From mboxrd@z Thu Jan  1 00:00:00 1970
From: Cyril Hrubis <chrubis@suse.cz>
Date: Thu, 1 Aug 2019 13:54:18 +0200
Subject: [LTP] [RFC PATCH 2/9] lib: Add a canary for guarded buffers
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References: <20190801092616.30553-1-chrubis@suse.cz>
 <20190801092616.30553-3-chrubis@suse.cz>
 <1516778317.3992530.1564656190448.JavaMail.zimbra@redhat.com>
Message-ID: <20190801115418.GB23916@rei>
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To: ltp@lists.linux.it

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.

-- 
Cyril Hrubis
chrubis@suse.cz