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From Hynek Schlawack <hynek+use...@hys.in-berlin.de>
Subject [users@httpd] Patching mod_cgi to prepend commandlines in Apache 2/Linux
Date Tue, 27 Jul 2004 09:30:49 GMT
Hi,

I'm afraid that I'm going to embarass myself but I simply can't work it
out:

I'm trying to patch mod_cgi of Apache 2 running on Linux so that it
prepends the commandline with another command (notably a wrapper that
does some stuff we need). I mostly only modificated
default_build_command (unfortunatly there will be more changes
necessary, so I can't just write an own build_command()), the result is
the following diff:

--- /home/hynek/src/httpd-2.0.49/modules/generators/mod_cgi.c   2004-02-09 21:53:17.000000000
+0100
+++ mod_vm_cgiwrap.c    2004-07-27 11:07:29.500405784 +0200
@@ -26,6 +26,8 @@
  * they fail.
  */
 
+#define WRAP_BIN "/home/hynek/t"
+
+
 #include "apr.h"
 #include "apr_strings.h"
 #include "apr_thread_proc.h"    /* for RLIMIT stuff */
@@ -485,7 +487,8 @@
     const char *args = NULL;
 
     if (e_info->process_cgi) {
-        *cmd = r->filename;
+       *cmd = apr_pstrdup(p, WRAP_BIN);
+
         /* Do not process r->args if they contain an '=' assignment 
          */
         if (r->args && r->args[0] && !ap_strchr_c(r->args, '='))
{
@@ -494,25 +497,26 @@
     }
 
     if (!args) {
-        numwords = 1;
+        numwords = 2;
     }
     else {
         /* count the number of keywords */
-        for (x = 0, numwords = 2; args[x]; x++) {
+        for (x = 0, numwords = 3; args[x]; x++) {
             if (args[x] == '+') {
                 ++numwords;
             }
         }
     }
-    /* Everything is - 1 to account for the first parameter 
-     * which is the program name.
+    /* Everything is - 2 to account for the first parameters 
+     * which are the wrapper and the program name.
      */ 
-    if (numwords > APACHE_ARG_MAX - 1) {
-        numwords = APACHE_ARG_MAX - 1;    /* Truncate args to prevent overrun */
+    if (numwords > APACHE_ARG_MAX - 2) {
+        numwords = APACHE_ARG_MAX - 2;    /* Truncate args to prevent overrun */
     }
     *argv = apr_palloc(p, (numwords + 2) * sizeof(char *));
     (*argv)[0] = *cmd;
-    for (x = 1, idx = 1; x < numwords; x++) {
+    (*argv)[1] = r->filename;
+    for (x = 2, idx = 2; x < numwords; x++) {
         w = ap_getword_nulls(p, &args, '+');
         ap_unescape_url(w);
         (*argv)[idx++] = ap_escape_shell_cmd(p, w);
@@ -1043,7 +1047,7 @@
     ap_hook_post_config(cgi_post_config, aszPre, NULL, APR_HOOK_REALLY_FIRST);
 }
 
-module AP_MODULE_DECLARE_DATA cgi_module =
+module AP_MODULE_DECLARE_DATA cgi_wrap_module =
 {
     STANDARD20_MODULE_STUFF,
     NULL,                        /* dir config creater */


t is in this case a binary that just prints "Content-Type:
text/html\n\nfoobar\n".

This modification works perfectly when the script is in a directory
defined by ScriptAlias(Match). However when I start it in a "normal"
directory that has +ExecCGI, I get a "[error] [client 127.0.0.1]
Premature end of script headers: test.pl" in the error_log.

I've been able to trace that the program is sucessfully "started" by the
module in run_cgi_child() (ie. the call to
ap_os_create_privileged_process() doesn't fail) however the program is
never started.

Could anyone give a a pointer where to look at? I'm starting to get
desperate as it seem trivial to me but it doesn't work. :( Google and
the other obvious ressources failed (or my imagination of
searching-keywords).

TIA,
Hynek

P.S. The setup is pretty vanilla, only mod_cgi is commented out. I'm certainly
not testing on a production server.


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