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Message-ID: <f008f8a2-8d5e-88ab-8d23-a2043ea5abe7@suse.cz>
Date:   Fri, 18 Nov 2022 12:34:01 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Kees Cook <keescook@...omium.org>
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>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2 2/2] slab: Remove special-casing of const 0 size
 allocations

On 11/18/22 04:51, Kees Cook wrote:
> Passing a constant-0 size allocation into kmalloc() or kmalloc_node()
> does not need to be a fast-path operation, so the static return value
> can be removed entirely. This is in preparation for making sure that
> all paths through the inlines result in a full extern function call,

So with the kmalloc_trace() already solved, we could now say it's not "in
preparation", but simply "makes sure", right? I can correct that while
picking this patch.

> where __alloc_size() hints will actually be seen[1] by GCC. (A constant
> return value of 0 means the "0" allocation size won't be propagated by
> the inline.)
> 
> [1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96503
> 
> Cc: Vlastimil Babka <vbabka@...e.cz>
> Cc: Christoph Lameter <cl@...ux.com>
> Cc: Pekka Enberg <penberg@...nel.org>
> Cc: David Rientjes <rientjes@...gle.com>
> Cc: Joonsoo Kim <iamjoonsoo.kim@....com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Roman Gushchin <roman.gushchin@...ux.dev>
> Cc: Hyeonggon Yoo <42.hyeyoo@...il.com>
> Cc: linux-mm@...ck.org
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
>  include/linux/slab.h | 12 ++----------
>  1 file changed, 2 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 9033937c758e..84be05208418 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -561,17 +561,13 @@ void *kmalloc_large_node(size_t size, gfp_t flags, int node) __assume_page_align
>  #ifndef CONFIG_SLOB
>  static __always_inline __alloc_size(1) void *kmalloc(size_t size, gfp_t flags)
>  {
> -	if (__builtin_constant_p(size)) {
> +	if (__builtin_constant_p(size) && size) {
>  		unsigned int index;
>  
>  		if (size > KMALLOC_MAX_CACHE_SIZE)
>  			return kmalloc_large(size, flags);
>  
>  		index = kmalloc_index(size);
> -
> -		if (!index)
> -			return ZERO_SIZE_PTR;
> -
>  		return kmalloc_trace(
>  				kmalloc_caches[kmalloc_type(flags)][index],
>  				flags, size);
> @@ -591,17 +587,13 @@ static __always_inline __alloc_size(1) void *kmalloc(size_t size, gfp_t flags)
>  #ifndef CONFIG_SLOB
>  static __always_inline __alloc_size(1) void *kmalloc_node(size_t size, gfp_t flags, int node)
>  {
> -	if (__builtin_constant_p(size)) {
> +	if (__builtin_constant_p(size) && size) {
>  		unsigned int index;
>  
>  		if (size > KMALLOC_MAX_CACHE_SIZE)
>  			return kmalloc_large_node(size, flags, node);
>  
>  		index = kmalloc_index(size);
> -
> -		if (!index)
> -			return ZERO_SIZE_PTR;
> -
>  		return kmalloc_node_trace(
>  				kmalloc_caches[kmalloc_type(flags)][index],
>  				flags, node, size);

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