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From Jim Patterson <Jim.Patter...@Cognos.COM>
Subject general/6890: Apache faults with certain CGIs
Date Sun, 26 Nov 2000 02:48:05 GMT

>Number:         6890
>Category:       general
>Synopsis:       Apache faults with certain CGIs
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    apache
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          sw-bug
>Submitter-Id:   apache
>Arrival-Date:   Sat Nov 25 18:50:00 PST 2000
>Closed-Date:
>Last-Modified:
>Originator:     Jim.Patterson@Cognos.COM
>Release:        2.0a8
>Organization:
apache
>Environment:
Windows 2000 SP1
Visual C++ 5.0 SP3
>Description:
Here's the traceback:

ap_http_filter(ap_filter_t * 0x00cc4ef0, ap_bucket_brigade * 0x00cbd228, int 0) line 965 +
17 bytes
ap_get_brigade(ap_filter_t * 0x00cc4ef0, ap_bucket_brigade * 0x00cbd228, int 0) line 211 +
20 bytes
ap_get_client_block(request_rec * 0x00cbcaf8, char * 0x016bded4, int 8192) line 2807 + 21
bytes
cgi_handler(request_rec * 0x00cbcaf8) line 605 + 21 bytes
ap_invoke_handler(request_rec * 0x00cbcaf8) line 358 + 10 bytes
process_request_internal(request_rec * 0x00cbcaf8) line 1335 + 9 bytes
ap_process_request(request_rec * 0x00cbcaf8) line 1362 + 9 bytes
ap_process_http_connection(conn_rec * 0x00cc4cd8) line 251 + 9 bytes
ap_run_process_connection(conn_rec * 0x00cc4cd8) line 85 + 78 bytes
ap_process_connection(conn_rec * 0x00cc4cd8) line 225
worker_main(int 0) line 1152
_threadstartex(void * 0x00412260) line 212 + 13 bytes
KERNEL32! 77e837cd()

The failing line is this one:

            if ((rv = ap_bucket_read(e, &ignore, &len, AP_BLOCK_READ)) != APR_SUCCESS)
{

ap_bucket_read is a macro which calls through a function pointer in the struct pointed to
by "e". In this case "e" has the value 0xdddddddd which I think indicates that it was fetched
from free'd memory. (This is a "Debug" build).

Problem appears to be use of a deleted struct. The pointer "e" iterates over the buckets in
the bucket brigade. At the end of the loop 'e' is advanced to the next bucket via 
  e = AP_BUCKET_NEXT(e)
but in some cases, 'e' is destroyed before this point e.g.
	AP_BUCKET_REMOVE(e)
	ap_bucket_destroy(e)
Because the debug malloc library overwrites deleted data, this bug is more likely to show
up in a debug build (one reason overwriting is done), but it is almost certain to occur in
release builds occasionally in a multi-threaded environment. Therefore, I think it is important
to correct this defect.
>How-To-Repeat:
Please review my fix. If you're not convinced I will attempt to create a consistent test case
(it happened when working with a large product which I cannot send to you).
>Fix:
Recommended fix: retrieve the "next" pointer before destroying 'e'

Here's a patch:

*** http_protocol.c-orig Sat Nov 25 20:31:48 2000
--- http_protocol.c Sat Nov 25 20:34:26 2000
***************
*** 960,965 ****
--- 960,966 ----
      if (f->c->remain) {
          e = AP_BRIGADE_FIRST(ctx->b);
          while (e != AP_BRIGADE_SENTINEL(ctx->b)) {
+             ap_bucket *next_e = AP_BUCKET_NEXT(e); // In case we destruct 'e'
              const char *ignore;
  
              if ((rv = ap_bucket_read(e, &ignore, &len, AP_BLOCK_READ)) != APR_SUCCESS)
{
***************
*** 986,992 ****
                  AP_BUCKET_REMOVE(e);
                  ap_bucket_destroy(e);
              }
!             e = AP_BUCKET_NEXT(e);
          }
          if (f->c->remain == 0) {
              ap_bucket *eos = ap_bucket_create_eos();
--- 987,993 ----
                  AP_BUCKET_REMOVE(e);
                  ap_bucket_destroy(e);
              }
!             e = next_e;
          }
          if (f->c->remain == 0) {
              ap_bucket *eos = ap_bucket_create_eos();
>Release-Note:
>Audit-Trail:
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