linux-kernel - Re: [PATCH 1/2][next] scsi: qla2xxx: Replace one-element array with DECLARE_FLEX

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Message-ID: <Y3iaLCY68E6Stgrp@work>
Date:   Sat, 19 Nov 2022 02:56:12 -0600
From:   "Gustavo A. R. Silva" <gustavoars@...nel.org>
To:     Christophe JAILLET <christophe.jaillet@...adoo.fr>
Cc:     "Martin K. Petersen" <martin.petersen@...cle.com>,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        GR-QLogic-Storage-Upstream@...vell.com,
        Nilesh Javali <njavali@...vell.com>,
        linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH 1/2][next] scsi: qla2xxx: Replace one-element array with
 DECLARE_FLEX_ARRAY() helper

On Sat, Nov 19, 2022 at 09:44:02AM +0100, Christophe JAILLET wrote:
> Le 19/11/2022 à 00:47, Gustavo A. R. Silva a écrit :
> > One-element arrays as fake flex arrays are deprecated and we are moving
> > towards adopting C99 flexible-array members, instead. So, replace
> > one-element array declaration in struct ct_sns_gpnft_rsp, which is
> > ultimately being used inside a union:
> > 
> > drivers/scsi/qla2xxx/qla_def.h:
> > 3240 struct ct_sns_gpnft_pkt {
> > 3241         union {
> > 3242                 struct ct_sns_req req;
> > 3243                 struct ct_sns_gpnft_rsp rsp;
> > 3244         } p;
> > 3245 };
> > 
> > Important to mention is that doing a build before/after this patch results
> > in no binary differences.
> 
> Hi,
> 
> even with the:
> 
> >   		rspsz = sizeof(struct ct_sns_gpnft_rsp) +
> > -			((vha->hw->max_fibre_devices - 1) *
> > +			(vha->hw->max_fibre_devices *
> >   			    sizeof(struct ct_sns_gpn_ft_data));
> 
> change ?

Yep; that change compensates for the removal of the 1 in the declaration
of entries[].

The above piece of code is a common idiom to calculate the size for an
allocation when a one-element array is involved. In the original code
(vha->hw->max_fibre_devices - 1) compensates for the _extra_ size of one
element of type struct ct_sns_gpn_ft_data in sizeof(struct ct_sns_gpnft_rsp).

--
Gustavo

> 
> CJ
> 
> > 
> > This help us make progress towards globally enabling
> > -fstrict-flex-arrays=3 [1].
> > 
> > Link: https://github.com/KSPP/linux/issues/245
> > Link: https://github.com/KSPP/linux/issues/193
> > Link: https://gcc.gnu.org/pipermail/gcc-patches/2022-October/602902.html [1]
> > Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
> > ---
> >   drivers/scsi/qla2xxx/qla_def.h | 4 ++--
> >   drivers/scsi/qla2xxx/qla_gs.c  | 2 +-
> >   2 files changed, 3 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/scsi/qla2xxx/qla_def.h b/drivers/scsi/qla2xxx/qla_def.h
> > index a26a373be9da..1eea977ef426 100644
> > --- a/drivers/scsi/qla2xxx/qla_def.h
> > +++ b/drivers/scsi/qla2xxx/qla_def.h
> > @@ -3151,12 +3151,12 @@ struct ct_sns_gpnft_rsp {
> >   		uint8_t vendor_unique;
> >   	};
> >   	/* Assume the largest number of targets for the union */
> > -	struct ct_sns_gpn_ft_data {
> > +	DECLARE_FLEX_ARRAY(struct ct_sns_gpn_ft_data {
> >   		u8 control_byte;
> >   		u8 port_id[3];
> >   		u32 reserved;
> >   		u8 port_name[8];
> > -	} entries[1];
> > +	}, entries);
> >   };
> >   /* CT command response */
> > diff --git a/drivers/scsi/qla2xxx/qla_gs.c b/drivers/scsi/qla2xxx/qla_gs.c
> > index 64ab070b8716..69d3bc795f90 100644
> > --- a/drivers/scsi/qla2xxx/qla_gs.c
> > +++ b/drivers/scsi/qla2xxx/qla_gs.c
> > @@ -4073,7 +4073,7 @@ int qla24xx_async_gpnft(scsi_qla_host_t *vha, u8 fc4_type, srb_t *sp)
> >   		sp->u.iocb_cmd.u.ctarg.req_size = GPN_FT_REQ_SIZE;
> >   		rspsz = sizeof(struct ct_sns_gpnft_rsp) +
> > -			((vha->hw->max_fibre_devices - 1) *
> > +			(vha->hw->max_fibre_devices *
> >   			    sizeof(struct ct_sns_gpn_ft_data));
> >   		sp->u.iocb_cmd.u.ctarg.rsp = dma_alloc_coherent(&vha->hw->pdev->dev,
>