Re: [PATCH] util: Set NS_FN_STACK_SIZE to one eighth of ulimit-reported maximum stack size

[David Gibson <david@gibson.dropbear.id.au>]

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On Mon, Oct 24, 2022 at 10:36:19AM +1100, David Gibson wrote:
> On Sat, Oct 22, 2022 at 10:15:35AM +0200, Stefano Brivio wrote:
> > On Sat, 22 Oct 2022 08:45:03 +0200
> > Stefano Brivio <sbrivio@redhat.com> wrote:
> > 
> > > ...instead of one fourth. On the main() -> conf() -> nl_sock_init()
> > > call path, LTO from gcc 12 on (at least) x86_64 decides to inline...
> > > everything: nl_sock_init() is effectively part of main(), after
> > > commit 3e2eb4337bc0 ("conf: Bind inbound ports with
> > > CAP_NET_BIND_SERVICE before isolate_user()").
> > > 
> > > This means we exceed the maximum stack size, and we get SIGSEGV,
> > > under any condition, at start time, as reported by Andrea on a recent
> > > build for CentOS Stream 9.
> > > 
> > > The calculation of NS_FN_STACK_SIZE, which is the stack size we
> > > reserve for clones, was previously obtained by dividing the maximum
> > > stack size by two, to avoid an explicit check on architecture (on
> > > PA-RISC, also known as hppa, the stack grows up, so we point the
> > > clone to the middle of this area), and then further divided by two
> > > to allow for any additional usage in the caller.
> > > 
> > > Well, if there are essentially no function calls anymore, this is
> > > not enough. Divide it by eight, which is anyway much more than
> > > possibly needed by any clone()d callee.
> > > 
> > > I think this is robust, so it's a fix in some sense. Strictly
> > > speaking, though, we have no formal guarantees that this isn't
> > > either too little or too much.
> > > 
> > > What we should do, eventually: check cloned() callees, there are just
> > > thirteen of them at the moment. Note down any stack usage (they are
> > > mostly small helpers), bonus points for an automated way at build
> > > time, quadruple that or so, to allow for extreme clumsiness, and use
> > > as NS_FN_STACK_SIZE. Perhaps introduce a specific condition for hppa.
> > > 
> > > Reported-by: Andrea Bolognani <abologna@redhat.com>
> > > Fixes: 3e2eb4337bc0 ("conf: Bind inbound ports with CAP_NET_BIND_SERVICE before isolate_user()")
> > > Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
> > > ---
> > 
> > I posted this in any case for (later) review, but I'm actually applying
> > it right away, given that some builds are completely unusable otherwise.
> 
> That's sensible, however, this patch confuses me.  I don't really
> understand how reducing the stack size is avoiding a SEGV, regardless
> of what LTO does.

Shortly after I wrote this, I realized what the issue was.  IIUC, the
problem is basically because we're allocating the stack of the
sub-thread as a buffer on the stack of the main thread, so the main
thread stack has to have room for both.

> The fact that we're basing the runtime stack size
> on a limit that's from build time also doesn't really make sense to
> me.

This aspect still seems pretty bogus to me.

> > I guess it would make sense to extend build ("distro") tests with some
> > compiler flags, common and less common.
> > 
> 



-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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