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

[Eric Blake <eblake@redhat.com>]

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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().

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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