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 <npitre@baylibre.com>
Link: https://lore.kernel.org/r/20250520171851.1219676-3-nico@fluxnic.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/vt/vt_ioctl.c b/drivers/tty/vt/vt_ioctl.c
index 1f2bdd2e1cc593ce80413207adeb9671f2c6c99c..61342e06970a00c024d8eceec7915bf685a18cb6 100644 (file)
--- a/drivers/tty/vt/vt_ioctl.c
+++ b/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;
        }

diff --git a/include/uapi/linux/vt.h b/include/uapi/linux/vt.h
index e9d39c48520af18b8a000c28e3e5b73155aa3d4a..e5b0c492aa184619df62d8a6fc790bfc11c66449 100644 (file)
--- a/include/uapi/linux/vt.h
+++ b/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 */