Re: [PATCH v3 linux-next] perf parse-events: Fix big-endian 'overwrite' by writing correct union member

[James Clark <james.clark@linaro.org>]

On 17/02/2026 8:13 am, Thomas Richter wrote:
> 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:
> +	}

Hi Thomas,

This still has nothing for the default label which clang doesn't like:

util/parse-events.c:1182:10: error: label at end of compound statement: 
expected statement
         default:

And then fixing that by replacing all the "t->val.val = val;" cases with 
just "default:" gives a new error. "val" in get_config_terms() is 
uninitialized (although unused, but the compiler doesn't know that):

util/parse-events.c:1269:8: error: variable 'val' is used uninitialized 
whenever switch case is taken [-Werror,-Wsometimes-uninitialized]
                 case PARSE_EVENTS__TERM_TYPE_RATIO_TO_PREV:
                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
util/parse-events.c:1296:42: note: uninitialized use occurs here
                                            str_type ? term->val.str : 
NULL, val);


> +
>   	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;