linux-kernel - Re: [PATCH] mm/slub: Fix incorrect checkings of s->offset

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Date:   Mon, 27 Apr 2020 06:38:07 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Changbin Du <changbin.du@...il.com>
Subject: Re: [PATCH] mm/slub: Fix incorrect checkings of s->offset

On Mon, Apr 27, 2020 at 09:29:41AM -0400, Waiman Long wrote:
> On 4/27/20 9:18 AM, Waiman Long wrote:
> > On 4/27/20 8:38 AM, Matthew Wilcox wrote:
> > > On Sun, Apr 26, 2020 at 10:02:12PM -0400, Waiman Long wrote:
> > > > In a couple of places in the slub memory allocator, the code uses
> > > > "s->offset" as a check to see if the free pointer is put right
> > > > after the
> > > > object. That check is no longer true with commit 3202fa62fb43 ("slub:
> > > > relocate freelist pointer to middle of object").
> > > > 
> > > > As a result, echoing "1" into the validate sysfs file, e.g. of dentry,
> > > > may cause a bunch of "Freepointer corrupt" error reports to appear with
> > > > the system in panic afterwards.
> > > > 
> > > > To fix it, use the check "s->offset == s->inuse" instead.
> > > I think a little refactoring would make this more clear.
> > > 
> > > unsigned int track_offset(const struct kmem_cache *s)
> > > {
> > >     return s->inuse + (s->offset == s->inuse) ? sizeof(void *) : 0;
> > > }
> > 
> > Yes, that was what I am thinking of doing in v2.
> 
> BTW, "+" has a higher priority than "?:". So we need a parenthesis around
> "?:".

That seems like a good reason to not use ?:

unsigned int track_offset(const struct kmem_cache *s)
{
	if (s->offset != s->inuse)
		return s->inuse;
	return s->inuse + sizeof(void *);
}

Also this needs a comment about why we're doing this ... something about
the freelist pointer, I think?