linux-hardening - Re: [PATCH] mm: Handle ksize() vs __alloc

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Date:   Mon, 28 Feb 2022 14:30:11 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Marco Elver <elver@...gle.com>
Cc:     Kees Cook <keescook@...omium.org>, llvm@...ts.linux.dev,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
        stable@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Christoph Lameter <cl@...ux.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Daniel Micay <danielmicay@...il.com>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] mm: Handle ksize() vs __alloc_size by forgetting size

On Mon, Feb 28, 2022 at 12:24:51PM +0100, Marco Elver wrote:
> 2. Somehow statically computing the size-class's size (kmalloc_index()
> might help here), removing __alloc_size from allocation functions and
> instead use some wrapper.

I don't think that's computable.  I have been thinking about a slab flag
that would say "speed is more important than size; if the smallest slab
for this size of allocation has no free objects, search larger slabs
to get memory instead of allocating a new slab".  If we did have such
a feature, it would be impossible to know how large ksize() would report.