Re: Prevent inconsistent CPU state after sequence of dlclose/dlopen

Mathieu Desnoyers
2025-01-10T17:14:01Z

On 2025-01-10 12:04, Florian Weimer wrote:
> * 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?

As I mentioned to Peter, we could add this barrier within mprotect(2)
and munmap(2) in the following cases:

- mprotect removes PROT_EXEC from a mapping,
- munmap unmaps a PROT_EXEC mapping.

We could even go further and batch this: we only need to
issue membarrier-sync-core on the following sequence for an mm:

On either of those, set current->mm->pending_membarrier_sync_core = true:
- mprotect removes PROT_EXEC from a mapping, or
- munmap unmaps a PROT_EXEC mapping,

And then, if current->mm->pending_membarrier_sync_core == true when:
- mmap is called to create a PROT_EXEC mapping, or
- mprotect sets PROT_EXEC on a mapping.

invoke membarrier sync-core and set
current->mm_pending_membarrier = false

Thoughts ?

Thanks,

Mathieu

>
> Thanks,
> Florian
>

--
Mathieu Desnoyers
EfficiOS Inc.