Re: [PATCH 2/3] perf tools: Make the JSON parser more conformant when in strict mode

[James Clark]

On 07/10/2021 18:52, Jiri Olsa wrote:
> On Thu, Oct 07, 2021 at 12:05:41PM +0100, James Clark wrote:
>> Return an error when a trailing comma is found or a new item is
>> encountered before a comma or an opening brace. This ensures that the
>> perf json files conform more closely to the spec at https://www.json.org
>>
>> Signed-off-by: James Clark <james.clark@xxxxxxx>
>> ---
>>  tools/perf/pmu-events/jsmn.c | 42 ++++++++++++++++++++++++++++++++++--
>>  1 file changed, 40 insertions(+), 2 deletions(-)
>>
>> diff --git a/tools/perf/pmu-events/jsmn.c b/tools/perf/pmu-events/jsmn.c
>> index 11d1fa18bfa5..8124d2d3ff0c 100644
>> --- a/tools/perf/pmu-events/jsmn.c
>> +++ b/tools/perf/pmu-events/jsmn.c
>> @@ -176,6 +176,14 @@ jsmnerr_t jsmn_parse(jsmn_parser *parser, const char *js, size_t len,
>>  	jsmnerr_t r;
>>  	int i;
>>  	jsmntok_t *token;
>> +#ifdef JSMN_STRICT
> 
> I might have missed some discussion on this, but do we need the
> JSMN_STRICT define, if you enable it in the next patch?
> why can't we be more strict by default.. do you plan to disable
> it in future?

I didn't plan on disabling it, I was just trying to keep to the existing style of the
jsmn project.

I could have added the trailing comma detection by default and not inside any
#ifdef JSMN_STRICT blocks, but I would like to enable JSMN_STRICT anyway, because it
enables some additional built in checking that was already there. So I thought it
made sense to put my new strict stuff inside the existing strict option.

One option would be to remove all (including the existing) #ifdef JSMN_STRICT blocks
and have everything strict by default. But it would be a further deviation from jsmn.

Thanks
James

> 
> thanks,
> jirka
> 
>> +	/*
>> +	 * Keeps track of whether a new object/list/primitive is expected. New items are only
>> +	 * allowed after an opening brace, comma or colon. A closing brace after a comma is not
>> +	 * valid JSON.
>> +	 */
>> +	int expecting_item = 1;
>> +#endif
>>  
>>  	for (; parser->pos < len; parser->pos++) {
>>  		char c;
>> @@ -185,6 +193,10 @@ jsmnerr_t jsmn_parse(jsmn_parser *parser, const char *js, size_t len,
>>  		switch (c) {
>>  		case '{':
>>  		case '[':
>> +#ifdef JSMN_STRICT
>> +			if (!expecting_item)
>> +				return JSMN_ERROR_INVAL;
>> +#endif
>>  			token = jsmn_alloc_token(parser, tokens, num_tokens);
>>  			if (token == NULL)
>>  				return JSMN_ERROR_NOMEM;
>> @@ -196,6 +208,10 @@ jsmnerr_t jsmn_parse(jsmn_parser *parser, const char *js, size_t len,
>>  			break;
>>  		case '}':
>>  		case ']':
>> +#ifdef JSMN_STRICT
>> +			if (expecting_item)
>> +				return JSMN_ERROR_INVAL;
>> +#endif
>>  			type = (c == '}' ? JSMN_OBJECT : JSMN_ARRAY);
>>  			for (i = parser->toknext - 1; i >= 0; i--) {
>>  				token = &tokens[i];
>> @@ -219,6 +235,11 @@ jsmnerr_t jsmn_parse(jsmn_parser *parser, const char *js, size_t len,
>>  			}
>>  			break;
>>  		case '\"':
>> +#ifdef JSMN_STRICT
>> +			if (!expecting_item)
>> +				return JSMN_ERROR_INVAL;
>> +			expecting_item = 0;
>> +#endif
>>  			r = jsmn_parse_string(parser, js, len, tokens,
>>  					      num_tokens);
>>  			if (r < 0)
>> @@ -229,11 +250,15 @@ jsmnerr_t jsmn_parse(jsmn_parser *parser, const char *js, size_t len,
>>  		case '\t':
>>  		case '\r':
>>  		case '\n':
>> -		case ':':
>> -		case ',':
>>  		case ' ':
>>  			break;
>>  #ifdef JSMN_STRICT
>> +		case ':':
>> +		case ',':
>> +			if (expecting_item)
>> +				return JSMN_ERROR_INVAL;
>> +			expecting_item = 1;
>> +			break;
>>  			/*
>>  			 * In strict mode primitives are:
>>  			 * numbers and booleans.
>> @@ -253,6 +278,9 @@ jsmnerr_t jsmn_parse(jsmn_parser *parser, const char *js, size_t len,
>>  		case 'f':
>>  		case 'n':
>>  #else
>> +		case ':':
>> +		case ',':
>> +			break;
>>  			/*
>>  			 * In non-strict mode every unquoted value
>>  			 * is a primitive.
>> @@ -260,6 +288,12 @@ jsmnerr_t jsmn_parse(jsmn_parser *parser, const char *js, size_t len,
>>  			/*FALL THROUGH */
>>  		default:
>>  #endif
>> +
>> +#ifdef JSMN_STRICT
>> +			if (!expecting_item)
>> +				return JSMN_ERROR_INVAL;
>> +			expecting_item = 0;
>> +#endif
>>  			r = jsmn_parse_primitive(parser, js, len, tokens,
>>  						 num_tokens);
>>  			if (r < 0)
>> @@ -282,7 +316,11 @@ jsmnerr_t jsmn_parse(jsmn_parser *parser, const char *js, size_t len,
>>  			return JSMN_ERROR_PART;
>>  	}
>>  
>> +#ifdef JSMN_STRICT
>> +	return expecting_item ? JSMN_ERROR_INVAL : JSMN_SUCCESS;
>> +#else
>>  	return JSMN_SUCCESS;
>> +#endif
>>  }
>>  
>>  /*
>> -- 
>> 2.28.0
>>
>