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Bug #49106 PHP incorrectly sets no_local_copy=1 on response as Apache 2 module
Submitted: 2009-07-30 02:40 UTC Modified: 2010-01-26 21:49 UTC
Votes:22
Avg. Score:4.4 ± 1.1
Reproduced:7 of 14 (50.0%)
Same Version:8 (114.3%)
Same OS:7 (100.0%)
From: n dot sherlock at gmail dot com Assigned:
Status: Analyzed Package: Apache2 related
PHP Version: 5.*, 6 OS: *
Private report: No CVE-ID: None
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 [2009-07-30 02:40 UTC] n dot sherlock at gmail dot com
Description:
------------
If PHP (5.3.0) is running as an (Apache 2) module, it currently sets no_local_copy to 1 on the response it sends to Apache (sapi/apache2handler/sapi_apache2.c:463). It looks like this flag was set to disallow Apache from erroneously creating its own "304 Not Modified" responses based on the ETag or Last-Modified-Date of the PHP scripts' sourcecode itself, which would result in stale pages being served if the scripts' output changes over time.

But there's a serious side effect of setting this flag in combination with Apache's mod_cache. If the browser makes a conditional request for a cached PHP document, but the document is expired in the cache, mod_cache correctly passes on the conditional request to PHP. If the PHP script responds with a "304 Not Modified" code, mod_cache should generate a 304 response for the browser. But due to no_local_copy, Apache is denied from creating a 304 code in response to a request for a PHP document. This forces it to resend the (still valid) body of the PHP document from the cache with a 200 code.

But setting "no_local_copy=1" is not needed anyway. Just below the r->no_local_copy=1 line in sapi_apache2.c is a series of calls to apr_table_unset which remove any headers that Apache might have generated based on the PHP source itself and could be using to accept conditional requests. Starting at line 468 in php_apache_request_ctor, we have:

apr_table_unset(r->headers_out, "Content-Length");
apr_table_unset(r->headers_out, "Last-Modified");
apr_table_unset(r->headers_out, "Expires");
apr_table_unset(r->headers_out, "ETag");

It seems to me that removing the r->no_local_copy=1 will therefore not result in erroneous "304 Not Modified" responses being sent by Apache for PHP scripts.

At the moment if you request a mod_cache'd PHP page which itself sends no special caching directives (e.g. an empty script), the reply from the server is (trimmed to include only cache-relevant directives):

Status=OK - 200
Date=Sun, 26 Jul 2009 10:07:58 GMT

PHP has correctly suppressed the generation of Last-Modified-Date and ETag headers based on the source of the script itself. No conditional request is possible and you won't get a stale page.

Now, if you request a PHP document that does set "ETag", such as the attached code "index.php", the response from the server is:

Status=OK - 200
Date=Sun, 26 Jul 2009 10:11:02 GMT
Etag="ComputedETag"
Expires=Tue, 25 Aug 2009 10:11:02 GMT

And the error log shows that the script correctly returned a 200 response code to Apache. Now if you press "refresh" in Firefox, the browser sends this request:

If-None-Match="ComputedETag"
Cache-Control=max-age=0

This is a conditional get which will also result in Apache revalidating its cache (since max-age=0). So Apache passes the conditional request onto PHP, and PHP sends back a 304 Not Modified response. But due to no_local_copy, mod_cache cannot send a 304, it responds to the browser with:

Status=OK - 200
Date=Sun, 26 Jul 2009 10:11:35 GMT
Etag="ComputedETag"
Expires=Tue, 25 Aug 2009 10:11:35 GMT

So, I removed the line that sets no_local_copy in my PHP. This had no impact on the way that the empty PHP document that sets no cache directives was served, Apache never served erroneous 304 responses because it never saw ETag or Last-Modified headers based on the PHP source. But the ETag-conditional request for index.php by the browser now gives the correct 304 response code:

Status=Not Modified - 304
Date=Sun, 26 Jul 2009 10:16:23 GMT
Etag="ComputedETag1"
Expires=Tue, 25 Aug 2009 10:16:23 GMT

Reproduce code:
---------------
<?php

$etag="\"ComputedETag\"";

header("Etag: $etag");
//Expires ages away
header("Expires: " . gmdate("D, d M Y H:i:s", time()
    + 60 * 60 * 24 * 30) . " GMT");

if (isset($_SERVER['HTTP_IF_NONE_MATCH']) &&
    $_SERVER['HTTP_IF_NONE_MATCH'] == $etag) {

    /* At a users' request, the cache has been bypassed, but the
     * document is still the same. Avoid costly response generation
     * and waste of bandwidth by just sending not-modified.
     * (it is illegal to send a response-body by the HTTP spec).
     */
    header('HTTP/1.0 304 Not Modified');
    
    error_log(date('r')." - Response: 304 Not Modified");
    exit(); //Don't generate or send the body
}   

error_log(date('r')." - Response: 200. Generated document.");

echo "Document body goes here";

?>


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csxigxvg (last revision 2015-06-19 06:26 UTC) by sample at email dot tst)
bstjkqgo (last revision 2015-06-19 06:23 UTC) by sample at email dot tst)

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 [2010-01-26 15:03 UTC] minfrin at sharp dot fm
The httpd mod_cache is designed to work as a self contained cache that bolts onto the front of the server (or with httpd v2.3+, can be inserted anywhere in the httpd filter stack for more targeted caching).

In theory, php shouldn't be touching any of the cache fields in request_rec at all, nor should php be trying to obscure any HTTP headers if it detects caching has been enabled.

It should be possible for a php script to support conditional requests, in other words php should be able to detect the If-None-Match header and respond with 200 OK or 304 Not Modified as appropriate.

Does anyone know what problem was being solved that made php want to care as to whether mod_cache was present?
 [2010-01-26 16:56 UTC] rasmus@php.net
I have been looking at this code a bit this morning.  It does indeed 
look like the no_local_copy is not needed here since both Last-Modified 
and ETag that may be present prior to PHP being executed are removed.

And to minfrin, I think PHP does need to remove any ETag or Last-
Modified headers that are generated prior to PHP execution.  It simply 
makes no sense for Apache to generate an ETag for a request prior to 
PHP executing on that request.  How could that possibly be valid?
 [2010-01-26 21:49 UTC] n dot sherlock at gmail dot com
minfrin, the caching headers ETag and Last-Modified are added by Apache before PHP gets to run, and whether mod_cache is turned on or not. They aren't mod_cache specific.
 
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