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Re: status-line trailing SP is missing ?

Sergey Kandaurov
August 30, 2023 08:22AM
> On 29 Aug 2023, at 08:33, Maxim Dounin <mdounin@mdounin.ru> wrote:
>
> Hello!
>
> On Mon, Aug 28, 2023 at 08:59:28PM +0400, Sergey Kandaurov wrote:
>
>>
>>> On 26 Aug 2023, at 18:21, Jérémy Lal <kapouer@melix.org> wrote:
>>>
>>> Hi,
>>>
>>> https://bugs.debian.org/1050571
>>> reports that the Status line doesn't always end with space, which seems contradictory to RFC9112 which states:
>>> "A server MUST send the space that separates the status-code from the reason-phrase even when the reason-phrase is absent (i.e., the status-line would end with the space)."
>>>
>>> Is it a documented nginx 1.24.0 behavior ?
>>>
>>
>> Note that the response line with empty reason phrase
>> is properly generated since nginx 1.5.6.
>> The exception remains is proxying FastCGI responses
>> as there is no distinguished response line in CGI syntax.
>>
>> The reason is that Status is a CGI header field, and hence
>> it is parsed by a generic routine that cuts trailing spaces.
>> But it can have a trailing space per RFC 3875, section 6.3.3.
>> So it needs a special treatment to preserve SP before empty reason
>> phrase. The below patch should help; although it doesn't look
>> efficient and can be polished, I think this is quite enough for
>> valid use cases.
>
> I very much doubt that RFC 3875 properly defines whitespace
> handling, see my response to the original report. In this
> particular case, it seems to define a header which cannot be
> correctly parsed if reason-phrase is empty.
>

Yes, it is quite dubious how this can be parsed correctly.
Although it is valid to have a trailing space in Status,
this contradicts to header field value syntax per RFC 3875:
field-content = *( token | separator | quoted-string )

>>
>> # HG changeset patch
>> # User Sergey Kandaurov <pluknet@nginx.com>
>> # Date 1693238094 -14400
>> # Mon Aug 28 19:54:54 2023 +0400
>> # Node ID f621c60dfa277774ab5fadb3c8b805957ca3f281
>> # Parent 2880f60a80c3e2706151dc7b48dc1267e39c47a9
>> FastCGI: preserved SP for empty Status header reason phrase.
>>
>> diff --git a/src/http/modules/ngx_http_fastcgi_module.c b/src/http/modules/ngx_http_fastcgi_module.c
>> --- a/src/http/modules/ngx_http_fastcgi_module.c
>> +++ b/src/http/modules/ngx_http_fastcgi_module.c
>> @@ -2048,7 +2048,25 @@ ngx_http_fastcgi_process_header(ngx_http
>> }
>>
>> u->headers_in.status_n = status;
>> - u->headers_in.status_line = *status_line;
>> +
>> + if (status_line->len == 3) {
>> + /* preserve SP for empty reason phrase */
>> +
>> + u->headers_in.status_line.data = ngx_pnalloc(r->pool,
>> + 5);
>> + if (u->headers_in.status_line.data == NULL) {
>> + return NGX_ERROR;
>> + }
>> +
>> + ngx_memcpy(u->headers_in.status_line.data,
>> + status_line->data, 3);
>> + u->headers_in.status_line.data[3] = ' ';
>> + u->headers_in.status_line.data[4] = '\0';
>> + u->headers_in.status_line.len = 4;
>> +
>> + } else {
>> + u->headers_in.status_line = *status_line;
>> + }
>>
>> } else if (u->headers_in.location) {
>> u->headers_in.status_n = 302;
>
> Do you think we really need this complexity here? I tend to think
> that

See above comment about efficiency.
I doubt empty reason phrase is seen often in the wild,
so this is a reasonable cost for kinda abusing protocol.
But this complexity also reflects the ugliness, that's
why I preferred the second version, also for its brevity.

>
> if (status_line->len > 3) {
> u->headers_in.status_line = *status_line;
> }
>
> would be enough, so nginx will generate status line by itself for
> such headers.

This is a nice trade-off between handling empty reason phrase
with extra allocation and dropping it altogether, I like it.

>
> The only downside I can see is that it will provide no way to
> actually generate a response with well-known status code, yet with
> an empty reason phrase.
>
> (Also note that uwsgi and scgi modules needs to be patched
> similarly.)

These modules use a distinct routine to parse status line,
that is ngx_http_parse_status_line, which respects spaces,
and they expect the first line to look like a status line
(similar to the proxy module).
So I don't see immediate necessity to patch them as well.

>
>> Alternatively, the reason-phrase value from FastCGI response
>> can be ignored, nginx will translate it with its own value:
>>
>> # HG changeset patch
>> # User Sergey Kandaurov <pluknet@nginx.com>
>> # Date 1693241054 -14400
>> # Mon Aug 28 20:44:14 2023 +0400
>> # Node ID f1a452cbd57ff4fba1caf3f36cb3624bd45411ea
>> # Parent 2880f60a80c3e2706151dc7b48dc1267e39c47a9
>> FastCGI: preserved SP for empty Status header reason phrase.
>>
>> As per RFC 3875 sec 6.3.3, the status code is always followed by SP.
>> The header is parsed by the common routine, and it can be challenging
>> to properly translate empty reason phrase to HTTP/1.1 status line, as
>> that would require an additional memory allocation. For simplicity,
>> the reason phrase is now ignored; the header filter will take care of
>> it by inserting its own reason phrase for well known status codes.
>>
>> diff --git a/src/http/modules/ngx_http_fastcgi_module.c b/src/http/modules/ngx_http_fastcgi_module.c
>> --- a/src/http/modules/ngx_http_fastcgi_module.c
>> +++ b/src/http/modules/ngx_http_fastcgi_module.c
>> @@ -2048,7 +2048,6 @@ ngx_http_fastcgi_process_header(ngx_http
>> }
>>
>> u->headers_in.status_n = status;
>> - u->headers_in.status_line = *status_line;
>>
>> } else if (u->headers_in.location) {
>> u->headers_in.status_n = 302;
>
> I don't think it's a good idea, since this always drops the
> reason phrase provided by the upstream server. It can contain
> some meaningful information which will be lost as a result, most
> notably for non-standard error codes.

It is little to no benefit to carry reason phrase,
also this is consistent with HTTP/1.1:
: A client SHOULD ignore the reason-phrase content because
: it is not a reliable channel for information

Note also that HTTP/2 and then HTTP/3 do not define a way
to carry the reason phrase that is included in HTTP/1.1.

Personally I'd proceed with your approach for fastcgi module
only, though I won't insist to patch other modules as well.
Although this would introduce an extra condition in these
other modules without a reason (as per my above analysis),
this would keep sources in sync between them, which is fine,
and will allow to stop using ngx_http_parse_status_line().

For the latter, uwsgi seems to use HTTP syntax in response,
so it is ok to call ngx_http_parse_status_line() here, but
scgi seems to follows CGI syntax where status line is
carried in the Status header field, so the call would
always return NGX_ERROR. Luckily this is already handled
by falling back to ngx_http_scgi_process_header().

--
Sergey Kandaurov
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Subject Author Posted

status-line trailing SP is missing ?

kapouer August 26, 2023 10:22AM

Re: status-line trailing SP is missing ?

Sergey Kandaurov August 28, 2023 01:00PM

Re: status-line trailing SP is missing ?

Maxim Dounin August 29, 2023 12:34AM

Re: status-line trailing SP is missing ?

Sergey Kandaurov August 30, 2023 08:22AM

Re: status-line trailing SP is missing ?

Maxim Dounin August 31, 2023 06:30AM

Re: status-line trailing SP is missing ?

Sergey Kandaurov August 31, 2023 07:46AM

Re: status-line trailing SP is missing ?

Maxim Dounin August 31, 2023 03:26PM

Re: status-line trailing SP is missing ?

Maxim Dounin August 29, 2023 12:16AM

Re: status-line trailing SP is missing ?

Sergey Kandaurov August 31, 2023 07:46AM

Re: status-line trailing SP is missing ?

Maxim Dounin August 31, 2023 04:14PM



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