Re: [PATCH v7 9/9] sparc64: Add support for ADI (Application Data Integrity)

[David Miller <davem@davemloft.net>]

From: Khalid Aziz <khalid.aziz@oracle.com>
Date: Wed, 30 Aug 2017 17:23:37 -0600

> That is an interesting idea. This would enable TSTATE_MCDE on all
> threads of a process as soon as one thread enables it. If we consider
> the case where the parent creates a shared memory area and spawns a
> bunch of threads. These threads access the shared memory without ADI
> enabled. Now one of the threads decides to enable ADI on the shared
> memory. As soon as it does that, we enable TSTATE_MCDE across all
> threads and since threads are all using the same TTE for the shared
> memory, every thread becomes subject to ADI verification. If one of
> the other threads was in the middle of accessing the shared memory, it
> will get a sigsegv. If we did not enable TSTATE_MCDE across all
> threads, it could have continued execution without fault. In other
> words, updating TSTATE_MCDE across all threads will eliminate the
> option of running some threads with ADI enabled and some not while
> accessing the same shared memory. This could be necessary at least for
> short periods of time before threads can communicate with each other
> and all switch to accessing shared memory with ADI enabled using same
> tag. Does that sound like a valid use case or am I off in the weeds
> here?

A threaded application needs to synchronize and properly orchestrate
access to shared memory.

When a change is made to a mappping, in this case setting ADI
attributes, it's being done for the address space not the thread.

And the address space is shared amongst threads.

Therefore ADI is not really a per-thread property but rather
a per-address-space property.