linux-kernel - Re: [RESEND PATCH v3 1/2] VT: Add KDFONTINFO ioctl

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Date: Wed, 10 Apr 2024 19:11:03 +0200
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: Alexey Gladkov <legion@...nel.org>
Cc: Jiri Slaby <jirislaby@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
	kbd@...ts.linux.dev, linux-api@...r.kernel.org,
	linux-fbdev@...r.kernel.org, linux-serial@...r.kernel.org,
	Helge Deller <deller@....de>
Subject: Re: [RESEND PATCH v3 1/2] VT: Add KDFONTINFO ioctl

On Wed, Apr 10, 2024 at 06:29:19PM +0200, Alexey Gladkov wrote:
> On Wed, Apr 03, 2024 at 07:27:55AM +0200, Jiri Slaby wrote:
> > On 02. 04. 24, 15:19, Alexey Gladkov wrote:
> > >>> --- a/include/uapi/linux/kd.h
> > >>> +++ b/include/uapi/linux/kd.h
> > ...
> > >>> +struct console_font_info {
> > >>> +	unsigned int min_width, min_height;	/* minimal font size */
> > >>> +	unsigned int max_width, max_height;	/* maximum font size */
> > >>> +	unsigned int flags;			/* KD_FONT_INFO_FLAG_* */
> > >>
> > >> This does not look like a well-defined™ and extendable uapi structure.
> > >> While it won't change anything here, still use fixed-length __u32.
> > >>
> > >> And you should perhaps add some reserved fields. Do not repeat the same
> > >> mistakes as your predecessors with the current kd uapi.
> > > 
> > > I thought about it, but I thought it would be overengineering.
> > 
> > It would not. UAPI structs are set in stone once released.
> > 
> > And in this case, it's likely you would want to know more info about 
> > fonts in the future.
> > 
> > > Can you suggest how best to do this?
> > 
> > Given you have flags in there already (to state that the structure 
> > contains more), just add an array of u32 reserved[] space. 3 or 5, I 
> > would say (to align the struct to 64bit).
> 
> struct console_font_info {
>        __u32 min_width, min_height;    /* minimal font size */
>        __u32 max_width, max_height;    /* maximum font size */
>        __u32  flags;                   /* KD_FONT_INFO_FLAG_* */
>        __u32 reserved[5];              /* This field is reserved forfuture use. Must be 0. */
> };
> 
> So, struct should be like this ?
> 
> I wouldn't add the version to the flags. Maybe it would be better to add a
> separate field with the version?

Versions do not work for ioctls, just use flags for stuff.  And you
might want to put flags first?

thanks,

greg k-h