USB: core: Use krealloc() in usb_cache_string()

Instead of "shrinking" the allocation by kmalloc()ing a new, smaller
buffer, utilize krealloc() to shrink the existing allocation. This saves
a memcpy(), as well as eliminates the temporary `smallbuf` allocation,
which guards against allocation failure under extreme memory pressure.

Signed-off-by: Bence Csókás <bence98@sch.bme.hu>
Link: https://patch.msgid.link/20260315-usb-krealloc-v2-1-32f83e090409@sch.bme.hu
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 11 insertions, 9 deletions

diff --git a/drivers/usb/core/message.c b/drivers/usb/core/message.c
index 2ab120ce2fa8..75e2bfd744a9 100644
--- a/drivers/usb/core/message.c
+++ b/drivers/usb/core/message.c
@@ -1063,7 +1063,7 @@ int usb_string(struct usb_device *dev, int index, char *buf, size_t size)
 }
 EXPORT_SYMBOL_GPL(usb_string);
 
-/* one UTF-8-encoded 16-bit character has at most three bytes */
+/* one 16-bit character, when UTF-8-encoded, has at most three bytes */
 #define MAX_USB_STRING_SIZE (127 * 3 + 1)
 
 /**
@@ -1084,16 +1084,18 @@ char *usb_cache_string(struct usb_device *udev, int index)
 		return NULL;
 
 	buf = kmalloc(MAX_USB_STRING_SIZE, GFP_NOIO);
-	if (buf) {
-		len = usb_string(udev, index, buf, MAX_USB_STRING_SIZE);
-		if (len > 0) {
-			smallbuf = kmalloc(++len, GFP_NOIO);
-			if (!smallbuf)
-				return buf;
-			memcpy(smallbuf, buf, len);
-		}
+	if (!buf)
+		return NULL;
+
+	len = usb_string(udev, index, buf, MAX_USB_STRING_SIZE);
+	if (len <= 0) {
 		kfree(buf);
+		return NULL;
 	}
+
+	smallbuf = krealloc(buf, len + 1, GFP_NOIO);
+	if (unlikely(!smallbuf))
+		return buf;
 	return smallbuf;
 }
 EXPORT_SYMBOL_GPL(usb_cache_string);