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* [PATCH v3 linux-next] perf parse-events: Fix big-endian 'overwrite' by writing correct union member
@ 2026-02-17  8:13 Thomas Richter
  2026-02-17 10:40 ` James Clark
  0 siblings, 1 reply; 2+ messages in thread
From: Thomas Richter @ 2026-02-17  8:13 UTC (permalink / raw)
  To: linux-kernel, linux-s390, linux-perf-users, acme, namhyung,
	irogers, james.clark
  Cc: agordeev, gor, sumanthk, hca, japo, Thomas Richter

The "Read backward ring buffer" test crashes on big-endian (e.g. s390x)
due to a NULL dereference when the backward mmap path isn't enabled.

Reproducer:
  # ./perf test -F 'Read backward ring buffer'
  Segmentation fault (core dumped)
  # uname -m
  s390x
  #

Root cause:
get_config_terms() stores into evsel_config_term::val.val (u64) while later
code reads boolean fields such as evsel_config_term::val.overwrite.
On big-endian the 1-byte boolean is left-aligned, so writing
evsel_config_term::val.val = 1 is read back as
evsel_config_term::val.overwrite = 0,
leaving backward mmap disabled and a NULL map being used.

Store values in the union member that matches the term type, e.g.:
  /* for OVERWRITE */
  new_term->val.overwrite = 1;  /* not new_term->val.val = 1 */
to fix this. Improve add_config_term() and add two more parameters for
string and value. Function add_config_term() now creates a complete node
element of type evsel_config_term and handles all evsel_config_term::val
union members.

Impact:
Enables backward mmap on big-endian and prevents the crash.
No change on little-endian.

Output after:
 # ./perf test -Fv 44
 --- start ---
 Using CPUID IBM,9175,705,ME1,3.8,002f
 mmap size 1052672B
 mmap size 8192B
 ---- end ----
 44: Read backward ring buffer                         : Ok
 #

Fixes: 159ca97cd97c ("perf parse-events: Refactor get_config_terms() to remove macros")
Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Reviewed-by: Jan Polensky <japo@linux.ibm.com>
Cc: James Clark <james.clark@linaro.org>
Cc: Ian Rogers <irogers@google.com>
---
 tools/perf/util/parse-events.c | 78 +++++++++++++++++++++++++++-------
 1 file changed, 62 insertions(+), 16 deletions(-)

diff --git a/tools/perf/util/parse-events.c b/tools/perf/util/parse-events.c
index b9efb296bba5..0a87987d8c6f 100644
--- a/tools/perf/util/parse-events.c
+++ b/tools/perf/util/parse-events.c
@@ -1117,7 +1117,7 @@ static int config_attr(struct perf_event_attr *attr,
 
 static struct evsel_config_term *add_config_term(enum evsel_term_type type,
 						 struct list_head *head_terms,
-						 bool weak)
+						 bool weak, char *str, u64 val)
 {
 	struct evsel_config_term *t;
 
@@ -1128,8 +1128,60 @@ static struct evsel_config_term *add_config_term(enum evsel_term_type type,
 	INIT_LIST_HEAD(&t->list);
 	t->type = type;
 	t->weak	= weak;
+
+	switch (type) {
+	case EVSEL__CONFIG_TERM_PERIOD:
+	case EVSEL__CONFIG_TERM_FREQ:
+	case EVSEL__CONFIG_TERM_STACK_USER:
+	case EVSEL__CONFIG_TERM_USR_CHG_CONFIG:
+	case EVSEL__CONFIG_TERM_USR_CHG_CONFIG1:
+	case EVSEL__CONFIG_TERM_USR_CHG_CONFIG2:
+	case EVSEL__CONFIG_TERM_USR_CHG_CONFIG3:
+	case EVSEL__CONFIG_TERM_USR_CHG_CONFIG4:
+		t->val.val = val;
+		break;
+	case EVSEL__CONFIG_TERM_TIME:
+		t->val.time = val;
+		break;
+	case EVSEL__CONFIG_TERM_INHERIT:
+		t->val.inherit = val;
+		break;
+	case EVSEL__CONFIG_TERM_OVERWRITE:
+		t->val.overwrite = val;
+		break;
+	case EVSEL__CONFIG_TERM_MAX_STACK:
+		t->val.max_stack = val;
+		break;
+	case EVSEL__CONFIG_TERM_MAX_EVENTS:
+		t->val.max_events = val;
+		break;
+	case EVSEL__CONFIG_TERM_PERCORE:
+		t->val.percore = val;
+		break;
+	case EVSEL__CONFIG_TERM_AUX_OUTPUT:
+		t->val.aux_output = val;
+		break;
+	case EVSEL__CONFIG_TERM_AUX_SAMPLE_SIZE:
+		t->val.aux_sample_size = val;
+		break;
+	case EVSEL__CONFIG_TERM_CALLGRAPH:
+	case EVSEL__CONFIG_TERM_BRANCH:
+	case EVSEL__CONFIG_TERM_DRV_CFG:
+	case EVSEL__CONFIG_TERM_RATIO_TO_PREV:
+	case EVSEL__CONFIG_TERM_AUX_ACTION:
+		if (str) {
+			t->val.str = strdup(str);
+			if (!t->val.str) {
+				zfree(&t);
+				return NULL;
+			}
+			t->free_str = true;
+		}
+		break;
+	default:
+	}
+
 	list_add_tail(&t->list, head_terms);
-
 	return t;
 }
 
@@ -1234,20 +1286,15 @@ static int get_config_terms(const struct parse_events_terms *head_config,
 			continue;
 		}
 
-		new_term = add_config_term(new_type, head_terms, term->weak);
+		/*
+		 * Note: Members evsel_config_term::val and
+		 * parse_events_term::val are unions and endianness needs
+		 * to be taken into account when changing such union members.
+		 */
+		new_term = add_config_term(new_type, head_terms, term->weak,
+					   str_type ? term->val.str : NULL, val);
 		if (!new_term)
 			return -ENOMEM;
-
-		if (str_type) {
-			new_term->val.str = strdup(term->val.str);
-			if (!new_term->val.str) {
-				zfree(&new_term);
-				return -ENOMEM;
-			}
-			new_term->free_str = true;
-		} else {
-			new_term->val.val = val;
-		}
 	}
 	return 0;
 }
@@ -1277,10 +1324,9 @@ static int add_cfg_chg(const struct perf_pmu *pmu,
 	if (bits) {
 		struct evsel_config_term *new_term;
 
-		new_term = add_config_term(new_term_type, head_terms, false);
+		new_term = add_config_term(new_term_type, head_terms, false, NULL, bits);
 		if (!new_term)
 			return -ENOMEM;
-		new_term->val.cfg_chg = bits;
 	}
 
 	return 0;
-- 
2.53.0


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2026-02-17  8:13 [PATCH v3 linux-next] perf parse-events: Fix big-endian 'overwrite' by writing correct union member Thomas Richter
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