Re: [PATCH] xgethostname: handle long hostnames

From: Jonathan Nieder <jrnieder@gmail.com>
Date: Apr 13, 2017, 22:05 UTC
Message-ID: <20170413220524.GE10084@aiede.mtv.corp.google.com>
In-Reply-To: <20170413192335.20679-1-dturner@twosigma.com>

Hi,

David Turner wrote:

> If the full hostname doesn't fit in the buffer supplied to
> gethostname, POSIX does not specify whether the buffer will be
> null-terminated, so to be safe, we should do it ourselves.

[...]

Show 17 quoted lines

> +++ b/wrapper.c
> @@ -655,3 +655,16 @@ void sleep_millisec(int millisec)
>  {
>  	poll(NULL, 0, millisec);
>  }
> +
> +int xgethostname(char *buf, size_t len)
> +{
> +	/*
> +	 * If the full hostname doesn't fit in buf, POSIX does not
> +	 * specify whether the buffer will be null-terminated, so to
> +	 * be safe, do it ourselves.
> +	 */
> +	int ret = gethostname(buf, len);
> +	if (!ret)
> +		buf[len - 1] = 0;
> +	return ret;

I wonder if after null-terminating we would want to report this as an error, instead of silently using a truncated result. I.e. something like

Show 6 quoted lines

> +	if (!ret)
> +		buf[len - 1] = 0;
> +	if (strlen(buf) >= len - 1) {
> +		errno = ENAMETOOLONG;
> +		return -1;
> +	}

(or EINVAL --- either is equally descriptive).

Also POSIX requires that hostnames are <= 255 bytes. Maybe we can force the buffer to be large enough.

Thoughts? Jonathan