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SubjectRe: News UBSAN warnings in aacraid
On Mon, Nov 27, 2017 at 8:17 PM, Meelis Roos <mroos@linux.ee> wrote:
> Tried 4.15-rc1 on an old 32-bit HP Netserver with aacraid card. Compared
> to 4.14, there are new UBSAN warnings with timer related backtraces, so
> the timespec64 change seems suspicious:

> [ 12.228155] UBSAN: Undefined behaviour in drivers/scsi/aacraid/commsup.c:2514:49
> [ 12.228229] signed integer overflow:
> [ 12.228283] 964297611 * 250 cannot be represented in type 'long int'

Thanks for reporting it! For reference, this is my change that got applied to
aac_command_thread:

@@ -2496,7 +2496,7 @@ int aac_command_thread(void *data)
}
if (!time_before(next_check_jiffies,next_jiffies)
&& ((difference = next_jiffies - jiffies) <= 0)) {
- struct timeval now;
+ struct timespec64 now;
int ret;

/* Don't even try to talk to adapter if its sick */
@@ -2506,15 +2506,15 @@ int aac_command_thread(void *data)
next_check_jiffies = jiffies
+ ((long)(unsigned)check_interval)
* HZ;
- do_gettimeofday(&now);
+ ktime_get_real_ts64(&now);

/* Synchronize our watches */
- if (((1000000 - (1000000 / HZ)) > now.tv_usec)
- && (now.tv_usec > (1000000 / HZ)))
- difference = (((1000000 - now.tv_usec) * HZ)
- + 500000) / 1000000;
+ if (((NSEC_PER_SEC - (NSEC_PER_SEC / HZ)) > now.tv_nsec)
+ && (now.tv_nsec > (NSEC_PER_SEC / HZ)))
+ difference = (((NSEC_PER_SEC -
now.tv_nsec) * HZ)
+ + NSEC_PER_SEC / 2) / NSEC_PER_SEC;
else {
- if (now.tv_usec > 500000)
+ if (now.tv_nsec > NSEC_PER_SEC / 2)
++now.tv_sec;

if (dev->sa_firmware)
The problem is that a microsecond number (0 to 999999) multiplied by
HZ (100 to 1024) always fits in a 32-bit integer, but the nanosecond
number doesn't.
We could make that a 64-bit division, but that would be fairly expensive.

I'm trying to understand the bigger picture now, rather than simply
attempting to do a simple conversion, but I don't see what we are
actually trying to compute in 'difference' here.

I think this chunk would solve the problem and result in the
same behavior as before:

--- a/drivers/scsi/aacraid/commsup.c
+++ b/drivers/scsi/aacraid/commsup.c
@@ -2511,8 +2511,8 @@ int aac_command_thread(void *data)
/* Synchronize our watches */
if (((NSEC_PER_SEC - (NSEC_PER_SEC / HZ)) > now.tv_nsec)
&& (now.tv_nsec > (NSEC_PER_SEC / HZ)))
- difference = (((NSEC_PER_SEC -
now.tv_nsec) * HZ)
- + NSEC_PER_SEC / 2) / NSEC_PER_SEC;
+ difference = HZ + HZ / 2 -
+ now.tv_nsec / (NSEC_PER_SEC / HZ);
else {
if (now.tv_nsec > NSEC_PER_SEC / 2)
++now.tv_sec;
but I don't see why we add in half a second here. Any ideas?

Arnd

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