Re: [usb-host] question about pointer dereference before null check

From: Gustavo A. R. Silva
Date: Tue May 02 2017 - 12:03:01 EST

Hi Alan,

Quoting Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>:

On Mon, 1 May 2017, Gustavo A. R. Silva wrote:

Hello everybody,

While taking a look into Coverity ID 100828 I ran into the following
piece of code at drivers/usb/host/ehci-sched.c:1096:

u32 addr;
int think_time;
int hs_transfers;

addr = dev->ttport << 24;
if (!ehci_is_TDI(ehci)
|| (dev->tt->hub !=
addr |= dev->tt->hub->devnum << 16;
addr |= epnum << 8;
addr |= dev->devnum;
stream->ps.usecs = HS_USECS_ISO(maxp);
think_time = dev->tt ? dev->tt->think_time : 0;

The issue here is that dev->tt is being dereferenced before null check.

I was thinking on placing think_time = dev->tt ? dev->tt->think_time :
0; just before the _if_ statement. But this doesn't address the
problem of dev->tt actually being NULL.

While looking into the callers of the function containing this piece
of code (iso_stream_init()) my impression is that dev->tt is not NULL
at the time this function is called and, a very simple patch like the
following can be applied in order to avoid the Coverity issue:

-think_time = dev->tt ? dev->tt->think_time : 0;
+think_time = dev->tt->think_time;

But I can't tell for sure, so in case dev->tt is NULL, a good strategy
to properly update the _addr_ variable would be needed.

What do you think?

I would really appreciate any comment on this,
Thank you!

You are right; udev->tt cannot ever be NULL when this section of code
runs. The test should be removed.

Thanks for confirming, I'll send a patch shortly.
Gustavo A. R. Silva