Hi,
we see times() return a really large positive value like 0x66693CF1 i.e.
signed int32 +1718172913 just a few minutes after booting.
Note that clock_gettime(CLOCK_MONOTONIC) works correctly and returns the
uptime. While times() +1718172913/100Hz time is in no relation to uptime.
Checking 'man 2 times': "Since Linux 2.6, this point is (2^32/HZ) - 300
(i.e., about 429 million) seconds before system boot time. [...] the
returned value may overflow the range of clock_t [...]"
Hence in theory on a 32-bit platform where clock_t is signed 32-bit, at
first times() should return negative values.
Then 300 seconds later they should become positive. Correct?
This should also happen via glibc times() call. There the times()
./sysdeps/unix/sysv/linux/times.c
simply does
clock_t ret = INTERNAL_SYSCALL (times, err, 1, buf);
with no extra "+- some constant".
In the kernel all jiffies and times syscall related handling is 64-bit:
---------
amn@multics:/home/etu/Desktop/linux-2.6.30.3/include$ fgrep INITIAL_JIFFIES
linux/jiffies.h:#define INITIAL_JIFFIES ((unsigned long)(unsigned int) (-300*HZ))
kernel/sys.c: return (long) jiffies_64_to_clock_t(get_jiffies_64());
kernel/time.c:u64 get_jiffies_64(void)
kernel/time.c: ret = jiffies_64;
---------
On 32-bit platforms and signed int32 clock_t it should take at least
octave:1> 2^31/(24*60*60 * 100)
ans = 248.55
octave:2> (2^31/100 - 300)/(24*60*60)
ans = 248.55
days until the positive times() return value wraps back to smallest
negative signed 32-bit int.
However, two bugs(?) :
1) times() return value after 1080 'uptime' seconds is 0x66693CF1, why?
Expected value would be (1080-300)*100Hz = 78000 = 0x000130B0 !
2) the times() wraps to negative only after 50 days uptime. Following the
intention of INITIAL_JIFFIES, how is this 50-day warp supposed to reveal
wrapping bugs right at 300 seconds after boot?
2) on 64-bit platforms, clock_t is 64-bit. There times() does not wrap
after 50 days, however it still starts at 0x000000006669**** !?
Any ideas?
- Jan
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