linux-kernel - Re: [PATCH] relay: Fix type mismatch when allocating memory in relay_create

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Date:   Tue, 29 Nov 2022 15:30:37 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Gavrilov Ilia <Ilia.Gavrilov@...otecs.ru>
Cc:     Colin Ian King <colin.i.king@...il.com>,
        wuchi <wuchi.zero@...il.com>, Jens Axboe <axboe@...nel.dk>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "lvc-project@...uxtesting.org" <lvc-project@...uxtesting.org>
Subject: Re: [PATCH] relay: Fix type mismatch when allocating memory in
 relay_create_buf()

On Tue, 29 Nov 2022 09:23:38 +0000 Gavrilov Ilia <Ilia.Gavrilov@...otecs.ru> wrote:

> The 'padding' field of the 'rchan_buf' structure is an array of 'size_t'
> elements, but the memory is allocated for an array of 'size_t *' elements.
> 
> Found by Linux Verification Center (linuxtesting.org) with SVACE.
> 
> --- a/kernel/relay.c
> +++ b/kernel/relay.c
> @@ -148,13 +148,13 @@ static struct rchan_buf *relay_create_buf(struct rchan *chan)
>  {
>  	struct rchan_buf *buf;
>  
> -	if (chan->n_subbufs > KMALLOC_MAX_SIZE / sizeof(size_t *))
> +	if (chan->n_subbufs > KMALLOC_MAX_SIZE / sizeof(size_t))
>  		return NULL;
>  
>  	buf = kzalloc(sizeof(struct rchan_buf), GFP_KERNEL);
>  	if (!buf)
>  		return NULL;
> -	buf->padding = kmalloc_array(chan->n_subbufs, sizeof(size_t *),
> +	buf->padding = kmalloc_array(chan->n_subbufs, sizeof(size_t),
>  				     GFP_KERNEL);

This is why I prefer kmalloc_array(N, sizeof(*(buf->padding)), ...)

Because the reviewer doesn't have to go check that the types match up,
and because the code doesn't need changing if the type of
*(buf->padding) is changed.

Others don't like this practice, but I forget why.