linux-kernel - Re: [PATCH] drm: mali-dp: Add check for kzalloc

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Message-ID: <17efaae0-9b6c-86ea-5fec-568d024d229f@arm.com>
Date:   Wed, 7 Dec 2022 13:59:04 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Jiasheng Jiang <jiasheng@...as.ac.cn>, liviu.dudau@....com,
        brian.starkey@....com, airlied@...il.com, daniel@...ll.ch
Cc:     linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH] drm: mali-dp: Add check for kzalloc

On 2022-12-07 09:21, Jiasheng Jiang wrote:
> As kzalloc may fail and return NULL pointer, it should be better to check
> the return value in order to avoid the NULL pointer dereference in
> __drm_atomic_helper_connector_reset.

This commit message is nonsense; if 
__drm_atomic_helper_connector_reset() would dereference the NULL implied 
by &mw_state->base, it would equally still dereference the explicit NULL 
pointer passed after this patch.

The current code works out OK because "base" is the first member of 
struct malidp_mw_connector_state, thus if mw_state is NULL then 
&mw_state->base == NULL + 0 == NULL. Now you *could* argue that this 
isn't robust if the layout of struct malidp_mw_connector_state ever 
changes, and that could be a valid justification for making this change, 
but the reason given certainly isn't.

Arithmetic on a (potentially) NULL pointer may well be a sign that it's 
worth a closer look to check whether it really is what the code intended 
to do, but don't automatically assume it has to be a bug. Otherwise, 
good luck with "fixing" every user of container_of() throughout the 
entire kernel.

Thanks,
Robin.

> Fixes: 8cbc5caf36ef ("drm: mali-dp: Add writeback connector")
> Signed-off-by: Jiasheng Jiang <jiasheng@...as.ac.cn>
> ---
>   drivers/gpu/drm/arm/malidp_mw.c | 6 +++++-
>   1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/arm/malidp_mw.c b/drivers/gpu/drm/arm/malidp_mw.c
> index ef76d0e6ee2f..fe4474c2ddcf 100644
> --- a/drivers/gpu/drm/arm/malidp_mw.c
> +++ b/drivers/gpu/drm/arm/malidp_mw.c
> @@ -72,7 +72,11 @@ static void malidp_mw_connector_reset(struct drm_connector *connector)
>   		__drm_atomic_helper_connector_destroy_state(connector->state);
>   
>   	kfree(connector->state);
> -	__drm_atomic_helper_connector_reset(connector, &mw_state->base);
> +
> +	if (mw_state)
> +		__drm_atomic_helper_connector_reset(connector, &mw_state->base);
> +	else
> +		__drm_atomic_helper_connector_reset(connector, NULL);
>   }
>   
>   static enum drm_connector_status