Re: [PATCH v7 01/11] libata: fix ata_host_alloc_pinfo()

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Mon, 2021-08-16 at 10:44 +0900, Damien Le Moal wrote:
> Avoid static checkers warnings about a potential NULL pointer
> dereference for the port info variable pi. To do so, test that at
> least
> one port info is available on entry to ata_host_alloc_pinfo() and
> start
> the ata port initialization for() loop with pi initialized to the
> first
> port info passed as argument (which is already checked to be non
> NULL).
> Within the for() loop, get the next port info, if it is not NULL,
> after initializing the ata port using the previous port info.
> 
> Reported-by: kernel test robot <lkp@intel.com>
> Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
> ---
>  drivers/ata/libata-core.c | 18 ++++++++++++++----
>  1 file changed, 14 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/ata/libata-core.c b/drivers/ata/libata-core.c
> index 61c762961ca8..b237a718ea0f 100644
> --- a/drivers/ata/libata-core.c
> +++ b/drivers/ata/libata-core.c
> @@ -5441,16 +5441,17 @@ struct ata_host *ata_host_alloc_pinfo(struct
> device *dev,
>  	struct ata_host *host;
>  	int i, j;
>  
> +	/* We must have at least one port info */
> +	if (!ppi[0])
> +		return NULL;

I've got to ask why on this one: most libata drivers use a static array
for the port info.  If the first element is NULL that's a coding
failure inside the driver, so WARN_ON would probably be more helpful to
the driver writer.

What makes the static checker think ppi isn't NULL?

> +
>  	host = ata_host_alloc(dev, n_ports);
>  	if (!host)
>  		return NULL;
>  
> -	for (i = 0, j = 0, pi = NULL; i < host->n_ports; i++) {
> +	for (i = 0, j = 0, pi = ppi[0]; i < host->n_ports; i++) {
>  		struct ata_port *ap = host->ports[i];
>  
> -		if (ppi[j])
> -			pi = ppi[j++];
> -
>  		ap->pio_mask = pi->pio_mask;
>  		ap->mwdma_mask = pi->mwdma_mask;
>  		ap->udma_mask = pi->udma_mask;
> @@ -5460,6 +5461,15 @@ struct ata_host *ata_host_alloc_pinfo(struct
> device *dev,
>  
>  		if (!host->ops && (pi->port_ops !=
> &ata_dummy_port_ops))
>  			host->ops = pi->port_ops;
> +
> +		/*
> +		 * Check that the next port info is not NULL.
> +		 * If it is, keep using the current one.
> +		 */
> +		if (j < n_ports - 1 && ppi[j + 1]) {
> +			j++;
> +			pi = ppi[j];
> +		}

This looks completely pointless: once you've verified ppi[0] is not
NULL above, there's no possible NULL deref in that loop and the static
checker should see it.  If it doesn't we need a new static checker
because we shouldn't be perturbing code for broken tools.

James