linux-kernel - [PATCH 10/12] usb: mon_text: Replace snprintf() with the safer scnprintf() variant

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Date:   Wed, 13 Dec 2023 16:42:39 +0000
From:   Lee Jones <lee@...nel.org>
To:     lee@...nel.org, gregkh@...uxfoundation.org
Cc:     linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org
Subject: [PATCH 10/12] usb: mon_text: 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.

Link: https://lwn.net/Articles/69419/
Link: https://github.com/KSPP/linux/issues/105
Signed-off-by: Lee Jones <lee@...nel.org>
---
 drivers/usb/mon/mon_text.c | 28 +++++-----------------------
 1 file changed, 5 insertions(+), 23 deletions(-)

diff --git a/drivers/usb/mon/mon_text.c b/drivers/usb/mon/mon_text.c
index 39cb141646526..2fe9b95bac1d5 100644
--- a/drivers/usb/mon/mon_text.c
+++ b/drivers/usb/mon/mon_text.c
@@ -352,7 +352,7 @@ static int mon_text_open(struct inode *inode, struct file *file)
 	rp->r.rnf_error = mon_text_error;
 	rp->r.rnf_complete = mon_text_complete;
 
-	snprintf(rp->slab_name, SLAB_NAME_SZ, "mon_text_%p", rp);
+	scnprintf(rp->slab_name, SLAB_NAME_SZ, "mon_text_%p", rp);
 	rp->e_slab = kmem_cache_create(rp->slab_name,
 	    sizeof(struct mon_event_text), sizeof(long), 0,
 	    mon_text_ctor);
@@ -700,46 +700,28 @@ static const struct file_operations mon_fops_text_u = {
 
 int mon_text_add(struct mon_bus *mbus, const struct usb_bus *ubus)
 {
-	enum { NAMESZ = 10 };
+	enum { NAMESZ = 12 };
 	char name[NAMESZ];
 	int busnum = ubus? ubus->busnum: 0;
-	int rc;
 
 	if (mon_dir == NULL)
 		return 0;
 
 	if (ubus != NULL) {
-		rc = snprintf(name, NAMESZ, "%dt", busnum);
-		if (rc <= 0 || rc >= NAMESZ)
-			goto err_print_t;
+		scnprintf(name, NAMESZ, "%dt", busnum);
 		mbus->dent_t = debugfs_create_file(name, 0600, mon_dir, mbus,
 							     &mon_fops_text_t);
 	}
 
-	rc = snprintf(name, NAMESZ, "%du", busnum);
-	if (rc <= 0 || rc >= NAMESZ)
-		goto err_print_u;
+	scnprintf(name, NAMESZ, "%du", busnum);
 	mbus->dent_u = debugfs_create_file(name, 0600, mon_dir, mbus,
 					   &mon_fops_text_u);
 
-	rc = snprintf(name, NAMESZ, "%ds", busnum);
-	if (rc <= 0 || rc >= NAMESZ)
-		goto err_print_s;
+	scnprintf(name, NAMESZ, "%ds", busnum);
 	mbus->dent_s = debugfs_create_file(name, 0600, mon_dir, mbus,
 					   &mon_fops_stat);
 
 	return 1;
-
-err_print_s:
-	debugfs_remove(mbus->dent_u);
-	mbus->dent_u = NULL;
-err_print_u:
-	if (ubus != NULL) {
-		debugfs_remove(mbus->dent_t);
-		mbus->dent_t = NULL;
-	}
-err_print_t:
-	return 0;
 }
 
 void mon_text_del(struct mon_bus *mbus)
-- 
2.43.0.472.g3155946c3a-goog