[Qemu-devel] arm-linux-user infinite SIGSEGV looping

[Thayne Harbaugh <thayne@c2.net>]

I've encountered what appears to be a race condition in arm-linux-user:
sometimes the program qemu is running will hang forever and sometimes it
will complete.  After digging about there appears to be at least two
problems:

1) An initial cause of a SIGSEGV

2) The incorrect handling of the SIGSEGV

When #2 happens the symptom is an infinite loop in handling the SIGSEGV.
Using GDB I've stepped through the signal handling and have cause to
question a section of code that I would appreciate comments on.  The
target-specific cpu-exec.c:handle_cpu_signal() appears to have a
different logical flow for arm than all other architectures and I wonder
if it is incorrectly implemented.

All architecture versions of handle_cpu_signal() will do one of four
things:

1) return 0 or 1 at the start for certain conditions

2) not return by calling (do_)raise_exception_err() which eventually
calls cpu_loop_exit()

3) not return by calling cpu_resume_from_signal() which eventually calls
longjmp()

4) call cpu_loop_exit() directly (which eventually calls longjmp())

After these various code paths there's usually a comment that says
"never comes here".

The arm target is an exception to the above outline.
raise_exception_err() has been comment disabled and the cpu_loop_exit()
is incorrectly indented giving a false impression that it will always be
called at the end of the function.  I'm hypothesizing that for arm
handle_cpu_signal() incorrectly returns and doesn't break an infinite
loop.

Please comment!

Below is the questionable handle_cpu_signal():

static inline int handle_cpu_signal(unsigned long pc, unsigned long address,
                                    int is_write, sigset_t *old_set,
                                    void *puc)
{
    TranslationBlock *tb;
    int ret;

    if (cpu_single_env)
        env = cpu_single_env; /* XXX: find a correct solution for multithread */
#if defined(DEBUG_SIGNAL)
    printf("qemu: SIGSEGV pc=0x%08lx address=%08lx w=%d oldset=0x%08lx\n", 
           pc, address, is_write, *(unsigned long *)old_set);
#endif
    /* XXX: locking issue */
    if (is_write && page_unprotect(h2g(address), pc, puc)) {
        return 1;
    }
    /* see if it is an MMU fault */
    ret = cpu_arm_handle_mmu_fault(env, address, is_write, 1, 0);
    if (ret < 0)
        return 0; /* not an MMU fault */
    if (ret == 0)
        return 1; /* the MMU fault was handled without causing real CPU fault */
    /* now we have a real cpu fault */
    tb = tb_find_pc(pc);
    if (tb) {
        /* the PC is inside the translated code. It means that we have
           a virtual CPU fault */
        cpu_restore_state(tb, env, pc, puc);
    }
    if( ret == 1 ) {
        sigprocmask(SIG_SETMASK, old_set, NULL);
        //raise_exception_err(env->exception_index, env->error_code);
    } else {
    /* we restore the process signal mask as the sigreturn should
       do it (XXX: use sigsetjmp) */
    sigprocmask(SIG_SETMASK, old_set, NULL);
    cpu_loop_exit();
    }
}