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n_tty: Use DIV_ROUND_UP() in room calculation 

When PARMRK is set, a character can result in up to 3 chars in the read
buffer. Receive code calculates for how many characters there (at
least) is room. Convert an opencoded rounding in the calculation to use
DIV_ROUND_UP().

Note: the room variable is decremented afterwards by one which ensures
the characters will fit into the buffer for real so the code is okay
despite rounding upwards.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20230309082035.14880-6-ilpo.jarvinen@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Ilpo Järvinen 2023-03-09 10:20:32 +02:00 committed by Greg Kroah-Hartman
parent 9db1be8405
commit 7e26c84d02
1 changed files with 2 additions and 1 deletions
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3
drivers/tty/n_tty.c
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@ -36,6 +36,7 @@
#include <linux/file.h>
#include <linux/interrupt.h>
#include <linux/major.h>
#include <linux/math.h>
#include <linux/mm.h>
#include <linux/module.h>
#include <linux/poll.h>
@ -1692,7 +1693,7 @@ n_tty_receive_buf_common(struct tty_struct *tty, const unsigned char *cp,
room = N_TTY_BUF_SIZE - (ldata->read_head - tail);
if (I_PARMRK(tty))
room = (room + 2) / 3;
room = DIV_ROUND_UP(room, 3);
room--;
if (room <= 0) {
overflow = ldata->icanon && ldata->canon_head == tail;