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vt: add VT_GETCONSIZECSRPOS to retrieve console size and cursor position

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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>
This commit is contained in:
Nicolas Pitre
2025-05-20 13:16:44 -04:00
committed by Greg Kroah-Hartman
parent 80fa7a0337
commit 81cf4d7d23
2 changed files with 27 additions and 0 deletions
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16
drivers/tty/vt/vt_ioctl.c
View File

@@ -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;
}

11
include/uapi/linux/vt.h
View File

@@ -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 */