c | 22 +++++++++++++---------
 1 file changed, 13 insertions(+), 9 deletions(-)

diff --git a/src/tracefs-hist.c b/src/tracefs-hist.c
index 9de70579a871..fefe251995ba 100644
--- a/src/tracefs-hist.c
+++ b/src/tracefs-hist.c
@@ -851,16 +851,19 @@ struct tracefs_synth *tracefs_synth_init(struct tep_handle *tep,
 
 static int add_synth_fields(struct tracefs_synth *synth,
 			    const struct tep_format_field *field,
-			    const char *name)
+			    const char *name, const char *var)
 {
 	char **list;
 	char *str;
 	int ret;
 
-	str = add_synth_field(field, name);
+	str = add_synth_field(field, name ? : field->name);
 	if (!str)
 		return -1;
 
+	if (!name)
+		name = var;
+
 	list = tracefs_list_add(synth->synthetic_fields, str);
 	free(str);
 	if (!list)
@@ -942,7 +945,7 @@ int tracefs_synth_add_match_field(struct tracefs_synth *synth,
 	if (ret < 0)
 		goto pop_lists;
 
-	ret = add_synth_fields(synth, key_field, name);
+	ret = add_synth_fields(synth, key_field, name, NULL);
 	if (ret < 0)
 		goto pop_lists;
 
@@ -1071,7 +1074,7 @@ int tracefs_synth_add_compare_field(struct tracefs_synth *synth,
 	if (ret < 0)
 		goto out_free;
 
-	ret = add_synth_fields(synth, start_field, name);
+	ret = add_synth_fields(synth, start_field, name, NULL);
 	if (ret < 0)
 		goto out_free;
 
@@ -1116,7 +1119,7 @@ __hidden int synth_add_start_field(struct tracefs_synth *synth,
 	if (ret)
 		goto out_free;
 
-	ret = add_synth_fields(synth, field, name);
+	ret = add_synth_fields(synth, field, name, NULL);
 	if (ret)
 		goto out_free;
 
@@ -1188,6 +1191,7 @@ int tracefs_synth_add_end_field(struct tracefs_synth *synth,
 				const char *name)
 {
 	const struct tep_format_field *field;
+	char *tmp_var = NULL;
 	int ret;
 
 	if (!synth || !end_field) {
@@ -1196,17 +1200,17 @@ int tracefs_synth_add_end_field(struct tracefs_synth *synth,
 	}
 
 	if (!name)
-		name = end_field;
+		tmp_var = new_arg(synth);
 
 	if (!trace_verify_event_field(synth->end_event, end_field, &field))
 		return -1;
 
-	ret = add_var(&synth->end_vars, name, end_field, false);
+	ret = add_var(&synth->end_vars, name ? : tmp_var, end_field, false);
 	if (ret)
 		goto out;
 
-	ret = add_synth_fields(synth, field, name);
-
+	ret = add_synth_fields(synth, field, name, tmp_var);
+	free(tmp_var);
  out:
 	return ret;
 }
-- 
2.30.2