linux-kernel - Re: [PATCH v1 1/4] ata: libata: Remove redundant sense

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Date: Mon, 17 Jun 2024 12:41:26 +0200
From: Niklas Cassel <cassel@...nel.org>
To: Igor Pylypiv <ipylypiv@...gle.com>
Cc: Damien Le Moal <dlemoal@...nel.org>, Tejun Heo <tj@...nel.org>,
	Hannes Reinecke <hare@...e.de>, linux-ide@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 1/4] ata: libata: Remove redundant sense_buffer memsets

On Fri, Jun 14, 2024 at 07:18:32PM +0000, Igor Pylypiv wrote:
> scsi_queue_rq() memsets sense_buffer before a command is dispatched.
> 
> Libata is not memsetting sense_buffer before setting sense data that
> was obtained from a disk so there should be no reason to do a memset
> for ATA PASS-THROUGH / ATAPI.
> 
> Memsetting the sense_buffer in ata_gen_passthru_sense() is erasing valid
> sense data that was previously obtained from a disk. A follow-up patch
> will modify ata_gen_passthru_sense() to stop generating sense data based
> on ATA status register bits if a valid sense data is already present.
> 
> Signed-off-by: Igor Pylypiv <ipylypiv@...gle.com>
> ---
>  drivers/ata/libata-eh.c   | 2 --
>  drivers/ata/libata-scsi.c | 4 ----
>  2 files changed, 6 deletions(-)
> 
> diff --git a/drivers/ata/libata-eh.c b/drivers/ata/libata-eh.c
> index 214b935c2ced..b5e05efe73f6 100644
> --- a/drivers/ata/libata-eh.c
> +++ b/drivers/ata/libata-eh.c
> @@ -1479,8 +1479,6 @@ unsigned int atapi_eh_request_sense(struct ata_device *dev,
>  	struct ata_port *ap = dev->link->ap;
>  	struct ata_taskfile tf;
>  
> -	memset(sense_buf, 0, SCSI_SENSE_BUFFERSIZE);
> -

Are you sure that this is safe?

atapi_eh_request_sense() is called both by:
ata_eh_analyze_tf():
tmp = atapi_eh_request_sense(.., qc->scsicmd->sense_buffer, ..)

and by:
atapi_eh_clear_ua():
atapi_eh_request_sense(.., sense_buffer, ..);
where sense_buffer is dev->link->ap->sector_buf.


Wouldn't a better fix be for ata_gen_* functions to return early if
ATA_QCFLAG_SENSE_VALID is set?


>  	/* initialize sense_buf with the error register,
>  	 * for the case where they are -not- overwritten
>  	 */
> diff --git a/drivers/ata/libata-scsi.c b/drivers/ata/libata-scsi.c
> index cdf29b178ddc..032cf11d0bcc 100644
> --- a/drivers/ata/libata-scsi.c
> +++ b/drivers/ata/libata-scsi.c
> @@ -858,8 +858,6 @@ static void ata_gen_passthru_sense(struct ata_queued_cmd *qc)
>  	unsigned char *desc = sb + 8;
>  	u8 sense_key, asc, ascq;
>  
> -	memset(sb, 0, SCSI_SENSE_BUFFERSIZE);
> -
>  	/*
>  	 * Use ata_to_sense_error() to map status register bits
>  	 * onto sense key, asc & ascq.
> @@ -953,8 +951,6 @@ static void ata_gen_ata_sense(struct ata_queued_cmd *qc)
>  	u64 block;
>  	u8 sense_key, asc, ascq;
>  
> -	memset(sb, 0, SCSI_SENSE_BUFFERSIZE);
> -
>  	if (ata_dev_disabled(dev)) {
>  		/* Device disabled after error recovery */
>  		/* LOGICAL UNIT NOT READY, HARD RESET REQUIRED */
> -- 
> 2.45.2.627.g7a2c4fd464-goog
>