netdev - Re: [patch iproute2-next v3 3/6] devlink: extend pr_out_nested

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Date: Fri, 27 Oct 2023 15:12:46 +0200
From: Petr Machata <me@...chata.org>
To: David Ahern <dsahern@...il.com>
Cc: Jiri Pirko <jiri@...nulli.us>, netdev@...r.kernel.org,
 stephen@...workplumber.org, daniel.machon@...rochip.com
Subject: Re: [patch iproute2-next v3 3/6] devlink: extend
 pr_out_nested_handle() to print object


David Ahern <dsahern@...il.com> writes:

> On 10/24/23 4:04 AM, Jiri Pirko wrote:
>> @@ -2861,6 +2842,38 @@ static void pr_out_selftests_handle_end(struct dl *dl)
>>  		__pr_out_newline();
>>  }
>>  
>> +static void __pr_out_nested_handle(struct dl *dl, struct nlattr *nla_nested_dl,
>> +				   bool is_object)
>> +{
>> +	struct nlattr *tb[DEVLINK_ATTR_MAX + 1] = {};
>> +	int err;
>> +
>> +	err = mnl_attr_parse_nested(nla_nested_dl, attr_cb, tb);
>> +	if (err != MNL_CB_OK)
>> +		return;
>> +
>> +	if (!tb[DEVLINK_ATTR_BUS_NAME] ||
>> +	    !tb[DEVLINK_ATTR_DEV_NAME])
>> +		return;
>> +
>> +	if (!is_object) {
>> +		char buf[64];
>> +
>> +		sprintf(buf, "%s/%s", mnl_attr_get_str(tb[DEVLINK_ATTR_BUS_NAME]),
>> +			mnl_attr_get_str(tb[DEVLINK_ATTR_DEV_NAME]));
>
> buf[64] - 1 for null terminator - 16 for IFNAMSIZ leaves 47. I do not
> see limits on bus name length, so how can you guarantee it is always <
> 47 characters?
>
> Make this snprintf, check the return and make sure buf is null terminated.

I was wondering whether somehing like this might make sense in the
iproute2 library:

	#define alloca_sprintf(FMT, ...) ({					\
		int xasprintf_n = snprintf(NULL, 0, (FMT), __VA_ARGS__);	\
		char *xasprintf_buf = alloca(xasprintf_n);			\
		sprintf(xasprintf_buf, (FMT), __VA_ARGS__);			\
		xasprintf_buf;							\
	})

	void foo() {
		const char *buf = alloca_sprintf("%x %y %z", etc.);
		printf(... buf ...);
	}

I'm not really happy with it -- because of alloca vs. array, and because
of the double evaluation. But all those SPRINT_BUF's peppered everywhere
make me uneasy every time I read or write them.

Or maybe roll something custom asprintf-like that can reuse and/or
realloc a passed-in buffer?

The sprintf story is pretty bad in iproute2 right now, IMHO.