linux-kernel - Re: [PATCH] usb: core: fix information leak to userland

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Date:	Sat, 6 Nov 2010 14:47:02 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	David Brownell <david-b@...bell.net>
cc:	Vasiliy Kulikov <segooon@...il.com>,
	<kernel-janitors@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Oliver Neukum <oliver@...kum.org>,
	Andi Kleen <ak@...ux.intel.com>,
	Chris Frey <cdfrey@...rsquare.net>,
	<linux-usb@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] usb: core: fix information leak to userland

On Sat, 6 Nov 2010, David Brownell wrote:

> --- On Sat, 11/6/10, Alan Stern <stern@...land.harvard.edu> wrote:
> 
> > Are you sure that adding an initializer
> > like this will zero out the 
> > padding bytes?  It might be safer just to call
> > memset.
> 
> ISTR the C standard says things get initted to
> zero in this case too ... and that compilers will
> as a rule use memset to do it.  One could look
> at the generated code to make sure of that.

Unfortunately I don't have a copy of the C standard here to consult.  
However...  Although I'm perfectly willing to believe that the standard
requires fields in a structure to be initialized to 0 if they
aren't mentioned explicitly in the initializer, I'm considerably more
doubtful that it also requires padding to be initialized!

And I certainly wouldn't want to depend on compilers _always_ using 
memset to do this initialization.

> There's certainly a fair amount of code I've seen
> that uses runtime initializers like that, to zero
> memory.  I can't believe i's _all_ broken!  ;)

Zeroing memory that belongs to a declared field is different from 
zeroing padding bytes.  Maybe what you remember seeing is the first and 
not the second.

Alan Stern

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