linux-kernel - Re: [PATCH 5/5] misc: mlx5ctl: Add umem reg/unreg ioctl

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Message-ID: <20231018115151.GP3952@nvidia.com>
Date:   Wed, 18 Oct 2023 08:51:51 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Saeed Mahameed <saeed@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, Leon Romanovsky <leonro@...dia.com>,
        Jiri Pirko <jiri@...dia.com>,
        Saeed Mahameed <saeedm@...dia.com>
Subject: Re: [PATCH 5/5] misc: mlx5ctl: Add umem reg/unreg ioctl

On Wed, Oct 18, 2023 at 11:30:24AM +0200, Arnd Bergmann wrote:

> > +struct mlx5ctl_umem_reg {
> > +	__aligned_u64 addr; /* user address */
> > +	__aligned_u64 len; /* user buffer length */
> > +	__aligned_u64 flags;
> > +	__u32 umem_id; /* returned device's umem ID */
> > +	__u32 reserved[7];
> > +};
> > +
>
> You have a 'flags' argument that is never accessed and can
> probably be removed. If the intention was to make the ioctl
> extensible for the future, this doesn't work unless you
> ensure that only known flags (i.e. none at this point)
> are set,

Yes, all the reserved fields and flags should be checked for 0 in the
ioctl paths to allow them to be used for something else someday.

> and it's probably a bad idea anyway, compared
> to creating a new ioctl command with new semantics.

This has been done successfully quite often in the kernel.

Thanks,
Jason