From 81cf4d7d2379df853a0cbb8486286783c7380ac3 Mon Sep 17 00:00:00 2001 From: Nicolas Pitre Date: Tue, 20 May 2025 13:16:44 -0400 Subject: 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 Link: https://lore.kernel.org/r/20250520171851.1219676-3-nico@fluxnic.net Signed-off-by: Greg Kroah-Hartman --- drivers/tty/vt/vt_ioctl.c | 16 ++++++++++++++++ include/uapi/linux/vt.h | 11 +++++++++++ 2 files changed, 27 insertions(+) diff --git a/drivers/tty/vt/vt_ioctl.c b/drivers/tty/vt/vt_ioctl.c index 1f2bdd2e1cc5..61342e06970a 100644 --- 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 e9d39c48520a..e5b0c492aa18 100644 --- 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 +#include /* * 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 */ -- cgit v1.2.3