linux-kernel - [PATCH 3/5] usb: fotg210-hcd: Replace snprintf() with the safer scnprintf() variant

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Message-ID: <20231130105459.3208986-4-lee@kernel.org>
Date:   Thu, 30 Nov 2023 10:54:37 +0000
From:   Lee Jones <lee@...nel.org>
To:     lee@...nel.org
Cc:     linux-kernel@...r.kernel.org,
        Linus Walleij <linus.walleij@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Yuan-Hsin Chen <yhchen@...aday-tech.com>,
        Feng-Hsin Chiang <john453@...aday-tech.com>,
        Po-Yu Chuang <ratbert.chuang@...il.com>,
        linux-usb@...r.kernel.org
Subject: [PATCH 3/5] usb: fotg210-hcd: Replace snprintf() with the safer scnprintf() variant

There is a general misunderstanding amongst engineers that {v}snprintf()
returns the length of the data *actually* encoded into the destination
array.  However, as per the C99 standard {v}snprintf() really returns
the length of the data that *would have been* written if there were
enough space for it.  This misunderstanding has led to buffer-overruns
in the past.  It's generally considered safer to use the {v}scnprintf()
variants in their place (or even sprintf() in simple cases).  So let's
do that.

The uses in this file both seem to assume that data *has been* written!

Link: https://lwn.net/Articles/69419/
Link: https://github.com/KSPP/linux/issues/105
Cc: Linus Walleij <linus.walleij@...aro.org>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Yuan-Hsin Chen <yhchen@...aday-tech.com>
Cc: Feng-Hsin Chiang <john453@...aday-tech.com>
Cc: Po-Yu Chuang <ratbert.chuang@...il.com>
Cc: linux-usb@...r.kernel.org
Signed-off-by: Lee Jones <lee@...nel.org>
---
 drivers/usb/fotg210/fotg210-hcd.c | 12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

diff --git a/drivers/usb/fotg210/fotg210-hcd.c b/drivers/usb/fotg210/fotg210-hcd.c
index 929106c16b29b..b2f8b53cc8ef5 100644
--- a/drivers/usb/fotg210/fotg210-hcd.c
+++ b/drivers/usb/fotg210/fotg210-hcd.c
@@ -404,9 +404,9 @@ static void qh_lines(struct fotg210_hcd *fotg210, struct fotg210_qh *qh,
 			else if (td->hw_alt_next != list_end)
 				mark = '/';
 		}
-		temp = snprintf(next, size,
-				"\n\t%p%c%s len=%d %08x urb %p",
-				td, mark, ({ char *tmp;
+		temp = scnprintf(next, size,
+				 "\n\t%p%c%s len=%d %08x urb %p",
+				 td, mark, ({ char *tmp;
 				switch ((scratch>>8)&0x03) {
 				case 0:
 					tmp = "out";
@@ -424,17 +424,13 @@ static void qh_lines(struct fotg210_hcd *fotg210, struct fotg210_qh *qh,
 				(scratch >> 16) & 0x7fff,
 				scratch,
 				td->urb);
-		if (size < temp)
-			temp = size;
 		size -= temp;
 		next += temp;
 		if (temp == size)
 			goto done;
 	}
 
-	temp = snprintf(next, size, "\n");
-	if (size < temp)
-		temp = size;
+	temp = scnprintf(next, size, "\n");
 
 	size -= temp;
 	next += temp;
-- 
2.43.0.rc1.413.gea7ed67945-goog