[ENBD] ENBD on 2.4.x

[Peter T. Breuer]

"A month of sundays ago jona@orac.ensor.org wrote:"
> Hi again.
> 
> Could somebody sanity check me here?  I'm still looking at issues
> with 2.4.x and ENBD and wanted to see how the Kernel module
> makes sure it doesn't merge requests to the point that it would
> overrun the client's buffer during a 'get_req' call (I suspect that
> I'm getting a SEGV in the ioctl(fd, MY_NBD_GET_REQ, 1000) call).
> 
> I see that the Kernel module tries to keep that from happening
> by looking at 'buf_sectors', but something still seems to be
> going wrong.
> 
> At line 1468 in nbd.c (kernel module), I added the following 
> test and warning if the kernel module tries to write a larger buffer
> than the user has allocated.  I'm not sure how 'request.len' is
> getting so big, but I got the warning when I tried to run my 'mke2fs' on
> the block device.
> 
>        if (request.len > NBD_MAX_SECTORS*512+1000) {
>                NBD_ALERT("Warning!  Want to write something larger than my buff\
> er %i\n", request.len);
>        }
>        else {
>        size = copy_to_user_from_req(req,
>                (char*)user+sizeof (struct nbd_request), request.len);
> 
>         // PTB we have been tracking where to write to in the buffer
>         buflen += size;
>        }
> 
> Log dump:
> 
> Mar 15 18:40:04 jarney kernel: NBD #3481[0]: init_module registered device at major 43
> Mar 15 18:40:18 jarney kernel: NBD #1892[0]: nbd_set_sock device nda not signed yet!
> Mar 15 18:40:27 jarney kernel: NBD #1469[0]: nbd_get_req Warning!  Want to write something larger than my buffer 131072
> 
> It would seem that we would want to do one of the following:
> 	a) Make sure the request NEVER gets bigger than NBD_MAX_SECTORS
> 	b) Break up large requests to the client daemon and inform it
> 	that it only got a partial.
> 
> The first is probably the simplest, but I'm not sure how we can
> limit the size of requests.
> 
> Ideas?
> 
> -Jon A.
> 
> 
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