Re: [PATCH] prevent timespec/timeval to ktime_t overflow

Andrew Morton
2006-09-02T19:32:43Z

On Sat, 02 Sep 2006 20:41:33 +0200
Thomas Gleixner <tglx@linutronix.de> wrote:

> On Fri, 2006-09-01 at 20:13 -0700, Andrew Morton wrote:
> > > Fun, here is a version with a bigabyte blocker.
> >
> > Your patch triggers waaaaaaay early.
> >
> > netconsole: remote IP 192.168.2.33
> > netconsole: remote ethernet address 00:0d:56:c6:c6:cc
> > Initializing CPU#0
> > PID hash table entries: 4096 (order: 12, 32768 bytes)
> > time.c: Using 14.318180 MHz WALL HPET GTOD HPET/TSC timer.
> > time.c: Detected 3400.238 MHz processor.
> > ktime_set: -1157140842 : 0
>
> -------------^ !!!!!!!
>
> > BUG: warning at include/linux/ktime.h:84/ktime_set()
> >
> > Call Trace:
> > <IRQ> [<ffffffff80247021>] hrtimer_run_queues+0x10e/0x211
>
> This seems to happen inside hrtimer_get_softirq_time().
> wall_to_monotonic is negative.
>
> Why does the check trigger ? We compare a "long", which contains a
> negative value against some positive constant.
>
> ktime_t ktime_set(const long secs, const unsigned long nsecs)
> {
> if (unlikely(secs >= KTIME_SEC_MAX)) {
>
> where KTIME_SEC_MAX is 0x7FFFFFFFFFFFFFFF / 1000 000 000 =
>
> 9223372036 == 0x225C17D04,
>
> which is compared against
>
> -1157140842 == 0xFFFFFFFFBB076E96
>
> This smells like gcc magic. Can you please disassemble the code in
> question ?
>

ktime_set:
.LFB522:
.file 1 "kernel/hrtimer.c"
.loc 1 884 0
.LVL0:
pushq %rbp #
.LCFI0:
movq %rsp, %rbp #,
.LCFI1:
.LBB2:
.LBB3:
.LBB4:
.file 2 "include/asm/current.h"
.loc 2 11 0
#APP
movq %gs:0,%rax #, t
.LVL1:
#NO_APP
.LBE4:
.LBE3:
.LBE2:
.loc 1 888 0
cmpq $0, 208(%rax) #, <variable>.mm
je .L2 #,
movabsq $9223372035, %rax #, tmp66
.LVL2:
cmpq %rax, %rdi # tmp66, secs
jbe .L2 #,
.loc 1 889 0
cmpl $0, no88bigabytes.11819(%rip) #, no88bigabytes
jne .L5 #,
.loc 1 890 0
movl $1, no88bigabytes.11819(%rip) #, no88bigabytes
.loc 1 891 0
movq %rsi, %rdx # nsecs, nsecs
movq %rdi, %rsi # secs, secs

--
VGER BF report: H 0.000301227