[LTP] [RFC PATCH 2/9] lib: Add a canary for guarded buffers

[Jan Stancek <jstancek@redhat.com>]

----- Original Message -----
> Hi!
> > > In a case that the buffer size is not a multiple of a page size there is
> > > unused space before the start of the buffer. Let's fill that with
> > > center mirrored random bytes and check that the buffer wasn't modified
> > > before we unmap it.
> > > 
> > >  void *tst_alloc(size_t size)
> > >  {
> > >  	size_t page_size = getpagesize();
> > > @@ -34,9 +61,13 @@ void *tst_alloc(size_t size)
> > >  	maps = map;
> > >  
> > >  	if (size % page_size)
> > > -		ret += page_size - (size % page_size);
> > > +		map->buf_shift = page_size - (size % page_size);
> > > +	else
> > > +		map->buf_shift = 0;
> > > +
> > > +	setup_canary(map);
> > >  
> > > -	return ret;
> > > +	return ret + map->buf_shift;
> > 
> > My concern here is alignment.
> 
> I'm aware of that. My reasoning here is that:
> 
> * The end of the page is aligned by definition to 2^page_order
> 
> * Any primitive types such as integer, etc. are hence aligned
> 
> * Structures are padded so that the total size is multiple of
>   the largest alignment required (because otherwise arrays of
>   structures would end up causing unaligned access as well).
> 
> That leaves out things such as buffers for direct I/O, the only way to
> allocate aligned buffers there is to make the size to be multiple of
> the block size.

I don't have concrete example at hand, but I foggily recall
s390 issue from couple years back, where it didn't like odd addresses.
Can't recall if it was data or code pointer.

Could we apply/enforce some minimum alignment, similar to what glibc
does for malloc?