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

[Stefan Hajnoczi <stefanha@gmail.com>]

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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 <pl@kamp.de>
> ---
>  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

> +               MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
> +    if (ptr == MAP_FAILED) {
> +        abort();
> +    }
> +
> +#if defined(HOST_IA64)
> +    /* separate register stack */
> +    guardpage = ptr + (((sz - pagesz) / 2) & ~pagesz);
> +#elif defined(HOST_HPPA)
> +    /* stack grows up */
> +    guardpage = ptr + sz - pagesz;
> +#else
> +    /* stack grows down */
> +    guardpage = ptr;
> +#endif
> +    if (mprotect(guardpage, pagesz, PROT_NONE) != 0) {
> +        abort();
> +    }
> +
> +    return ptr;
> +}
> +
> +void qemu_free_stack(void *stack, size_t sz)
> +{
> +    sz = adjust_stack_size(sz);
> +    munmap(stack, sz);
> +}
> -- 
> 1.9.1
> 
> 

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