Re: For review: documentation of clone3() system call

[Jann Horn]

On Sat, Nov 9, 2019 at 9:10 AM Michael Kerrisk (man-pages)
<mtk.manpages@xxxxxxxxx> wrote:
[...]
> On 11/7/19 4:19 PM, Christian Brauner wrote:
> > On Fri, Oct 25, 2019 at 06:59:31PM +0200, Michael Kerrisk (man-pages) wrote:
[...]
> >>        The stack argument specifies the location of the stack used by the
> >>        child process.  Since the child and calling process may share memâ
> >>        ory,  it  is  not possible for the child process to execute in the
> >>        same stack as the  calling  process.   The  calling  process  must
> >>        therefore  set  up  memory  space  for  the child stack and pass a
> >>        pointer to this space to clone().  Stacks  grow  downward  on  all
> >
> > It might be a good idea to advise people to use mmap() to create a
> > stack. The "canonical" way of doing this would usually be something like
> >
> > #define DEFAULT_STACK_SIZE (4 * 1024 * 1024) /* 8 MB usually on Linux */
> > void *stack = mmap(NULL, DEFAULT_STACK_SIZE, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS | MAP_STACK, -1, 0);
> >
> > (Yes, the MAP_STACK is usally a noop but people should always include it
> >  in case some arch will have weird alignment requirement in which case
> >  this flag can be changed to actually do something...)
>
> So, I'm getting a little bit of an education here, and maybe you are
> going to further educate me. Long ago, I added the documentation of
> MAP_STACK to mmap(2), but I never quite connected the dots.
>
> However, you say MAP_STACK is *usually* a noop. As far as I can see,
> in current kernels it is *always* a noop. And AFAICS, since it was first
> added in 2.6.27 (2008), it has always been a noop.
>
> I wonder if it will always be a noop.
[...]
> So, my understanding from the above is that MAP_STACK was added to
> allow a possible fix on some old architectures, should anyone decide it
> was worth doing the work of implementing it. But so far, after 12 years,
> no one did. It kind of looks like no one ever will (since those old
> architectures become less and less relevant).
>
> So, AFAICT, while it's not wrong to tell people to use mmap(MAP_STACKED),
> it doesn't provide any benefit (and perhaps never will), and it is a
> more clumsy than plain old malloc().
>
> But, it could well be that there's something I still don't know here,
> and I'd be interested to get further education.

Not on Linux, but on OpenBSD, they do use MAP_STACK now AFAIK; this
was announced here:
<http://openbsd-archive.7691.n7.nabble.com/stack-register-checking-td338238.html>.
Basically they periodically check whether the userspace stack pointer
points into a MAP_STACK region, and if not, they kill the process. So
even if it's a no-op on Linux, it might make sense to advise people to
use the flag to improve portability? I'm not sure if that's something
that belongs in Linux manpages.

Another reason against malloc() is that when setting up thread stacks
in proper, reliable software, you'll probably want to place a guard
page (in other words, a 4K PROT_NONE VMA) at the bottom of the stack
to reliably catch stack overflows; and you probably don't want to do
that with malloc, in particular with non-page-aligned allocations.