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From:   Marco Elver <elver@google.com>
Date:   Wed, 13 Jul 2022 12:33:31 +0200
Message-ID: <CANpmjNPbbugrbCFADy1C7PgaU-4PMd9UK90QiHKS-Md0ocqa3w@mail.gmail.com>
Subject: Re: [PATCH 16/16] mm/sl[au]b: check if large object is valid in __ksize()
To:     Christoph Lameter <cl@gentwo.de>
Cc:     Hyeonggon Yoo <42.hyeyoo@gmail.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Joe Perches <joe@perches.com>,
        Vasily Averin <vasily.averin@linux.dev>,
        Matthew WilCox <willy@infradead.org>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
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On Wed, 13 Jul 2022 at 12:07, Christoph Lameter <cl@gentwo.de> wrote:
>
> On Wed, 13 Jul 2022, Hyeonggon Yoo wrote:
>
> > > Why return 0 if there is an error and why bother the callers with these
> > > checks. BUG()?
> >
> > I thought BUG should be used when there is no other solution.
>
> Spurios returns of 0 that the caller has to check for is a solution?

It's never really been about the caller checking for 0, see below.

> > > I guess this is an error since the order-0 page cannot come from slab
> > > allocations.
> >
> > comment in ksize() says:
> >       "The caller must guarantee that objp points to a valid object
> >       previously allocated with either kmalloc() or kmem_cache_alloc()."
> >
> > It should not be used on order-0 page that is not allocated from slab. No?
>
> I guess we would need to check. Code could exist that does this.
>
> Getting a 0 size would be surprising too here. BUG()? Or WARN() and return
> PAGE_SIZE.

We shouldn't crash, so it should be WARN(), but also returning
PAGE_SIZE is bad. The intuition behind returning 0 is to try and make
the buggy code cause less harm to the rest of the kernel.

From [1]:

> Similarly, if you are able to tell if the passed pointer is not a
> valid object some other way, you can do something better - namely,
> return 0. The intuition here is that the caller has a pointer to an
> invalid object, and wants to use ksize() to determine its size, and
> most likely access all those bytes. Arguably, at that point the kernel
> is already in a degrading state. But we can try to not let things get
> worse by having ksize() return 0, in the hopes that it will stop
> corrupting more memory. It won't work in all cases, but should avoid
> things like "s = ksize(obj); touch_all_bytes(obj, s)" where the size
> bounds the memory accessed corrupting random memory.

[1] https://lore.kernel.org/all/CANpmjNNYt9AG8RrGF0pq2dPbFc=vw2kaOnL2k5+8kfJeEMGuwg@mail.gmail.com/

Thanks,
-- Marco