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

Adhemerval Zanella Netto
2025-01-10T17:24:23Z

On 10/01/25 14:15, Mathieu Desnoyers wrote:
> On 2025-01-10 12:10, Florian Weimer wrote:
>> * Mathieu Desnoyers:
>>
>>> On 2025-01-10 11:54, Peter Zijlstra wrote:
>>>> On Fri, Jan 10, 2025 at 10:55:36AM -0500, Mathieu Desnoyers wrote:
>>>>> Hi,
>>>>>
>>>>> 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.
>>>> What is the exact race? Should not munmap() invalidate the TLBs
>>>> before
>>>> it allows overlapping mmap() to complete?
>>>
>>> The race Mark mentioned (on ARM) is AFAIU the following scenario:
>>>
>>> CPU 0                     CPU 1
>>>
>>> - dlopen()
>>>    - mmap PROT_EXEC @addr
>>>                            - fetch insn @addr, CPU state expects unchanged insn.
>>>                            - execute unrelated code
>>> - dlclose(addr)
>>>    - munmap @addr
>>> - dlopen()
>>>    - mmap PROT_EXEC @addr
>>>                            - fetch new insn @addr. Incoherent CPU state.
>>
>> Unmapping an object while code is executing in it is undefined.
>
> That's not the scenario though. In this scenario, CPU 1 executes
> _unrelated code_ while we unmap @addr.

But in this scenario you still a concurrent dlclose while you have a running
thread executing code from that module, right? Or am I still missing something
here?

Or, are you saying that even after dlopen returns (assuming the scenario where
it maps the code in a previous used mapping), the CPU is in an inconsistent
state unless MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE is issued?