POSIX Simulator: Clear SIGRESUME (SIGUSR1) when exiting xPortStartScheduler

[johnboiles]

Clear SIGRESUME (SIGUSR1) when exiting xPortStartScheduler

Description

When attempting to stop the scheduler via vTaskEndScheduler, an unhandled SIGUSR1 (aka SIGRESUME) happens when restoring the scheduler thread's signals with pthread_sigmask. This crashes the program.

I'm not sure why this is happening since sigwait is supposed to consume the pending signals. It's possible there's something platform-specific about this. I'm testing on macOS. If the behavior is different on other platforms however, it probably can't hurt to double check. What's particularly interesting is if I modify this code like this:

    while( xSchedulerEnd != pdTRUE )
    {
        int result = sigwait( &xSignals, &iSignal );
        printf("Signal received %d result=%d\n", iSignal, result);

        sigset_t set;
        sigpending( &set );
        if( sigismember( &set, SIG_RESUME ) )
        {
            int result = sigwait( &xSignals, &iSignal );
            printf("Signal received %d result=%d\n", iSignal, result);
        }
    }

I then get this:

Signal received 0 result=0
Signal received 30 result=0

Which means sigwait the first time is returning an invalid signal 0 but also a return code suggesting that it succeeded! Strange!

Test Steps

Testing with this example (full code: freertos-pr1224.zip ):

#include <FreeRTOS.h>
#include <task.h>
#include <stdio.h>
#include <stdlib.h>
#include <assert.h>

extern "C" {
    void vAssertCalled(const char *file, int line) {
        fprintf(stderr, "Assertion failed in file %s:%d\n", file, line);
        abort();
    }
}

int main() {
    TaskHandle_t task;
    xTaskCreate(
        [](void *param) {
        printf("FreeRTOS scheduler started\n");
        vTaskDelay(pdMS_TO_TICKS(1000));
        printf("Task Done, ending scheduler\n");
        vTaskEndScheduler();
        assert(false && "After scheduler ended (SHOULD NOT GET HERE)");
    }, "start", 10000, nullptr, 1, &task);
    printf("Starting FreeRTOS scheduler\n");
    vTaskStartScheduler();
    printf("FreeRTOS scheduler exited\n");
    vTaskDelete(task);
}

Without this fix, I see in lldb:

Process 9032 stopped
* thread #1, name = 'Scheduler', queue = 'com.apple.main-thread', stop reason = signal SIGUSR1
    frame #0: 0x00000001815d6620 libsystem_kernel.dylib`__pthread_sigmask + 8
libsystem_kernel.dylib`__pthread_sigmask:
->  0x1815d6620 <+8>:  b.lo   0x1815d6640    ; <+40>
    0x1815d6624 <+12>: pacibsp
    0x1815d6628 <+16>: stp    x29, x30, [sp, #-0x10]!
    0x1815d662c <+20>: mov    x29, sp
Target 0: (freertos_posix_example) stopped.
(lldb) bt
* thread #1, name = 'Scheduler', queue = 'com.apple.main-thread', stop reason = signal SIGUSR1
  * frame #0: 0x00000001815d6620 libsystem_kernel.dylib`__pthread_sigmask + 8
    frame #1: 0x000000018160f680 libsystem_pthread.dylib`pthread_sigmask + 16
    frame #2: 0x000000010000f028 freertos_posix_example`xPortStartScheduler at port.c:322:14
    frame #3: 0x000000010000902c freertos_posix_example`vTaskStartScheduler at tasks.c:3761:18
    frame #4: 0x0000000100003ce8 freertos_posix_example`main at main.cpp:25:5
    frame #5: 0x0000000181290274 dyld`start + 2840
(lldb) f 2
frame #2: 0x000000010000f028 freertos_posix_example`xPortStartScheduler at port.c:322:14
   319 	    #endif /* __APPLE__*/
   320
   321 	    /* Restore original signal mask. */
-> 322 	    ( void ) pthread_sigmask( SIG_SETMASK, &xSchedulerOriginalSignalMask, NULL );
   323
   324 	    prvDestroyThreadKey();
   325

Checklist:

• I have tested my changes. No regression in existing tests.
• I have modified and/or added unit-tests to cover the code changes in this Pull Request.

[johnboiles]

I'll undo this change in my local fork of FreeRTOS and see if I still have issues. It might be that this was solved by #1223

[johnboiles]

I still have this issue, though I don't fully understand why sigwait isn't clearing that signal. There may be a more root-cause-y solution to this, but this is all I've been able to figure out.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[aggarg]

As you already mentioned, this seems like a hack. Do I need a mac to repro it or is it reproducible on Linux?

[johnboiles]

@aggarg I haven't gotten to trying on Linux yet but I can give it a shot. I'm reproducing in macOS with LLDB. I wonder if LLDB could be triggering this strange behavior somehow.

[aggarg]

Can you try running without the debugger?

[kar-rahul-aws]

I tried to reproduce the issue on Ubuntu 20.04 , but running it with or without the debugger, the issue is not reproduced.

Code used

#include <FreeRTOS.h>
#include <task.h>
#include <stdio.h>
#include <stdlib.h>
#include <assert.h>

void task_function(void *param) {
    ( void ) param;
    printf("FreeRTOS scheduler started\n");
    vTaskDelay(pdMS_TO_TICKS(1000));
    printf("Task Done, ending scheduler\n");
    vTaskEndScheduler();
    assert(0 && "After scheduler ended (SHOULD NOT GET HERE)");
}

int main_test() {
    TaskHandle_t task;
    xTaskCreate( task_function, "start", 10000, NULL, 1, &task );
    printf("Starting FreeRTOS scheduler\n");
    vTaskStartScheduler();
    printf("FreeRTOS scheduler exited\n");
    vTaskDelete(task);
    return 0;
}

Results after running without debugger

Starting FreeRTOS scheduler
FreeRTOS scheduler started
Task Done, ending scheduler
FreeRTOS scheduler exited

Results after running with debugger

Starting FreeRTOS scheduler
FreeRTOS scheduler started
Task Done, ending scheduler
FreeRTOS scheduler exited
[1] + Done                       "/usr/bin/gdb" --interpreter=mi --tty=${DbgTerm} 0<"/tmp/Microsoft-MIEngine-In-ogdc4hd5.xao" 1>"/tmp/Microsoft-MIEngine-Out-e1s1jg33.5yj"

I also tried your change in xPortStartScheduler which resulted in an invalid signal on MacOS, but the same code returns a valid signal in Ubuntu , and runs only once.

    while( xSchedulerEnd != pdTRUE )
    {
        int result = sigwait( &xSignals, &iSignal );
        printf("Signal received %d result=%d\n", iSignal, result);

        sigset_t set;
        sigpending( &set );
        if( sigismember( &set, SIG_RESUME ) )
        {
            int result = sigwait( &xSignals, &iSignal );
            printf("Signal received %d result=%d\n", iSignal, result);
        }
    }

Result on Ubuntu:

Starting FreeRTOS scheduler
FreeRTOS scheduler started
Task Done, ending scheduler
Signal received 10 result=0
FreeRTOS scheduler exited