linux-kernel - Re: [PATCH v2 1/2] mm: krealloc: consider spare memory for __GFP

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Message-ID: <65fd31b2-51c1-458c-a3fa-fadbde5039fb@kernel.org>
Date: Tue, 20 Aug 2024 01:23:00 +0200
From: Danilo Krummrich <dakr@...nel.org>
To: akpm@...ux-foundation.org
Cc: cl@...ux.com, penberg@...nel.org, rientjes@...gle.com,
 iamjoonsoo.kim@....com, vbabka@...e.cz, roman.gushchin@...ux.dev,
 42.hyeyoo@...il.com, linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2 1/2] mm: krealloc: consider spare memory for __GFP_ZERO

On 8/13/24 12:34 AM, Danilo Krummrich wrote:

Gentle reminder on this one.

> As long as krealloc() is called with __GFP_ZERO consistently, starting
> with the initial memory allocation, __GFP_ZERO should be fully honored.
> 
> However, if for an existing allocation krealloc() is called with a
> decreased size, it is not ensured that the spare portion the allocation
> is zeroed. Thus, if krealloc() is subsequently called with a larger size
> again, __GFP_ZERO can't be fully honored, since we don't know the
> previous size, but only the bucket size.
> 
> Example:
> 
> 	buf = kzalloc(64, GFP_KERNEL);
> 	memset(buf, 0xff, 64);
> 
> 	buf = krealloc(buf, 48, GFP_KERNEL | __GFP_ZERO);
> 
> 	/* After this call the last 16 bytes are still 0xff. */
> 	buf = krealloc(buf, 64, GFP_KERNEL | __GFP_ZERO);
> 
> Fix this, by explicitly setting spare memory to zero, when shrinking an
> allocation with __GFP_ZERO flag set or init_on_alloc enabled.
> 
> Acked-by: Vlastimil Babka <vbabka@...e.cz>
> Signed-off-by: Danilo Krummrich <dakr@...nel.org>

I think I forgot to add:

Cc: stable@...r.kernel.org

Not sure if there is a useful commit for "Fixes" though. AFAICT, this has been 
broken since forever.

> ---
>   mm/slab_common.c | 7 +++++++
>   1 file changed, 7 insertions(+)
> 
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 40b582a014b8..cff602cedf8e 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -1273,6 +1273,13 @@ __do_krealloc(const void *p, size_t new_size, gfp_t flags)
>   
>   	/* If the object still fits, repoison it precisely. */
>   	if (ks >= new_size) {
> +		/* Zero out spare memory. */
> +		if (want_init_on_alloc(flags)) {
> +			kasan_disable_current();
> +			memset((void *)p + new_size, 0, ks - new_size);
> +			kasan_enable_current();
> +		}
> +
>   		p = kasan_krealloc((void *)p, new_size, flags);
>   		return (void *)p;
>   	}
> 
> base-commit: b8dbbb7fe1db26c450a9d2c3302013154b3431df