linux-kernel - [usb-host] question about pointer dereference before null check

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Date:   Mon, 01 May 2017 18:35:07 -0500
From:   "Gustavo A. R. Silva" <garsilva@...eddedor.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alan Stern <stern@...land.harvard.edu>
Cc:     linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        Peter Senna Tschudin <peter.senna@...il.com>
Subject: [usb-host] question about pointer dereference before null check

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 !=
                         ehci_to_hcd(ehci)->self.root_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!
--
Gustavo A. R. Silva