Re: [net-next 09/17] can: length: can_fd_len2dlc(): simplify length calculcation
From: Vincent MAILHOL <hidden>
Date: 2021-01-15 01:58:02
Also in:
linux-can
On Fri. 15 Jan 2021 at 02:03, Oliver Hartkopp [off-list ref] wrote:
quoted hunk ↗ jump to hunk
On 14.01.21 10:16, Vincent MAILHOL wrote:quoted
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() */
Ack
/* map the sanitized data length to an appropriate data length code */
u8 can_fd_len2dlc(u8 len)
{
- if (unlikely(len > 64))
+ if (len > 48)I personally prefer the use of macros instead of hardcoded values. 48 is the last index of the table, i.e. it is ARRAY_SIZE(len2dlc) - 1. For me, it is like doing this: for (i = 0; i <= harcoded_value_representing_last_index_of_array; i++) instead of this: for (i = 0; i < ARRAY_SIZE(array); i++) Definitely prefer the later and (len >= ARRAY_SIZE(len2dlc)) is nothing less than the negation of the i < ARRAY_SIZE(array) that we usually see in a for loop. I recognize below patterns to be correct: i < ARRAY_SIZE(array): check that variable is inbound. i >= ARRAY_SIZE(array): check that variable is outbound. Anything which deviates from those patterns is fishy and it is actually how I spotted the bug. If we don’t use ARRAY_SIZE() we lose that recognizable pattern and we need to be aware of the actual content of len2dlc[] to understand the code. (And I know that the table is just above the function and that this makes my argument weaker). So IMO, checks done against the array size should use the ARRAY_SIZE() macro in order 1/ to make it a recognizable pattern and 2/ to make it work regardless of the actual size of the table (i.e. no hardcoded value).
return 0xF;
I would also prefer to use the CANFD_MAX_DLC macro here. Yours sincerely, Vincent
return len2dlc[len]; }