linux-kernel - [PATCH 27/44] drivers/tty/vt: use umin() instead of min

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Message-Id: <20251119224140.8616-28-david.laight.linux@gmail.com>
Date: Wed, 19 Nov 2025 22:41:23 +0000
From: david.laight.linux@...il.com
To: linux-kernel@...r.kernel.org,
	linux-serial@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jirislaby@...nel.org>,
	David Laight <david.laight.linux@...il.com>
Subject: [PATCH 27/44] drivers/tty/vt: use umin() instead of min_t(u16, ...) for row/col limits

From: David Laight <david.laight.linux@...il.com>

The row/column bounds (for a screen window box) are changed from
'offset one' to 'offset zero' and bound to the screen size using:
	v->xs = min_t(u16, v->xs - 1, vc->vc_cols - 1);
This has the side effect of converting zero to the limit.

A check I'm adding to min_t() reports that (u16)(v->xs - 1) (etc)
discards signiticant bits (because v->xs is promoted to 'int' before
the addition).
If v->xs is zero (it comes from userspace) it converts -1 to 0xffff.
This is then bounded to 'vc->vc_cols - 1' which will be fine.

Replace with:
	v->xs = umin(v->xs - 1, vc->vc_cols - 1);
which again converts a -1 to unsigned - this time to 0xffffffff,
with the same overall effect.

Whether zero is meant to mean the 'maximum size' is unknown.
I can't find any documentation for the ioctl and it pre-dates git.

Detected by an extra check added to min_t().

Signed-off-by: David Laight <david.laight.linux@...il.com>
---
 drivers/tty/vt/selection.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/tty/vt/selection.c b/drivers/tty/vt/selection.c
index 07d3b93975d3..13f4e48b4142 100644
--- a/drivers/tty/vt/selection.c
+++ b/drivers/tty/vt/selection.c
@@ -348,10 +348,11 @@ static int vc_selection(struct vc_data *vc, struct tiocl_selection *v,
 		return 0;
 	}
 
-	v->xs = min_t(u16, v->xs - 1, vc->vc_cols - 1);
-	v->ys = min_t(u16, v->ys - 1, vc->vc_rows - 1);
-	v->xe = min_t(u16, v->xe - 1, vc->vc_cols - 1);
-	v->ye = min_t(u16, v->ye - 1, vc->vc_rows - 1);
+	/* Historically 0 => max value */
+	v->xs = umin(v->xs - 1, vc->vc_cols - 1);
+	v->ys = umin(v->ys - 1, vc->vc_rows - 1);
+	v->xe = umin(v->xe - 1, vc->vc_cols - 1);
+	v->ye = umin(v->ye - 1, vc->vc_rows - 1);
 
 	if (mouse_reporting() && (v->sel_mode & TIOCL_SELMOUSEREPORT)) {
 		mouse_report(tty, v->sel_mode & TIOCL_SELBUTTONMASK, v->xs,
-- 
2.39.5