Re: Should we add a compiler warning for large stack frames?

Hi,

On 2024-04-11 15:07:11 -0700, Andres Freund wrote:
> On 2024-04-11 16:35:58 -0400, Tom Lane wrote:
> > Indeed. I recall reading, not long ago, some Linux kernel docs to the
> > effect that automatic stack growth is triggered by a reference into
> > the page just below what is currently mapped as your stack, and
> > therefore allocating a stack frame greater than one page has the
> > potential to cause SIGSEGV rather than the desired stack extension.
> > (If you feel like digging in the archives, I think this was brought
> > up in the last round of lets-add-some-more-check_stack_depth-calls.)
>
> I think it's more than a single page, but I'm not entirely sure either. I
> think some compilers inject artificial stack accesses when extending the stack
> by a lot, but I don't remember the details.
>
> There certainly was the issue that strict memory overcommit does not reliably
> work with larger stack extensions.
>
> Could be worth writing a test program for...

It looks like it's a mess.

In the good cases the kernel doesn't map anything within ulimit -R of the
stack, and the stack is extended whenever memory in that range is accessed.
Nothing is mapped into that region unless MAP_FIXED is used.

However, in some cases linux maps the heap and the stack fairly close to each
other at program startup. I've observed this with an executable compiled with
-static-pie and executed with randomization disabled (via setarch -R). In
that case the the layout at program start is

...
7ffff7fff000-7ffff8021000 rw-p 00000000 00:00 0 [heap]
7ffffffdd000-7ffffffff000 rw-p 00000000 00:00 0 [stack]

Here the start of the heap and the end of the stack are only 128MB appart. The
heap grows upwards, the stack downwards.

Which means that if glibc allocates a bunch of memory via sbrk() and the stack
grows, they clash into each other.

I think this may be a glibc bug. If I compile with musl instead, this doesn't
happen, because musl stops using sbrk() style allocations before stack and
program break get too close to each other.

Greetings,

Andres Freund