Re: [Qemu-devel] [PATCH 1/6] oslib-posix: add helpers for stack alloc and free

[Peter Lieven <pl@kamp.de>]

Am 01.07.2016 um 22:49 schrieb Richard Henderson:
> On 07/01/2016 01:12 PM, Richard Henderson wrote:
>> On 06/30/2016 12:37 AM, Peter Lieven wrote:
>>> +void *qemu_alloc_stack(size_t sz)
>>> +{
>>> +    /* allocate sz bytes plus one extra page for a guard
>>> +     * page at the bottom of the stack */
>>> +    void *ptr = mmap(NULL, sz + getpagesize(), PROT_NONE,
>>> +                     MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
>>> +    if (ptr == MAP_FAILED) {
>>> +        abort();
>>> +    }
>>> +    if (mmap(ptr + getpagesize(), sz, PROT_READ | PROT_WRITE,
>>> +        MAP_PRIVATE | MAP_ANONYMOUS | MAP_FIXED, -1, 0) == MAP_FAILED) {
>>> +        abort();
>>> +    }
>
> Rare platforms now, but fwiw, this is incorrect for hppa and ia64.
>
> For hppa, stack grows up, so the guard page needs to be at the top.
>
> For ia64, there are two stacks, the "normal" program stack (grows down) and the register window stack (grows up).  The guard page goes in between.
>
> See e.g. glibc/nptl/allocatestack.c
>
> #ifdef NEED_SEPARATE_REGISTER_STACK
>           char *guard = mem + (((size - guardsize) / 2) & ~pagesize_m1);
> #elif _STACK_GROWS_DOWN
>           char *guard = mem;
> #elif _STACK_GROWS_UP
>           char *guard = (char *) (((uintptr_t) pd - guardsize) & ~pagesize_m1);
> #endif
>           if (mprotect (guard, guardsize, PROT_NONE) != 0)

It seems that ia64 needs even more care when allocating a stack, right?
Would you think it is ok to only handle _STACK_GROWS_DOWN and _STACK_GROWS_UP ?

Peter