netdev - Re: [PATCH 1/2] bpf: do not use KMALLOC_SHIFT

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Message-ID: <20161216180209.GA77597@ast-mbp.thefacebook.com>
Date:   Fri, 16 Dec 2016 10:02:10 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-mm@...ck.org, Cristopher Lameter <cl@...ux.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Alexei Starovoitov <ast@...nel.org>, netdev@...r.kernel.org,
        Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [PATCH 1/2] bpf: do not use KMALLOC_SHIFT_MAX

On Thu, Dec 15, 2016 at 05:47:21PM +0100, Michal Hocko wrote:
> From: Michal Hocko <mhocko@...e.com>
> 
> 01b3f52157ff ("bpf: fix allocation warnings in bpf maps and integer
> overflow") has added checks for the maximum allocateable size. It
> (ab)used KMALLOC_SHIFT_MAX for that purpose. While this is not incorrect
> it is not very clean because we already have KMALLOC_MAX_SIZE for this
> very reason so let's change both checks to use KMALLOC_MAX_SIZE instead.
> 
> Cc: Alexei Starovoitov <ast@...nel.org>
> Signed-off-by: Michal Hocko <mhocko@...e.com>

Nack until the patches 1 and 2 are reversed.

The bug that patch 2 fixes was the reason we used KMALLOC_SHIFT_MAX - 1 here
instead of KMALLOC_MAX_SIZE,
so you have to fix the kmalloc vs __alloc_pages_slowpath discrepancy first.

> ---
>  kernel/bpf/arraymap.c | 2 +-
>  kernel/bpf/hashtab.c  | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/bpf/arraymap.c b/kernel/bpf/arraymap.c
> index a2ac051c342f..229a5d5df977 100644
> --- a/kernel/bpf/arraymap.c
> +++ b/kernel/bpf/arraymap.c
> @@ -56,7 +56,7 @@ static struct bpf_map *array_map_alloc(union bpf_attr *attr)
>  	    attr->value_size == 0 || attr->map_flags)
>  		return ERR_PTR(-EINVAL);
>  
> -	if (attr->value_size >= 1 << (KMALLOC_SHIFT_MAX - 1))
> +	if (attr->value_size > KMALLOC_MAX_SIZE)
>  		/* if value_size is bigger, the user space won't be able to
>  		 * access the elements.
>  		 */
> diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
> index ad1bc67aff1b..c5ec7dc71c84 100644
> --- a/kernel/bpf/hashtab.c
> +++ b/kernel/bpf/hashtab.c
> @@ -181,7 +181,7 @@ static struct bpf_map *htab_map_alloc(union bpf_attr *attr)
>  		 */
>  		goto free_htab;
>  
> -	if (htab->map.value_size >= (1 << (KMALLOC_SHIFT_MAX - 1)) -
> +	if (htab->map.value_size >= KMALLOC_MAX_SIZE -
>  	    MAX_BPF_STACK - sizeof(struct htab_elem))
>  		/* if value_size is bigger, the user space won't be able to
>  		 * access the elements via bpf syscall. This check also makes
> -- 
> 2.10.2
>