Open Radar

 
11-Jan-2013 09:57 PM Landon Fuller:
Summary:

The kernel appears to have stopped respecting sigaltstack()/SA_ONSTACK.

Steps to Reproduce:

Test code that will demonstrate this bug. It will configure an alternative stack, a signal handler, and then trigger a stack overflow. If you uncomment the raise() call, it will instead simply raise SIGABRT.

#import <Foundation/Foundation.h>

#include <signal.h>
#include <stdio.h>
#include <stdint.h>
#include <string.h>
#include <stdlib.h>
#include <execinfo.h>
#include <unistd.h>
#include <errno.h>

static void signal_handler (int signal, siginfo_t *info, void *uap) {
    /*
     * If we're actually running on a new stack, this should:
     * A) Not crash, if a stack overflow occured.
     * B) Show an empty stack trace.
     *
     * Yes, this is not async-safe. It's demo code.
     */
    NSLog(@"Handler ran\n");
    NSLog(@"%@", [NSThread callStackSymbols]);
    _exit(0);
}

void stackFrame (void) {
    stackFrame();
}

int main (int argc, char *argv[]) {
    /* Configure signal stack */
    stack_t sigstk;
    sigstk.ss_size = SIGSTKSZ;
    sigstk.ss_sp = malloc(sigstk.ss_size);
    sigstk.ss_flags = 0;
    if (sigaltstack(&sigstk, 0) < 0) {
        fprintf(stderr, "sigaltstack() failed: %s\n", strerror(errno));
        _exit(1);
    }
    
    /* Configure action */
    struct sigaction sa;
    memset(&sa, 0, sizeof(sa));
    sa.sa_flags = SA_SIGINFO|SA_ONSTACK;
    sigemptyset(&sa.sa_mask);
    sa.sa_sigaction = &signal_handler;
    sigaction(SIGABRT, &sa, NULL);
    sigaction(SIGSEGV, &sa, NULL);
    sigaction(SIGBUS, &sa, NULL);

    //raise(SIGABRT);
    stackFrame();

    return 0;
}

Expected Results:

The signal handler executes, printing an empty call stack.

Actual Results:

On iOS, the stack is left unchanged, and the signal handler either doesn't run (in the case of a stack overflow), or runs on the old stack (in the case of SIGABRT).

On Mac OS X, the thread's stack is switched to the alternative stack, and the test passes successfully.

Regression:

I believe the tests pass on iOS 4.x and earlier. I know they pass in iOS 2.x.

Notes:

I've previously reproduced this on earlier devices, including the iPhone 4.