Stack buffer size issue

[evilsocket]

Hello to all, i'm trying to develop a kernel module that accepts an 
ioctl with this structure :

typedef struct{
   /* [INPUT]  */
   long   process;
   unsigned long address;
  
   /* [OUTPUT] */   
   long mm_size;
}
mg_query_t;

where :

process : is the pid of a process .
address : is the address of a buffer on the stack of that process .

mm_size : *should* be the return value of the ioctl, indicating the size 
of that buffer, as an example (userspace test application):

char * abuffer[123];
mg_query_t query;
  
query.process = getpid();
query.address = abuffer;
  
if( ioctl( fd, IOCTL_MTABLE_BY_PID, &query ) < 0 ){
   close(fd);
   perror( "IOCTL_MTABLE_BY_PID" );
   return -1;
}

printf( "SIZE : %d\n", query.mm_size );



This *should* give the output :

SIZE : 123

I'm using the struct task_struct in the kernel module, looping the mmap 
to find the vm area the address resides in and then to set

mm_size = vm_end - vm_start

But doing so i obtaing only the size of the vm page the buffer resides .

Any hints ?

Thanks in advantage .
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