krb5_free_data

[Markus Moeller]

Hi Greg,

I see this in krb5 1.8.3

(gdb) where
#0  0x00007ffff5f911b5 in *__GI_raise (sig=<value optimized out>) at 
../nptl/sysdeps/unix/sysv/linux/raise.c:64
#1  0x00007ffff5f93fc0 in *__GI_abort () at abort.c:92
#2  0x00007ffff5fc75bb in __libc_message (do_abort=<value optimized out>, 
fmt=<value optimized out>) at ../sysdeps/unix/sysv/linux/libc_fatal.c:189
#3  0x00007ffff5fd0e16 in malloc_printerr (action=3, str=0x7ffff6088748 
"double free or corruption (fasttop)", ptr=<value optimized out>)
    at malloc.c:6267
#4  0x00007ffff5fd5b8c in *__GI___libc_free (mem=<value optimized out>) at 
malloc.c:3739
#5  0x00007ffff7939472 in krb5_free_data (context=<value optimized out>, 
val=0x6171f0) at ../../../../src/lib/krb5/krb/kfree.c:253
#6  0x00000000004051a4 in get_ad_groups (ad_groups=0x7fffffffaff0 "", 
context=0x60f9e0, pac=0x0) at negotiate_kerberos_pac.cc:464
#7  0x0000000000403265 in main (argc=5, argv=0x7fffffffe0e8) at 
negotiate_kerberos_auth.cc:419

and line 253 is free(val->data)

The code I have is the following and I get an error in krb5_pac_get_buffer: 
Invalid argument


ad_data = (krb5_data *)xmalloc(sizeof(krb5_data));

#define KERB_LOGON_INFO 1
    ret = krb5_pac_get_buffer(context, pac, KERB_LOGON_INFO, ad_data);
    if (check_k5_err(context, "krb5_pac_get_buffer", ret))
        goto k5clean;


k5clean:
    krb5_free_data(context, ad_data);

Maybe there is something wrong. I usually do not get this error, but I don't 
know why it happens sometimes.

Markus

-----Original Message----- 
From: Greg Hudson
Sent: Saturday, June 21, 2014 11:36 PM
To: Markus Moeller ; krbdev at mit.edu
Subject: Re: krb5_free_data

On 06/21/2014 05:17 PM, Markus Moeller wrote:
> Sorry I do not understand. Right now I get crashes as val->data is NULL.

free(NULL) does not crash.  See section 7.20.3.2 of:

    http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf

or:

    http://pubs.opengroup.org/onlinepubs/009695399/functions/free.html

We make this assumption all over the krb5 code base, as do many other
programs.  Perhaps the crashes you are seeing have another cause?