Re: Memory corruption after fork, only on AMD CPUs

[Michael Pratt <mpratt%google.com@localhost>]

On Tue, Dec 14, 2021 at 1:06 PM Michael Pratt <mpratt%google.com@localhost> wrote:
>
> [This is a reply to
> https://mail-index.netbsd.org/tech-kern/2021/12/01/msg027830.html. I
> just joined the mailing list and can't seem to find the metadata
> required for a proper reply. Apologies.]
>
> I filed https://gnats.netbsd.org/56535 for this a while ago, which has
> an even simpler reproducer: a direct fork() call with a child that
> immediately exits sometimes causes memory corruption in the parent
> process.
>
> We've kept looking since filing https://gnats.netbsd.org/56535 but
> haven't had luck on further simplification. No C reproducer yet,
> unfortunately. (No crashes if the Go parent process is single-threaded
> either.)

I spoke too soon here, we managed to get a reproducer in C today,
which I've posted at
https://github.com/golang/go/issues/34988#issuecomment-994115345.

>
> This feels like a bug in memory management somewhere (TLB invalidation
> issue, bug in copy-on-write?). Fundamentally, we have the parent
> process getting corrupt memory after calling fork with an
> (effectively) no-op child. That just shouldn't happen.
>
> I think we need someone familiar with NetBSD memory management
> internals to help take a look. Otherwise I'm afraid we won't figure it
> out and will have to declare that Go doesn't work on NetBSD on AMD
> CPUs.
>
> gdt: that does sound like a different issue to me. It may be worth
> filing a bug at https://github.com/golang/go/issues with the crash
> details.
>
> Thanks,
> Michael