On 14.01.21 10:16, Vincent MAILHOL wrote:

On Tue. 14 Jan 2021 at 17:23, Oliver Hartkopp [off-list ref] wrote:

quoted

On 14.01.21 02:59, Vincent MAILHOL wrote:

quoted

On Tue. 14 Jan 2021 at 06:14, Marc Kleine-Budde [off-list ref] wrote:

quoted

If the length paramter in len2dlc() exceeds the size of the len2dlc array, we
return 0xF. This is equal to the last 16 members of the array.

This patch removes these members from the array, uses ARRAY_SIZE() for the
length check, and returns CANFD_MAX_DLC (which is 0xf).

Reviewed-by: Vincent Mailhol <redacted>
Link: https://lore.kernel.org/r/20210111141930.693847-9-mkl@pengutronix.de (local)
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
---
   drivers/net/can/dev/length.c | 6 ++----
   1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/drivers/net/can/dev/length.c b/drivers/net/can/dev/length.c
index 5e7d481717ea..d695a3bee1ed 100644
--- a/drivers/net/can/dev/length.c
+++ b/drivers/net/can/dev/length.c

@@ -27,15 +27,13 @@ static const u8 len2dlc[] = {
          13, 13, 13, 13, 13, 13, 13, 13, /* 25 - 32 */
          14, 14, 14, 14, 14, 14, 14, 14, /* 33 - 40 */
          14, 14, 14, 14, 14, 14, 14, 14, /* 41 - 48 */
-       15, 15, 15, 15, 15, 15, 15, 15, /* 49 - 56 */
-       15, 15, 15, 15, 15, 15, 15, 15  /* 57 - 64 */
   };

   /* map the sanitized data length to an appropriate data length code */
   u8 can_fd_len2dlc(u8 len)
   {
-       if (unlikely(len > 64))
-               return 0xF;
+       if (len > ARRAY_SIZE(len2dlc))

Sorry but I missed an of-by-one issue when I did my first
review. Don't know why but it popped to my eyes this morning when
casually reading the emails.

Oh, yes.

The fist line is 0 .. 8 which has 9 bytes.

I also looked on it (from the back), and wondered if it was correct. But
didn't see it either at first sight.

quoted

ARRAY_SIZE(len2dlc) is 49. If len is between 0 and 48, use the
array, if len is greater *or equal* return CANFD_MAX_DLC.

All these changes and discussions make it very obviously more tricky to
understand that code.

I don't really like this kind of improvement ...

Before that it was pretty clear that we only catch an out of bounds
value and usually grab the value from the table.

I understand your point: all three of us initially missed that
bug. But now that it is fixed, I would still prefer to keep
Marc's patch.

No, I'm still against it as it is now.

Even

	if (len >= ARRAY_SIZE(len2dlc))

would need some comment that values > 48 lead to a DLC = 15.

This is not intuitively understandable from that value 
"ARRAY_SIZE(len2dlc)" !

Using ARRAY_SIZE() is a bad choice IMO.

If it's really worth to save 16 bytes I would suggest this:

diff --git a/drivers/net/can/dev.c b/drivers/net/can/dev.c
index 3486704c8a95..0b0a5a16943a 100644
--- a/drivers/net/can/dev.c
+++ b/drivers/net/can/dev.c

@@ -42,18 +42,17 @@ static const u8 len2dlc[] = {0, 1, 2, 3, 4, 5, 6, 7, 

8,             /* 0 - 8 */
                              10, 10, 10, 10,                    /* 13 - 
16 */
                              11, 11, 11, 11,                    /* 17 - 
20 */
                              12, 12, 12, 12,                    /* 21 - 
24 */
                              13, 13, 13, 13, 13, 13, 13, 13,    /* 25 - 
32 */
                              14, 14, 14, 14, 14, 14, 14, 14,    /* 33 - 
40 */
-                            14, 14, 14, 14, 14, 14, 14, 14,    /* 41 - 
48 */
-                            15, 15, 15, 15, 15, 15, 15, 15,    /* 49 - 
56 */
-                            15, 15, 15, 15, 15, 15, 15, 15};   /* 57 - 
64 */
+                            14, 14, 14, 14, 14, 14, 14, 14};   /* 41 - 
48 */
+                            /* 49 - 64 is checked in  can_fd_len2dlc() */

  /* map the sanitized data length to an appropriate data length code */
  u8 can_fd_len2dlc(u8 len)
  {
-       if (unlikely(len > 64))
+       if (len > 48)
                 return 0xF;

         return len2dlc[len];
  }
  EXPORT_SYMBOL_GPL(can_fd_len2dlc);

Regards,
Oliver

Yours sincerely,
Vincent

quoted

quoted

In short, replace > by >=:
+       if (len >= ARRAY_SIZE(len2dlc))

quoted

+               return CANFD_MAX_DLC;

          return len2dlc[len];
   }