For "clean" semantics of a simulative execution of the kernel, it feels
erroneous to advance time even if this value is only read once.
In my experiments timer_read was called much more often than I
anticipated (e.g., filesystem code).
Yeah, that does not really sound like something we would want (and it
will also not help with performance with time-travel=ext).
Looking at the old discussion, it doesn't seem that Johannes was
against the idea of doing the time insertion only in more specific
scenarios. So, we "just" need a reasonably elegant solution.
If we accept writing a list of syscalls, then maybe we could just do it
within handle_syscall and do a um_udelay(1) for any syscall that takes
a timeout parameter (select, pselect6, poll, ...)? It is going to be a
pretty long list, but could still be reasonable.
That's actually not what my "hack" did. I filtered out all syscalls,
that give some information about the current timestamp of the system.
Actually, I think, timeouts are no problem, if we can assure, that a
timeout is never rounded down to 0. Mostly a direct input of 0 have
special meanings, or provokes wrong behavior in the first place from
user space program.
Since time-travel mode has a very limited niche, I would not try to
prevent every possible dumb behavior that bad user space programs could
have. I think busy-waiting on a system clock advancement is not the best
style, but acceptable.
So my list was:
sys_getitimer
sys_gettimeofday
sys_time
sys_timer_gettime
sys_clock_gettime
sys_timerfd_gettime
While overthinking it, I see the possibility to read the access
timestamps of a file to create an endless loop, so maybe the stat
syscalls may be included, although this makes me a bit uncomfortable
again. I tend to say, this "bad" behavior of asking the same information
over and over again, should only be punished, if it happens multiple times.
I was thinking about, storing the PID of a busy-looped process, and only
increase time, if the same PID is "suspicious". However, this "hack"
becomes more and more costly, which is on the other hand not important
for timetravel mode.
One neat side effect is that if reading time does not actually cost
time, then we could implement clock_gettime in the VDSO.
That would exactly not work, because of my comment from before.
