Prevent inconsistent CPU state after sequence of dlclose/dlopen

[Florian Weimer]

* Mathieu Desnoyers:

> I was discussing with Mark Rutland recently, and he pointed out that a
> sequence of dlclose/dlopen mapping new code at the same addresses in
> multithreaded environments is an issue on ARM, and possibly on Intel/AMD
> with the newer TLB broadcast maintenance.
>
> I maintain the membarrier(2) system call, which provides a
> MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE command for this
> purpose. It's been there since Linux 4.16. It can be configured
> out (CONFIG_MEMBARRIER=n), but it's enabled by default.
>
> Calling this after dlclose() in glibc would prevent this issue.
>
> Is it handled in some other way, or should we open a bugzilla
> entry to track this ?

There is nothing special about dlopen/dlclose, we just use mmap/munmap.
If there is a synchronization problem, we'd have to add to add barriers
to mmap and munmap.

But why isn't it up to the kernel to handle this correctly?

Thanks,
Florian