Re: [Qemu-devel] [PATCH V5 1/6] oslib-posix: add helpers for stack alloc and free

[Peter Lieven]

Am 08.08.2016 um 12:37 schrieb Stefan Hajnoczi:
> On Tue, Jul 12, 2016 at 06:23:01PM +0200, Peter Lieven wrote:
>> the allocated stack will be adjusted to the minimum supported stack size
>> by the OS and rounded up to be a multiple of the system pagesize.
>> Additionally an architecture dependent guard page is added to the stack
>> to catch stack overflows.
>>
>> Signed-off-by: Peter Lieven <address@hidden>
>> ---
>>  include/sysemu/os-posix.h | 23 +++++++++++++++++++++++
>>  util/oslib-posix.c        | 46 
>> ++++++++++++++++++++++++++++++++++++++++++++++
>>  2 files changed, 69 insertions(+)
>>
>> diff --git a/include/sysemu/os-posix.h b/include/sysemu/os-posix.h
>> index 9c7dfdf..7630665 100644
>> --- a/include/sysemu/os-posix.h
>> +++ b/include/sysemu/os-posix.h
>> @@ -60,4 +60,27 @@ int qemu_utimens(const char *path, const qemu_timespec 
>> *times);
>>  
>>  bool is_daemonized(void);
>>  
>> +/**
>> + * qemu_alloc_stack:
>> + * @sz: size of required stack in bytes
>> + *
>> + * Allocate memory that can be used as a stack, for instance for
>> + * coroutines. If the memory cannot be allocated, this function
>> + * will abort (like g_malloc()).
>> + *
>> + * The allocated stack must be freed with qemu_free_stack().
>> + *
>> + * Returns: pointer to (the lowest address of) the stack memory.
>> + */
>> +void *qemu_alloc_stack(size_t sz);
>> +
>> +/**
>> + * qemu_free_stack:
>> + * @stack: stack to free
>> + * @sz: size of stack in bytes
>> + *
>> + * Free a stack allocated via qemu_alloc_stack().
>> + */
>> +void qemu_free_stack(void *stack, size_t sz);
>> +
>>  #endif
>> diff --git a/util/oslib-posix.c b/util/oslib-posix.c
>> index e2e1d4d..2303ca6 100644
>> --- a/util/oslib-posix.c
>> +++ b/util/oslib-posix.c
>> @@ -497,3 +497,49 @@ pid_t qemu_fork(Error **errp)
>>      }
>>      return pid;
>>  }
>> +
>> +static size_t adjust_stack_size(size_t sz)
>> +{
>> +#ifdef _SC_THREAD_STACK_MIN
>> +    /* avoid stacks smaller than _SC_THREAD_STACK_MIN */
>> +    sz = MAX(MAX(sysconf(_SC_THREAD_STACK_MIN), 0), sz);
>> +#endif
>> +    /* adjust stack size to a multiple of the page size */
>> +    sz = ROUND_UP(sz, getpagesize());
>> +    return sz;
>> +}
>> +
>> +void *qemu_alloc_stack(size_t sz)
>> +{
>> +    void *ptr, *guardpage;
>> +    size_t pagesz = getpagesize();
>> +    sz = adjust_stack_size(sz);
>> +
>> +    ptr = mmap(NULL, sz, PROT_READ | PROT_WRITE,
> It's cleaner to count for the guard page separately and give the caller
> the sz bytes they expected:
>
>   sz + pagesz

I don't mind to change that, but the glibc stack allocation routine
does it exactly this way. (see glibc/nptl/allocatestack.c).
As we already used other parts (minimal stack size, positioning
of the guard page), we mimicked this as well.

Peter