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

[Stefano Brivio <sbrivio@redhat.com>]

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.

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

-- 
Stefano