# Re: \[PATCH v2 1/2\] HID: u2fzero: explicitly check for errors

- From: Alan Stern \<stern@rowland.harvard.edu\>
- Date: 2021-10-18 14:15:46 
- List: linux-input
- Page: https://ratatoskr.run/linux-input/2021/10/6886038
- Thread: https://ratatoskr.run/linux-input/2021/10/6886027/t.md

```text
On Mon, Oct 18, 2021 at 02:21:43PM +0200, Andrej Shadura wrote:
> The previous commit fixed handling of incomplete packets but broke error
> handling: offsetof returns an unsigned value (size_t), but when compared
> against the signed return value, the return value is interpreted as if
> it were unsigned, so negative return values are never less than the
> offset.
> 
> Fixes: 22d65765f211 ("HID: u2fzero: ignore incomplete packets without data")
> Fixes: 42337b9d4d95 ("HID: add driver for U2F Zero built-in LED and RNG")
> Signed-off-by: Andrej Shadura <redacted>
> ---
>  drivers/hid/hid-u2fzero.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/hid/hid-u2fzero.c b/drivers/hid/hid-u2fzero.c
> index d70cd3d7f583..5145d758bea0 100644
> --- a/drivers/hid/hid-u2fzero.c
> +++ b/drivers/hid/hid-u2fzero.c
> @@ -200,7 +200,7 @@ static int u2fzero_rng_read(struct hwrng *rng, void *data,
>  	ret = u2fzero_recv(dev, &req, &resp);
>  
>  	/* ignore errors or packets without data */
> -	if (ret < offsetof(struct u2f_hid_msg, init.data))
> +	if (ret < 0 || ret < offsetof(struct u2f_hid_msg, init.data))

Although the patch description does a good job of explaining what's 
happening, someone merely reading the code will most likely not 
understand.

One alternative is to add a comment.  Another is simply to force a 
signed integer comparison:

	if (ret < (ssize_t) offsetof(...

Alan Stern
```