Subject: Re: [PATCH RFC iproute2] libnetlink: allow reading more than one message from extack

From: Stephen Hemminger <[email-masked]>
Date: Mon, 19 Mar 2018 09:09:50 -0700

On Fri, 16 Mar 2018 16:23:09 -0300
Marcelo Ricardo Leitner <[email-masked]> wrote:

> This patch introduces support for reading more than one message from
> extack's and to adjust their level (warning/error) accordingly.
> 
> Yes, there is a FIXME tag in the callback call for now.
> 
> Signed-off-by: Marcelo Ricardo Leitner <[email-masked]>

Make sense, can hold off until kernel supports warnings.

> ---
>  lib/libnetlink.c | 55 ++++++++++++++++++++++++++++++++++++++++---------------
>  1 file changed, 40 insertions(+), 15 deletions(-)
> 
> diff --git a/lib/libnetlink.c b/lib/libnetlink.c
> index 928de1dd16d84b7802c06d0a659f5d73e1bbcb4b..7c4c81e11e02ea857888190eb5e7a9e99d159bb3 100644
> --- a/lib/libnetlink.c
> +++ b/lib/libnetlink.c
> @@ -43,9 +43,16 @@ static const enum mnl_attr_data_type extack_policy[NLMSGERR_ATTR_MAX + 1] = {
>  	[NLMSGERR_ATTR_OFFS]	= MNL_TYPE_U32,
>  };
>  
> +#define NETLINK_MAX_EXTACK_MSGS 8

Would rather not have fixed maximums

> +struct extack_args {
> +	const struct nlattr *msg[NETLINK_MAX_EXTACK_MSGS];
> +	const struct nlattr *offs;
> +	int msg_count;
> +};

If you put msg[] last in structure, it could be variable length.

>  static int err_attr_cb(const struct nlattr *attr, void *data)
>  {
> -	const struct nlattr **tb = data;
> +	struct extack_args *tb = data;
>  	uint16_t type;
>  
>  	if (mnl_attr_type_valid(attr, NLMSGERR_ATTR_MAX) < 0) {
> @@ -60,19 +67,23 @@ static int err_attr_cb(const struct nlattr *attr, void *data)
>  		return MNL_CB_ERROR;
>  	}
>  
> -	tb[type] = attr;
> +	if (type == NLMSGERR_ATTR_OFFS)
> +		tb->offs = attr;
> +	else if (tb->msg_count < NETLINK_MAX_EXTACK_MSGS)
> +		tb->msg[tb->msg_count++] = attr;
>  	return MNL_CB_OK;
>  }
>  
>  /* dump netlink extended ack error message */
>  int nl_dump_ext_ack(const struct nlmsghdr *nlh, nl_ext_ack_fn_t errfn)
>  {
> -	struct nlattr *tb[NLMSGERR_ATTR_MAX + 1] = {};
> +	struct extack_args tb = {};
>  	const struct nlmsgerr *err = mnl_nlmsg_get_payload(nlh);
>  	const struct nlmsghdr *err_nlh = NULL;
>  	unsigned int hlen = sizeof(*err);
> -	const char *msg = NULL;
> +	const char *msg[NETLINK_MAX_EXTACK_MSGS] = {};
>  	uint32_t off = 0;
> +	int ret, i;
>  
>  	/* no TLVs, nothing to do here */
>  	if (!(nlh->nlmsg_flags & NLM_F_ACK_TLVS))
> @@ -82,14 +93,14 @@ int nl_dump_ext_ack(const struct nlmsghdr *nlh, nl_ext_ack_fn_t errfn)
>  	if (!(nlh->nlmsg_flags & NLM_F_CAPPED))
>  		hlen += mnl_nlmsg_get_payload_len(&err->msg);
>  
> -	if (mnl_attr_parse(nlh, hlen, err_attr_cb, tb) != MNL_CB_OK)
> +	if (mnl_attr_parse(nlh, hlen, err_attr_cb, &tb) != MNL_CB_OK)
>  		return 0;
>  
> -	if (tb[NLMSGERR_ATTR_MSG])
> -		msg = mnl_attr_get_str(tb[NLMSGERR_ATTR_MSG]);
> +	for (i = 0; i < NETLINK_MAX_EXTACK_MSGS && tb.msg[i]; i++)
> +		msg[i] = mnl_attr_get_str(tb.msg[i]);
>  
> -	if (tb[NLMSGERR_ATTR_OFFS]) {
> -		off = mnl_attr_get_u32(tb[NLMSGERR_ATTR_OFFS]);
> +	if (tb.offs) {
> +		off = mnl_attr_get_u32(tb.offs);
>  
>  		if (off > nlh->nlmsg_len) {
>  			fprintf(stderr,
> @@ -100,21 +111,35 @@ int nl_dump_ext_ack(const struct nlmsghdr *nlh, nl_ext_ack_fn_t errfn)
>  	}
>  
>  	if (errfn)
> -		return errfn(msg, off, err_nlh);
> +		return errfn(*msg, off, err_nlh); /* FIXME */
>  
> -	if (msg && *msg != '\0') {
> +	ret = 0;
> +	for (i = 0; i < NETLINK_MAX_EXTACK_MSGS && msg[i]; i++) {
>  		bool is_err = !!err->error;
> +		const char *_msg = msg[i];
> +
> +		/* Message tagging has precedence.
> +		 * KERN_WARNING = ASCII Start Of Header ('\001') + '4'
> +		 */
> +		if (!strncmp(_msg, "\0014", 2)) {
> +			is_err = false;
> +			_msg += 2;
> +		}

If you are going to have an API that has levels, it must be the same
as existing syslog kernel log format and maybe even get some code reuse.

> +		/* But we can't have Error if it didn't fail. */
> +		if (is_err && !err->error)
> +			is_err = 0;
>  
>  		fprintf(stderr, "%s: %s",
> -			is_err ? "Error" : "Warning", msg);
> -		if (msg[strlen(msg) - 1] != '.')
> +			is_err ? "Error" : "Warning", _msg);
> +		if (_msg[strlen(_msg) - 1] != '.')
>  			fprintf(stderr, ".");
>  		fprintf(stderr, "\n");
>  
> -		return is_err ? 1 : 0;
> +		if (is_err)
> +			ret = 1;
>  	}
>  
> -	return 0;
> +	return ret;
>  }
>  #else
>  #warning "libmnl required for error support"