Re: [PATCH] usb: host: ehci-q: make qtd_fill() return 'u16'

[Alan Stern <stern@rowland.harvard.edu>]

On Wed, Feb 16, 2022 at 10:33:15PM +0000, David Laight wrote:
> From: Sergey Shtylyov
> > Sent: 16 February 2022 20:19
> > 
> > At the end of qtd_fill(), we assign the 'int count' variable to the 'size_t
> > length' field of 'struct ehci_qtd'.  In order not to mix the *signed* and
> > *unsigned* values let's make that variable and the function's result 'u16'
> > as qTD's maximum length is a 15-bit quantity anyway...
> 
> Except that you really don't want to be doing arithmetic on sub-register
> sized values.
> On everything except x86 the compiler will have to add instructions
> to mask the value to 16 bits (unless its logic can detect that overflow
> can never happen).
> 
> There is a similar problem with parameters and return values.
> They need masking one side of the call (or maybe both).
> 
> > Found by Linux Verification Center (linuxtesting.org) with the SVACE static
> > analysis tool.
> 
> Which clearly doesn't understand the implications of its reports.
> 
> 	David

Agreed.  It would be acceptable to change the types to "unsigned int", 
but there's no reason to make them "u16".

In general, the only situation where a size should be smaller than the 
native register size is when you're defining fields in a structure or 
union, or doing memory-mapped I/O (which often involves the same 
thing).

Alan Stern