Re: [Qemu-devel] [PATCH V4 1/6] oslib-posix: add helpers for stack alloc and free

[Peter Lieven <pl@kamp.de>]

Am 11.07.2016 um 18:39 schrieb Eric Blake:
> On 07/11/2016 03:07 AM, 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 <pl@kamp.de>
>> ---
>>  include/sysemu/os-posix.h | 23 +++++++++++++++++++++++
>>  util/oslib-posix.c        | 44 ++++++++++++++++++++++++++++++++++++++++++++
>>  2 files changed, 67 insertions(+)
>>
>> +
>> +static size_t adjust_stack_size(size_t sz)
>> +{
>> +    /* avoid stacks smaller than _SC_THREAD_STACK_MIN */
>> +    sz = MAX(sz, sysconf(_SC_THREAD_STACK_MIN));
> sz is unsigned, but sysconf() is signed.  Furthermore, sysconf() is
> permitted to return -1 if there is no such minimum.  MAX() would then
> operate on the common integral promotion between the two arguments,
> which may treat (unsigned)(-1) as the larger of the two values, and give
> you the wrong results.
>
> I think it is theoretical (all platforms that we compile on have a
> working sysconf(_SC_THREAD_STACK_MIN), right?), but still may be worth
> being sure that sysconf() returned a positive value before computing MAX().
>

If you feel more comfortable I can surround it by a

if (sysconf(_SC_THREAD_STACK_MIN) > 0) { }

I wonder if the _SC_THREAD_STACK_MIN constant exists if there is no minimum?

Peter