Re: [PATCH] fs: select: fix information leak to userspace

[Andrew Morton <akpm@linux-foundation.org>]

On Sat, 13 Nov 2010 14:38:19 -0700 Andreas Dilger <adilger.kernel@dilger.ca> wrote:

> On 2010-11-12, at 13:08, Andrew Morton wrote:
> > On Wed, 10 Nov 2010 23:38:02 +0300
> > Vasiliy Kulikov <segooon@gmail.com> wrote:
> >> On some architectures __kernel_suseconds_t is int.
> > 
> > On sparc and parisc.  On all other architectures this patch is a waste
> > of cycles.
> > 
> > --- a/fs/select.c~fs-select-fix-information-leak-to-userspace-fix
> > +++ a/fs/select.c
> > @@ -306,7 +306,8 @@ static int poll_select_copy_remaining(st
> > 		rts.tv_sec = rts.tv_nsec = 0;
> > 
> > 	if (timeval) {
> > -		memset(&rtv, 0, sizeof(rtv));
> > +		if (sizeof(rtv) > sizeof(rtv.tv_sec) + sizeof(rtv.tv_usec))
> > +			memset(&rtv, 0, sizeof(rtv));
> > 		rtv.tv_sec = rts.tv_sec;
> > 		rtv.tv_usec = rts.tv_nsec / NSEC_PER_USEC;
> > 
> > _
> > 
> > 
> > The `if' gets eliminated at compile time.  With this approach we add
> > four bytes of text to the sparc64 build and zero bytes of text to the
> > x86_64 build.
> 
> It's nice to have comments (or at least a good commit message) for unusual code like this, so that in the future it is clear when this kind of workaround can be removed (e.g. if the time_t is changed to always be a 64-bit value for Y2038 issues, even on 32-bit arches). 
> 

Well, I'm the resident comment fanatic, but I thought this was all
sufficiently obvious to not need one.  But I'll add one ;)