https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=261781
Bug ID: 261781
Summary: VDSO time calculation integer overflow
Product: Base System
Version: CURRENT
Hardware: arm
OS: Any
Status: New
Severity: Affects Only Me
Priority: ---
Component: kern
Assignee: [email protected]
Reporter: [email protected]
I have been tinkering with clock subsystem for more or less tickless based
approach. I am not sure whether calculation in
"lib/libc/sys/__vdso_gettimeofday.c" for "binuptime" function is correct.
Currently the code looks like this:
scale = th->th_scale;
#ifdef _LP64
scale_bits = ffsl(scale);
#else
scale_bits = ffsll(scale);
#endif
if (__predict_false(scale_bits + fls(delta) > 63)) {
x = (scale >> 32) * delta;
scale &= 0xffffffff;
bt->sec += x >> 32;
bintime_addx(bt, x << 32);
}
Example outputs from two time points (time is measured by ARM Generic Timer,
but that's just 64bit counter masked to 32bits):
th->th_boottime = 1640852968.ff886104742783f9
timecounter delta = 29015463
th->th_scale = 295147905178
th->th_offset_count = 539967626
bintime_addx(bt, scale * delta = 0x76D8EB0A9A877676) => 9.4522dbb32c111955
th->th_boottime = 1640852968.ff886104742783f9
timecounter delta = 64100295
th->th_scale = 295147905178
th->th_offset_count = 539967626
bintime_addx(bt, scale * delta) => 8.d4d7f89392515095
Multiplication of scale and delta will overflow 64 bits.
I am not sure whether the scale_bits should rather be calculated by flsl/flsll
to detect the overflow.
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