ctdb debug vasprintf memory overwrite in ib

Peter Somogyi psomogyi at gamax.hu
Fri Apr 20 11:09:49 GMT 2007

Hi Volker,

Accidentally I've run into a memory overwrite problem with your debug code 

# gdb bin/ibwrapper_test #-i 1 -d -t 0 -n 1000
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This GDB was configured as "i586-suse-linux"...Using host libthread_db 
library "/lib/libthread_db.so.1".

(gdb) break ibwrapper.c:559
Breakpoint 1 at 0x805697b: file ib/ibwrapper.c, line 559.
(gdb) run -i 1 -d -t 0 -n 1000
Starting program: /root/psomogyi/ctdb/bin/ibwrapper_test -i 1 -d -t 0 -n 1000
[Thread debugging using libthread_db enabled]
[New Thread -1209997120 (LWP 9731)]
[Switching to Thread -1209997120 (LWP 9731)]

Breakpoint 1, ibw_event_handler_cm (ev=0x80691c8, fde=0x8069868, flags=1, 
private_data=0x8069258) at ib/ibwrapper.c:559
559             if (event!=NULL && (rc=rdma_ack_cm_event(event))) {
(gdb) print cma_id->context
$1 = (void *) 0x806b788
(gdb) next
563             DEBUG(0, ("cm event handler: %s", ibw_lasterr));
(gdb) print cma_id->context
$2 = (void *) 0x806b788
(gdb) next
1177065484.603498 [9731]: cm event handler: RDMA_CM_EVENT_REJECTED, error 8
565             if (cma_id!=pctx->cm_id) {
(gdb) print cma_id->context
$3 = (void *) 0x6e616820

More precisely, after the call vasprintf gets the memory overwritten (shows me 
gdb by stepping).
When I change vasprintf usage to vsprintf/vsnprintf with fixed buffer then it 
runs fine. Also runs fine when I comment its line.

BTW. man says vasprintf is a GNU extension, not in POSIX or C. It tells 
something also about #define _GNU_SOURCE.

I'm still not aware of *why* this overwrite happens, but on my system somehow 
it occures. (vasprintf returns 50 which is ok, so we don't have failure.)

(glibc version is 2.4-31.2 on my SLES10)

Picking another vasprintf implementation like this solves my problem:

int vasprintf(char **ptr, const char *format, va_list ap)
    int ret;
    va_list tmp_ap;

    va_copy(tmp_ap, ap);
    ret = vsnprintf(NULL, 0, format, tmp_ap);
    if (ret <= 0) return ret;

    (*ptr) = (char *)malloc(ret+1);
    if (!*ptr) return -1;
    ret = vsnprintf(*ptr, ret+1, format, ap);

    return ret;
(code is from jerry at samba.org 

Do you agree changing do_debug this way?

Peter Somogyi
Gamax Kft
Bartok Bela ut 15/D
H-1114, Budapest, Hungary
e-mail: psomogyi at gamax.hu

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