Re: [PATCH RFC v3 2/6] riscv/mm: add untagged_addr_remote_unlocked()

[Usama Arif <usama.arif@linux.dev>]

On 20/07/2026 18:34, Rik van Riel wrote:
> On Mon, 2026-07-20 at 17:46 +0100, Usama Arif wrote:
>>
>>
>> On 20/07/2026 16:08, Rik van Riel wrote:
>>> On Mon, 2026-07-20 at 04:57 -0700, Usama Arif wrote:
>>>> On Fri, 17 Jul 2026 13:00:32 -0400 Rik van Riel
>>>> <riel@surriel.com>
>>>> wrote:
>>>>
>>>>
>>>>>
>>>>> mm->context.pmlen is set only through PR_SET_TAGGED_ADDR_CTRL
>>>>> and
>>>>> is stable
>>>>> afterwards, so it can be read without the mmap lock, as it
>>>>> already
>>>>> is from
>>>>> untagged_addr() and mm_untag_mask().
>>>>>
>>>>
>>>> I think it might not be stable? set_tagged_addr_ctrl() can change
>>>> it
>>>> repeatedly until a CLONE_VM operation sets MM_CONTEXT_LOCK_PMLEN.
>>>
>>> You're right, ARM tagged addresses seem to work a
>>
>> ah do you mean RISCV here?
> 
> Ugh, I looked at the wrong one.  RISCV is like
> x86, with a dynamic (though a limited number
> of options?) mask.
> 
> Having said that, won't a process have all of
> its VMAs in the masked-off area, so coming in
> from a remote process to a valid memory address
> should already give us a valid address?
> 

So my understanding from exploring this code is, and hopefully someone
in CC from riscv can correct me, for example:

Tagged pointer:  0xabcd000012345678
PMLEN 16:        0x0000000012345678
PMLEN 7:         0xffcd000012345678

The target VMA might be at 0x12345678, but applying PMLEN 7
to that tagged pointer does not produce that address.

Previously, the order was:

Take mmap read lock.
Read pmlen.
Untag the address.
Look up the VMA.

Changing PMLEN takes the mmap write lock. The read and write
operations were therefore serialized.

The new order is:

Read pmlen without mmap lock.
Untag the address.
Attempt the per-VMA lookup.
Possibly take mmap lock later.
Continue using the already-untagged address.

A concurrent PMLEN change can occur between those operations?