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

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))
 		return 0;
 
 	/* only take the minimum amount of data it is safe to take */

-- 
2.33.0

The wait_for_completion_timeout function returns 0 if timed out or a
positive value if completed. Hence, "less than zero" comparison always
misses timeouts and doesn't kill the URB as it should, leading to
re-sending it while it is active.

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 5145d758bea0..562da98cfb82 100644
--- a/drivers/hid/hid-u2fzero.c
+++ b/drivers/hid/hid-u2fzero.c

@@ -132,7 +132,7 @@ static int u2fzero_recv(struct u2fzero_device *dev,
 
 	ret = (wait_for_completion_timeout(
 		&ctx.done, msecs_to_jiffies(USB_CTRL_SET_TIMEOUT)));
-	if (ret < 0) {
+	if (ret == 0) {
 		usb_kill_urb(dev->urb);
 		hid_err(hdev, "urb submission timed out");
 	} else {

-- 
2.33.0

On Mon, Oct 18, 2021 at 02:21:43PM +0200, Andrej Shadura wrote:

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

On 18/10/2021 16:15, Alan Stern wrote:

On Mon, Oct 18, 2021 at 02:21:43PM +0200, Andrej Shadura wrote:

quoted

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(...

I have considered that, but I thought that is actually less readable 
than having two conditions. I’m curious that you say "ignore errors or 
packets without data" is not clear enough — how would you reword that 
without inflating it too much?

-- 
Cheers,
   Andrej

On Mon, Oct 18, 2021 at 04:17:57PM +0200, Andrej Shadura wrote:

On 18/10/2021 16:15, Alan Stern wrote:

quoted

On Mon, Oct 18, 2021 at 02:21:43PM +0200, Andrej Shadura wrote:

quoted

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(...

I have considered that, but I thought that is actually less readable than
having two conditions. I’m curious that you say "ignore errors or packets
without data" is not clear enough — how would you reword that without
inflating it too much?

You misunderstand.  The existing comment is clear enough.  But the code 
itself is misleading:

	if (ret < 0 || ret < offsetof(...

looks redundant.  Someone reading it for the first time will 
automatically think: "If ret < 0 then certainly it is < the offset of 
some internal field.  So why perform two comparisons when one is 
enough?"

To help such a reader understand what is happening, you could add a 
comment like:

	/*
	 * offsetof returns an unsigned value, so the comparison with
	 * ret uses unsigned arithmetic and won't detect a negative
	 * error value.  We need a separate test for errors.
	 */

If you think a comment like this is preferable to a typecast, fine.

Another alternative is:

	ret -= offsetof(...);
	if (ret < 0)
		return 0;

which may look more complicated but allows you to simplify the max3 
computation in the next line.

Alan Stern