Subject: Re: [PATCH net-next v3 01/11] devlink: retain error in struct devlink_fmsg

From: Simon Horman <[email-masked]>
Date: Thu, 19 Oct 2023 15:00:37 +0200

On Wed, Oct 18, 2023 at 10:26:37PM +0200, Przemek Kitszel wrote:
> Retain error value in struct devlink_fmsg, to relieve drivers from
> checking it after each call.
> Note that fmsg is an in-memory builder/buffer of formatted message,
> so it's not the case that half baked message was sent somewhere.
> 
> We could find following scheme in multiple drivers:
>   err = devlink_fmsg_obj_nest_start(fmsg);
>   if (err)
>   	return err;
>   err = devlink_fmsg_string_pair_put(fmsg, "src", src);
>   if (err)
>   	return err;
>   err = devlink_fmsg_something(fmsg, foo, bar);
>   if (err)
> 	return err;
>   // and so on...
>   err = devlink_fmsg_obj_nest_end(fmsg);
> 
> With retaining error API that translates to:
>   devlink_fmsg_obj_nest_start(fmsg);
>   devlink_fmsg_string_pair_put(fmsg, "src", src);
>   devlink_fmsg_something(fmsg, foo, bar);
>   // and so on...
>   devlink_fmsg_obj_nest_end(fmsg);
> 
> What means we check error just when is time to send.
> 
> Possible error scenarios are developer error (API misuse) and memory
> exhaustion, both cases are good candidates to choose readability
> over fastest possible exit.
> 
> Note that this patch keeps returning errors, to allow per-driver conversion
> to the new API, but those are not needed at this point already.
> 
> This commit itself is an illustration of benefits for the dev-user,
> more of it will be in separate commits of the series.
> 
> Reviewed-by: Jesse Brandeburg <[email-masked]>
> Reviewed-by: Jiri Pirko <[email-masked]>
> Signed-off-by: Przemek Kitszel <[email-masked]>

...

> @@ -1027,14 +934,12 @@ int devlink_fmsg_binary_pair_put(struct devlink_fmsg *fmsg, const char *name,

Hi Przemek,

The line before this hunk is:

		err = devlink_fmsg_binary_put(fmsg, value + offset, data_size);

And, as of this patch, the implementation of
devlink_fmsg_binary_pair_nest_start() looks like this:

int devlink_fmsg_binary_put(struct devlink_fmsg *fmsg, const void *value,
                            u16 value_len)
{
        if (!fmsg->putting_binary)
                return -EINVAL;

        return devlink_fmsg_put_value(fmsg, value, value_len, NLA_BINARY);
}

Which may return an error, if the if condition is met, without setting
fmsg->err.

>  		if (err)
>  			break;
>  		/* Exit from loop with a break (instead of
> -		 * return) to make sure putting_binary is turned off in
> -		 * devlink_fmsg_binary_pair_nest_end
> +		 * return) to make sure putting_binary is turned off
>  		 */
>  	}
>  
> -	end_err = devlink_fmsg_binary_pair_nest_end(fmsg);
> -	if (end_err)
> -		err = end_err;

Prior to this patch, the value of err from the loop above was preserved,
unless devlink_fmsg_binary_pair_nest_end generated an error.

> +	err = devlink_fmsg_binary_pair_nest_end(fmsg);

But now it looks like this is only the case if fmsg->err corresponds to err
when the loop was exited.

Or in other words, the err returned by devlink_fmsg_binary_put()
is not propagated to the caller if !fmsg->putting_binary.

If so, is this intentional?

> +	fmsg->putting_binary = false;
>  
>  	return err;
>  }
> -- 
> 2.38.1
>