[PATCH] HID: hid-debug: clean up snprintf() checks in hid_resolv_usage()

The snprintf() limits are complicated and slightly wrong when it does:

	max(0, HID_DEBUG_BUFSIZE - len - 1)

The "- 1" should not be there.  It means we can't use the last
byte of the buffer.  If we change the first snprintf() to scnprintf()
then we can remove the max().

At the start of the function the strlen(buf) is going always going to
be < HID_DEBUG_BUFSIZE so that is safe.  If it were > HID_DEBUG_BUFSIZE
then that would result in a WARN().

Signed-off-by: Dan Carpenter <redacted>
---
 drivers/hid/hid-debug.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/drivers/hid/hid-debug.c b/drivers/hid/hid-debug.c
index fa57d05badf7..3f62fe3b0a49 100644
--- a/drivers/hid/hid-debug.c
+++ b/drivers/hid/hid-debug.c

@@ -486,8 +486,7 @@ char *hid_resolv_usage(unsigned usage, struct seq_file *f) {
 
 	if (!f) {
 		len = strlen(buf);
-		snprintf(buf+len, max(0, HID_DEBUG_BUFSIZE - len), ".");
-		len++;
+		len += scnprintf(buf + len, HID_DEBUG_BUFSIZE - len, ".");
 	}
 	else {
 		seq_printf(f, ".");

@@ -498,7 +497,7 @@ char *hid_resolv_usage(unsigned usage, struct seq_file *f) {
 				if (p->usage == (usage & 0xffff)) {
 					if (!f)
 						snprintf(buf + len,
-							max(0,HID_DEBUG_BUFSIZE - len - 1),
+							HID_DEBUG_BUFSIZE - len,
 							"%s", p->description);
 					else
 						seq_printf(f,

@@ -509,8 +508,8 @@ char *hid_resolv_usage(unsigned usage, struct seq_file *f) {
 			break;
 		}
 	if (!f)
-		snprintf(buf + len, max(0, HID_DEBUG_BUFSIZE - len - 1),
-				"%04x", usage & 0xffff);
+		snprintf(buf + len, HID_DEBUG_BUFSIZE - len, "%04x",
+			 usage & 0xffff);
 	else
 		seq_printf(f, "%04x", usage & 0xffff);
 	return buf;

-- 
2.20.1

On Thu, 16 Sep 2021, Dan Carpenter wrote:

The snprintf() limits are complicated and slightly wrong when it does:

	max(0, HID_DEBUG_BUFSIZE - len - 1)

The "- 1" should not be there.  It means we can't use the last
byte of the buffer.  If we change the first snprintf() to scnprintf()
then we can remove the max().

At the start of the function the strlen(buf) is going always going to
be < HID_DEBUG_BUFSIZE so that is safe.  If it were > HID_DEBUG_BUFSIZE
then that would result in a WARN().

Applied, thanks Dan.

-- 
Jiri Kosina
SUSE Labs