On 16/09/20 11:49 am, Petko Manolov wrote:

On 20-09-16 10:35:40, Anant Thazhemadam wrote:

quoted

get_registers() copies whatever memory is written by the
usb_control_msg() call even if the underlying urb call ends up failing.

Not true, memcpy() is only called if "ret" is positive.

Right. I'm really sorry I fumbled and messed up the commit message
there. Thank you for pointing that out.

quoted

If get_registers() fails, or ends up reading 0 bytes, meaningless and junk 
register values would end up being copied over (and eventually read by the 
driver), and since most of the callers of get_registers() don't check the 
return values of get_registers() either, this would go unnoticed.

usb_control_msg() returns negative on error (look up usb_internal_control_msg() 
to see for yourself) so it does not go unnoticed.

When I said "this would go unnoticed", I meant get_register() failing would
go unnoticed, not that usb_control_msg() failing would go unnoticed.
I agree that get_registers() notices usb_control_msg() failing, and
appropriately returns the return value from usb_control_msg().
But there are many instances where get_registers() is called but the return
value of get_registers() is not checked, to see if it failed or not; hence, "this
would go unnoticed".

If for some reason it return zero, nothing is copied.  Also, if usb transfer fail 
no register values are being copied anywhere.

True.
Now consider set_ethernet_addr(), and suppose get_register() fails when
invoked from inside set_ethernet_addr().
As you said, no value is copied back, which means no value is copied back
into node_id, which leaves node_id uninitialized. This node_id (still
uninitialized) is then blindly copied into dev->netdev->dev_addr; which
is less than ideal and could also quickly prove to become an issue, right?

Your patch also allows for memcpy() to be called with 'size' either zero or 
greater than the allocated buffer size. Please, look at the code carefully.

Oh. I apologize for this. This can be reverted relatively easily.

quoted

It might be a better idea to try and mirror the PCI master abort
termination and set memory to 0xFFs instead in such cases.

I wasn't aware drivers are now responsible for filling up the memory with 
anything.  Does not sound like a good idea to me.

Since we copy the correct register values when get_register() doesn't fail,
I thought it might be a slightly better alternative to fill node_id with 0xFFs,
instead of leaving it go uninitialized in case get_registers() fails.

Also, what are the odds that a successful get_register() call would see
0xFFs being copied?
If that's very real scenario, then I admit this doesn't work at all.

The only other alternative approach I can think of that can handle the
issue I highlighted above, is to introduce checking for get_registers()'s
return values nearly everywhere it gets called.
Would that be a more preferable and welcome approach?

Thank you for your time.

Thanks,
Anant