Confusion in `setitimer`

[aeryz]

Hey, this is a followup of this. The bug and my investigation is explained in detail there. I wanted to fix the issue but I need some clarification first.

From the Linux man page you see that the timer is defined as:

struct itimerval {
    struct timeval it_interval; /* next value */
    struct timeval it_value;    /* current value */
};

So in the case where it_value = 2 secs and it_interval = 5 secs, this would result in SIGALARM to be fired after 2 seconds, then repeated signals are delivered every 5 seconds. However, since proc_raise_interval only supports a single interval value, this behaviour is not possible. And this creates a problem because for example: alarm function creates a delayed single shot signal. Hence it uses it_value only. This causes the signal to be never fired in WASIX.

Another confusion im seeing is although both the setitimer in non-WASIX libc and proc_raise_interval allows non-repeated signals, repetition is forcibly enabled in the WASIX implementation.

Possible paths

1. Support it_value and it_interval in proc_raise_interval

Change proc_raise_interval from:

  (@interface func (export "proc_raise_interval")
    (param $sig $signal)
    (param $interval $timestamp)
    (param $repeat $bool)
    (result $error (expected (error $errno)))
  )

to:

  (@interface func (export "proc_raise_interval")
    (param $sig $signal)
    (param $value $timestamp)
    (param $interval $timestamp)
    (result $error (expected (error $errno)))
  )

The new implementation will raise the signal after value milliseconds (disarm the timer if 0). If the interval is zero, the prior interval for that signal is cancelled, otherwise the signal is repeated every interval milliseconds.

Pros:

• The logic is almost identical to setitimer.

Cons:

• API change.
• More state to be tracked in WasiSignalInterval. (not much work tbf)

2. Allow it_value == it_interval

Instead of only parsing the interval in the setitimer implementation and hardcoding repeat to be true, do:

repeat = it_interval != 0;
int ts = 0;
if it_value == 0:
    ts = 0; // disarm any timers
else if it_interval != 0:
    ts = it_interval;
else:
    ts = it_value;
int ret = __wasi_proc_raise_interval((__wasi_signal_t)__WASI_SIGNAL_ALRM, ts, repeat);

Pros:

• Supports both the single shot and repeated signals with interval.
• Requires no change to the WASIX API.

Cons:

• No support for different it_value and it_interval which might cause unexpected effects in the programs that rely on this.

[zebreus]

Agree, it seems like proc_raise_interval is missing some functionality to implement setitimer cleanly. However, we can't really change the interface of existing WASIX syscalls, so I think introducing a proc_raise_interval2 with an improved signature is the way to go. Maybe also add two pointers for the return values to the signature, so we can return the old value like setitimer does.

[Arshia001]

I'm with @zebreus on the need to introduce a second function, existing wasix syscalls can't be changed. However, we don't need two pointers for the return value, just a single pointer to a struct that parallels itimerval.