Re: [PATCH v4 1/8] http: read HTTP WWW-Authenticate response headers

From: Victoria Dye <vdye@github.com>
Date: Dec 14, 2022, 23:15 UTC
Message-ID: <c255896d-637d-f7b0-8698-10a2112852c1@github.com>
In-Reply-To: <b5b56ccd9419353a4bf5bc9d751a711af07d2197.1670880984.git.gitgitgadget@gmail.com>

Matthew John Cheetham via GitGitGadget wrote:

Show 37 quoted lines

> +static size_t fwrite_wwwauth(char *ptr, size_t eltsize, size_t nmemb, void *p)
> +{
> +	size_t size = eltsize * nmemb;
> +	struct strvec *values = &http_auth.wwwauth_headers;
> +	struct strbuf buf = STRBUF_INIT;
> +	const char *val;
> +	const char *z = NULL;
> +
> +	/*
> +	 * Header lines may not come NULL-terminated from libcurl so we must
> +	 * limit all scans to the maximum length of the header line, or leverage
> +	 * strbufs for all operations.
> +	 *
> +	 * In addition, it is possible that header values can be split over
> +	 * multiple lines as per RFC 2616 (even though this has since been
> +	 * deprecated in RFC 7230). A continuation header field value is
> +	 * identified as starting with a space or horizontal tab.
> +	 *
> +	 * The formal definition of a header field as given in RFC 2616 is:
> +	 *
> +	 *   message-header = field-name ":" [ field-value ]
> +	 *   field-name     = token
> +	 *   field-value    = *( field-content | LWS )
> +	 *   field-content  = <the OCTETs making up the field-value
> +	 *                    and consisting of either *TEXT or combinations
> +	 *                    of token, separators, and quoted-string>
> +	 */
> +
> +	strbuf_add(&buf, ptr, size);
> +
> +	/* Strip the CRLF that should be present at the end of each field */
> +	strbuf_trim_trailing_newline(&buf);
> +
> +	/* Start of a new WWW-Authenticate header */
> +	if (skip_iprefix(buf.buf, "www-authenticate:", &val)) {
> +		while (isspace(*val))
> +			val++;

Per the RFC [1]:

> The field value MAY be preceded by any amount of LWS, though a single SP
> is preferred.

And LWS (linear whitespace) is defined as:

> CRLF           = CR LF 
> LWS            = [CRLF] 1*( SP | HT )

and 'isspace()' includes CR, LF, SP, and HT [2].

Looks good!

[1] https://datatracker.ietf.org/doc/html/rfc2616#section-4-2 [2] https://linux.die.net/man/3/isspace

> +
> +		strvec_push(values, val);

I had the same question about "what happens with an empty 'val' here?" as Stolee did earlier [3], but I *think* the "zero length" (i.e., single null terminator) will be copied successfully. It's probably worth testing that explicitly, though (I see you set up tests in later patches - ideally a "www-authenticate:<mix of whitespace>" line could be tested there).

[3] https://lore.kernel.org/git/9fded44b-c503-f8e5-c6a6-93e882d50e27@github.com/

Show 12 quoted lines

> +		http_auth.header_is_last_match = 1;
> +		goto exit;
> +	}
> +
> +	/*
> +	 * This line could be a continuation of the previously matched header
> +	 * field. If this is the case then we should append this value to the
> +	 * end of the previously consumed value.
> +	 */
> +	if (http_auth.header_is_last_match && isspace(*buf.buf)) {
> +		const char **v = values->v + values->nr - 1;
> +		char *append = xstrfmt("%s%.*s", *v, (int)(size - 1), ptr + 1);

In this case (where the line is a continuation of a 'www-authenticate' header), it looks like the code here expects *exactly* one LWS at the start of the line ('isspace(*buf.buf)' requiring at least one space to append the header, 'ptr + 1' skipping no more than one). But, according to the RFC, it could be more than one:

> Header fields can be extended over multiple lines by preceding each extra
> line with at least one SP or HT.

So I think 'buf.buf' might need to have all preceding spaces removed, like you did in the "Start of a new WWW-Authenticate header" block.

Also, if you're copying 'ptr' into 'buf' to avoid issues from a missing null terminator, wouldn't you want to use 'buf.buf' (instead of 'ptr') in 'xstrfmt()'?

> +
> +		free((void*)*v);
> +		*v = append;

I was about to suggest (optionally) rewriting this to use 'strvec_pop()' and 'strvec_push_nodup()':

	strvec_pop(values); 
	strvec_push_nodup(values, append);

to maybe make this a bit easier to follow, but unfortunately 'strvec_push_nodup()' isn't available outside of 'strvec.c'. If you did want to use 'strvec' functions, you could remove the 'static' from 'strvec_push_nodup()' and add it to 'strvec.h' it in a later reroll, but I don't consider that change "blocking" or even important enough to warrant its own reroll.

Show 16 quoted lines

> +
> +		goto exit;
> +	}
> +
> +	/* This is the start of a new header we don't care about */
> +	http_auth.header_is_last_match = 0;
> +
> +	/*
> +	 * If this is a HTTP status line and not a header field, this signals
> +	 * a different HTTP response. libcurl writes all the output of all
> +	 * response headers of all responses, including redirects.
> +	 * We only care about the last HTTP request response's headers so clear
> +	 * the existing array.
> +	 */
> +	if (skip_iprefix(buf.buf, "http/", &z))
> +		strvec_clear(values);

The comments describing the intended behavior (as well as the commit message) are clear and explain the somewhat esoteric (at least to my untrained eye ;) ) code. Thanks!

Show 18 quoted lines

> +
> +exit:
> +	strbuf_release(&buf);
> +	return size;
> +}
> +
>  size_t fwrite_null(char *ptr, size_t eltsize, size_t nmemb, void *strbuf)
>  {
>  	return nmemb;
> @@ -1864,6 +1940,8 @@ static int http_request(const char *url,
>  					 fwrite_buffer);
>  	}
>  
> +	curl_easy_setopt(slot->curl, CURLOPT_HEADERFUNCTION, fwrite_wwwauth);
> +
>  	accept_language = http_get_accept_language_header();
>  
>  	if (accept_language)