vt: add VT_GETCONSIZECSRPOS to retrieve console size and cursor position

The console dimension and cursor position are available through the
/dev/vcsa interface already. However the /dev/vcsa header format uses
single-byte fields therefore those values are clamped to 255.

As surprizing as this may seem, some people do use 240-column 67-row
screens (a 1920x1080 monitor with 8x16 pixel fonts) which is getting
close to the limit. Monitors with higher resolution are not uncommon
these days (3840x2160 producing a 480x135 character display) and it is
just a matter of time before someone with, say, a braille display using
the Linux VT console and BRLTTY on such a screen reports a bug about
missing and oddly misaligned screen content.

Let's add VT_GETCONSIZECSRPOS for the retrieval of console size and cursor
position without byte-sized limitations. The actual console size limit as
encoded in vt.c is 32767x32767 so using a short here is appropriate. Then
this can be used to get the cursor position when /dev/vcsa reports 255.

The screen dimension may already be obtained using TIOCGWINSZ and adding
the same information to VT_GETCONSIZECSRPOS might be redundant. However
applications that care about cursor position also care about display
size and having 2 separate system calls to obtain them separately is
wasteful. Also, the cursor position can be queried by writing "\e[6n" to
a tty and reading back the result but that may be done only by the actual
application using that tty and not a sideline observer.

Signed-off-by: Nicolas Pitre <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: Nicolas Pitre

drivers/tty/vt/vt_ioctl.c

@@ -951,6 +951,22 @@ int vt_ioctl(struct tty_struct *tty,
 					(unsigned short __user *)arg);
 	case VT_WAITEVENT:
 		return vt_event_wait_ioctl((struct vt_event __user *)arg);
+
+	case VT_GETCONSIZECSRPOS:
+	{
+		struct vt_consizecsrpos concsr;
+
+		console_lock();
+		concsr.con_cols = vc->vc_cols;
+		concsr.con_rows = vc->vc_rows;
+		concsr.csr_col = vc->state.x;
+		concsr.csr_row = vc->state.y;
+		console_unlock();
+		if (copy_to_user(up, &concsr, sizeof(concsr)))
+			return -EFAULT;
+		return 0;
+	}
+
 	default:
 		return -ENOIOCTLCMD;
 	}

include/uapi/linux/vt.h

@@ -2,6 +2,8 @@
 #ifndef _UAPI_LINUX_VT_H
 #define _UAPI_LINUX_VT_H
 
+#include <linux/ioctl.h>
+#include <linux/types.h>
 
 /*
  * These constants are also useful for user-level apps (e.g., VC
@@ -84,4 +86,13 @@ struct vt_setactivate {
 
 #define VT_SETACTIVATE	0x560F	/* Activate and set the mode of a console */
 
+/* get console size and cursor position */
+struct vt_consizecsrpos {
+	__u16 con_rows;		/* number of console rows */
+	__u16 con_cols;		/* number of console columns */
+	__u16 csr_row;		/* current cursor's row */
+	__u16 csr_col;		/* current cursor's column */
+};
+#define VT_GETCONSIZECSRPOS	_IOR('V', 0x10, struct vt_consizecsrpos)
+
 #endif /* _UAPI_LINUX_VT_H */