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Re: [Qemu-devel] [PATCH] oslib-posix: Use MAP_STACK in qemu_alloc_stack(
From: |
Brad Smith |
Subject: |
Re: [Qemu-devel] [PATCH] oslib-posix: Use MAP_STACK in qemu_alloc_stack() on OpenBSD |
Date: |
Thu, 11 Oct 2018 17:20:59 -0400 |
User-agent: |
Mutt/1.10.1 (2018-07-13) |
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 <address@hidden> 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();
- [Qemu-devel] [PATCH] oslib-posix: Use MAP_STACK in qemu_alloc_stack() on OpenBSD, Brad Smith, 2018/10/07
- Re: [Qemu-devel] [PATCH] oslib-posix: Use MAP_STACK in qemu_alloc_stack() on OpenBSD, Kamil Rytarowski, 2018/10/09
- Re: [Qemu-devel] [PATCH] oslib-posix: Use MAP_STACK in qemu_alloc_stack() on OpenBSD, Peter Maydell, 2018/10/09
- Re: [Qemu-devel] [PATCH] oslib-posix: Use MAP_STACK in qemu_alloc_stack() on OpenBSD, Kamil Rytarowski, 2018/10/09
- Re: [Qemu-devel] [PATCH] oslib-posix: Use MAP_STACK in qemu_alloc_stack() on OpenBSD, Brad Smith, 2018/10/10
- Re: [Qemu-devel] [PATCH] oslib-posix: Use MAP_STACK in qemu_alloc_stack() on OpenBSD, Peter Maydell, 2018/10/11
- Re: [Qemu-devel] [PATCH] oslib-posix: Use MAP_STACK in qemu_alloc_stack() on OpenBSD, Kamil Rytarowski, 2018/10/11
- Re: [Qemu-devel] [PATCH] oslib-posix: Use MAP_STACK in qemu_alloc_stack() on OpenBSD, Brad Smith, 2018/10/11
- Re: [Qemu-devel] [PATCH] oslib-posix: Use MAP_STACK in qemu_alloc_stack() on OpenBSD, Kamil Rytarowski, 2018/10/11
- Re: [Qemu-devel] [PATCH] oslib-posix: Use MAP_STACK in qemu_alloc_stack() on OpenBSD,
Brad Smith <=
- Re: [Qemu-devel] [PATCH] oslib-posix: Use MAP_STACK in qemu_alloc_stack() on OpenBSD, Kamil Rytarowski, 2018/10/13
Re: [Qemu-devel] [PATCH] oslib-posix: Use MAP_STACK in qemu_alloc_stack() on OpenBSD, Brad Smith, 2018/10/09