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Date:   Mon, 28 Feb 2022 15:16:45 -0800
From:   Kees Cook <keescook@chromium.org>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     llvm@lists.linux.dev, Marco Elver <elver@google.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Vlastimil Babka <vbabka@suse.cz>, linux-mm@kvack.org,
        stable@vger.kernel.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Christoph Lameter <cl@linux.com>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Daniel Micay <danielmicay@gmail.com>,
        linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org
Subject: Re: [PATCH] mm: Handle ksize() vs __alloc_size by forgetting size
Message-ID: <202202251823.45E09CF@keescook>
References: <20220225221625.3531852-1-keescook@chromium.org>
 <20220225154518.0d1159fdc6f37ee38e39e90c@linux-foundation.org>
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On Fri, Feb 25, 2022 at 03:45:18PM -0800, Andrew Morton wrote:
> On Fri, 25 Feb 2022 14:16:25 -0800 Kees Cook <keescook@chromium.org> wrote:
> 
> > If ksize() is used on an allocation, the compiler cannot make any
> > assumptions about its size any more (as hinted by __alloc_size). Force
> > it to forget.
> > 
> > One caller was using a container_of() construction that needed to be
> > worked around.
> 
> Please, when fixing something do fully explain what that thing is.  I,
> for one, simply cannot understand why this change is being proposed.
> 
> Especially when proposing a -stable backport!  Tell readers what was
> the end-user impact of the bug.
> 
> > Link: https://github.com/ClangBuiltLinux/linux/issues/1599
> 
> Even that didn't tell me.  Is it just a clang warning?  Does the kernel
> post your private keys on reddit then scribble all over your disk
> drive?  I dunno.

Yup, sorry. I tend to get so deep changes like this that I forget to
give an appropriately detailed summary. As others have mentioned, this
is trying to fix a miscompilation issue, triggered by what can be
considered either a mis-application of __alloc_size, or a failure to
correctly disable compiler optimizations in the face of ksize().

-- 
Kees Cook