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[198.0.35.241]) by smtp.gmail.com with ESMTPSA id dw16-20020a17090b095000b00260cce91d20sm2026175pjb.33.2023.07.11.09.21.36 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 11 Jul 2023 09:21:36 -0700 (PDT) Date: Tue, 11 Jul 2023 09:21:36 -0700 From: Kees Cook To: Vlastimil Babka Cc: Christoph Lameter , David Rientjes , Pekka Enberg , Joonsoo Kim , Hyeonggon Yoo <42.hyeyoo@gmail.com>, Roman Gushchin , linux-mm@kvack.org, patches@lists.linux.dev, linux-kernel@vger.kernel.org, Matteo Rizzo , Jann Horn , Andrey Konovalov , Marco Elver , Alexander Potapenko , kasan-dev@googlegroups.com, linux-hardening@vger.kernel.org Subject: Re: [PATCH 2/2] mm/slub: remove freelist_dereference() Message-ID: <202307110917.DEED145F0@keescook> References: <20230711134623.12695-3-vbabka@suse.cz> <20230711134623.12695-4-vbabka@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230711134623.12695-4-vbabka@suse.cz> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jul 11, 2023 at 03:46:25PM +0200, Vlastimil Babka wrote: > freelist_dereference() is a one-liner only used from get_freepointer(). > Remove it and make get_freepointer() call freelist_ptr_decode() > directly to make the code easier to follow. > > Signed-off-by: Vlastimil Babka > --- > mm/slub.c | 16 ++++++---------- > 1 file changed, 6 insertions(+), 10 deletions(-) > > diff --git a/mm/slub.c b/mm/slub.c > index 07edad305512..c4556a5dab4b 100644 > --- a/mm/slub.c > +++ b/mm/slub.c > @@ -397,18 +397,14 @@ static inline void *freelist_ptr_decode(const struct kmem_cache *s, > return decoded; > } > > -/* Returns the freelist pointer recorded at location ptr_addr. */ > -static inline void *freelist_dereference(const struct kmem_cache *s, > - void *ptr_addr) > -{ > - return freelist_ptr_decode(s, *(freeptr_t *)(ptr_addr), > - (unsigned long)ptr_addr); > -} > - > static inline void *get_freepointer(struct kmem_cache *s, void *object) > { > - object = kasan_reset_tag(object); > - return freelist_dereference(s, (freeptr_t *)(object + s->offset)); > + unsigned long ptr_addr; > + freeptr_t p; > + > + ptr_addr = ((unsigned long)kasan_reset_tag(object)) + s->offset; > + p = *(freeptr_t *)(ptr_addr); > + return freelist_ptr_decode(s, p, ptr_addr); > } > > #ifndef CONFIG_SLUB_TINY > -- > 2.41.0 > I like reducing the complexity here, but I find dropping the "object" reassignment makes this a bit harder to read. What about: object = kasan_reset_tag(object); unsigned long ptr_addr = (unsigned long)object + s->offset; freeptr_t p = *(freeptr_t *)(ptr_addr); return freelist_ptr_decode(s, p, ptr_addr); ? They're the same result, so either way: Acked-by: Kees Cook -- Kees Cook