Re: [Qemu-devel] [PATCH] oslib-posix: Use MAP_STACK in qemu_alloc_stack() on OpenBSD

[Brad Smith <brad@comstyle.com>]

On Thu, Oct 11, 2018 at 09:31:23PM +0200, Kamil Rytarowski wrote:
> On 11.10.2018 16:25, Brad Smith wrote:
> > On 10/11/2018 5:41 AM, Kamil Rytarowski wrote:
> > 
> >> On 11.10.2018 11:36, Peter Maydell wrote:
> >>> On 11 October 2018 at 00:55, Brad Smith <brad@comstyle.com> wrote:
> >>>> And from FreeBSD...
> >>>>
> >>>> ?????????? MAP_STACK MAP_STACK implies MAP_ANON, and offset of 0.?? The fd
> >>>> argument must be -1 and prot must include at least
> >>>> PROT_READ and PROT_WRITE.
> >>>>
> >>>> This option creates a memory region that grows to at
> >>>> most len bytes in size, starting from the stack top
> >>>> and growing down.?? The stack top is the starting
> >>>> address returned by the call, plus len bytes.?? The
> >>>> bottom of the stack at maximum growth is the starting
> >>>> address returned by the call.
> >>>>
> >>>> Stacks created with MAP_STACK automatically grow.
> >>>> Guards prevent inadvertent use of the regions into
> >>>> which those stacks can grow without requiring mapping
> >>>> the whole stack in advance.
> >>> Hmm. That "automatically growing" part sounds like
> >>> behaviour we definitely do not want for our use case.
> >>> So we're going to need to make this OS-specific :-(
> >>>
> >> I propose to restrict MAP_STACK it to OpenBSD (with a comment in the
> >> code). Once it will be needed by someone else will be able to enable it
> >> for other OSes.
> > 
> > I was going to propose doing something like that but you had replied
> > before I did.
> > What sort of comment did you have in mind?
> > 
> 
> Why do we want it only on OpenBSD and its either unneeded or meaning
> something else on other OSes.
> 
> #ifdef __OpenBSD__
> flags |= MAP_STACK;
> #endif

How about the following?


diff --git a/util/oslib-posix.c b/util/oslib-posix.c
index fbd0dc8c57..7814e61114 100644
--- a/util/oslib-posix.c
+++ b/util/oslib-posix.c
@@ -596,6 +596,7 @@ pid_t qemu_fork(Error **errp)
 void *qemu_alloc_stack(size_t *sz)
 {
     void *ptr, *guardpage;
+    int flags;
 #ifdef CONFIG_DEBUG_STACK_USAGE
     void *ptr2;
 #endif
@@ -610,8 +611,15 @@ void *qemu_alloc_stack(size_t *sz)
     /* allocate one extra page for the guard page */
     *sz += pagesz;
 
-    ptr = mmap(NULL, *sz, PROT_READ | PROT_WRITE,
-               MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+    flags = MAP_PRIVATE | MAP_ANONYMOUS;
+#if defined(MAP_STACK) && defined(__OpenBSD__)
+    /* Only enable MAP_STACK on OpenBSD. Other OS's such
+       as Linux/FreeBSD/NetBSD have a flag with the same
+       name but have differing functionality. */
+    flags |= MAP_STACK;
+#endif
+
+    ptr = mmap(NULL, *sz, PROT_READ | PROT_WRITE, flags, -1, 0);
     if (ptr == MAP_FAILED) {
         perror("failed to allocate memory for stack");
         abort();