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Full Discussion: Problem about Kernel oops
Top Forums Programming Problem about Kernel oops Post 302588222 by Corona688 on Saturday 7th of January 2012 04:39:04 PM
Old 01-07-2012
Quote:
Originally Posted by homeboy
hello,everyone,I'm reading LDD3.Topics about oops,it said the code bellow would cause a fault condition because "this method copies a string to a local variable,unfortunately,the string is longer than the destination array".Well,hard to understand....Is that right?I thought the fault should be caused by the wrong memset,it overwrite the function stack,the value of __user is 0xff now.will anyone tell me what happened here?Thanks;
Code:
size_t fauly_read(struct file *filp, char __user *buf, size_t count, loff_t *pos)
{
      int ret;
      char stack_buf[4];

      memset(stack_buf, 0xff, 20);
      if(count > 4)
           count  = 4;

      ret = copy_to_user(buf, stack_buf, count);
      if(!ret)
          return count;

      return ret;
}

It's writing 20 bytes to an array 4 bytes long, rampaging 16 more bytes across memory it shouldn't, trashing things like local variable values, function parameters, and return locations, all of which are kept in the same general area of stack here.

You're both right, in other words. There's many possible problems that could be caused by this, not just corruption of the return vector. If you're lucky, it crashes immediately. If you're unlucky, it puts bizarre values into your local variables and function parameters, causing behavior that's very difficult to rationally explain.
This User Gave Thanks to Corona688 For This Post:
homeboy
 

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LEARN ABOUT REDHAT

netdump

NETDUMP-CLIENT(8)						  System Programs						 NETDUMP-CLIENT(8)

NAME

       netdump - send oops data and memory dumps over the network

SYNOPSIS

       /etc/init.d/netdump (start|stop|status|restart|propagate)

DESCRIPTION

       Loads and configures the netdump kernel modules. Once these are loaded, when the kernel crashes it will send the oops message and a dump of
       physical memory to the machine that runs the netdump-server. This can then be used to debug the problem using gdb and a kernel image.

OPTIONS

       start  Starts the netdump client service. Loads the kernel modules with the right options.

       stop   Unloads the kernel modules.

       status Shows the status of the netdump client

       restart
	      Restarts the netclient modules with new configurations.

       propagate
	      Sends the ssh public key needed to update the random key for each netdump session to the netdump server.

FILES

       /etc/sysconfig/netdump
	      Configuration for the netdump client. You need to at least specify the address of the machine running the netdump server here.

       /etc/sysconfig/netdump_id_dsa.pub
	      Public key for this machine. Used by the server to authenticate the client.  This must be installed on the server, possibly by using
	      the the propagate capability of the netdump init script, which does:
	      cat /etc/sysconfig/netdump_id_dsa.pu | ssh netdump@$NETDUMPADDR cat '>>' /var/crash/.ssh/authorized_keys2
	      Where $NETDUMPADDR is the ip or name of the machine running the netdump server.

SEE ALSO

       netdump-server(8), init(8)

BUGS

       Report any bugs you find to http://bugzilla.redhat.com/bugzilla

AUTHOR

       Alexander Larsson <alexl@redhat.com>

Linux								    14 Feb 2002 						 NETDUMP-CLIENT(8)
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