[PATCH v1] arm64: poe: fix stale POR_EL0 values for ptrace

[Kevin Brodsky]

On 21/01/2026 16:37, Mark Rutland wrote:
> On Wed, Jan 21, 2026 at 03:59:22PM +0100, Kevin Brodsky wrote:
>> On 21/01/2026 14:56, Joey Gouly wrote:
>>> If a process wrote to POR_EL0 and then crashed before a context switch
>>> happened, the coredump would contain an incorrect value for POR_EL0.
>> Isn't that also a problem if using ptrace(PTRACE_GETREGSET, REGSET_POE)?
> In the case of manipulating a tracee (i.e. target != current), the core
> code ensures that the tracee is stopped (has context-switched out, an
> hence has saved its registrer contents to memory) before the relevant
> regset functions can be called.

Right, hadn't thought that through!

>> Just like for fpsimd, etc.
> Just FYI, The FPSIMD/SVE/SME registers are a special case relative to
> all the other regsets.
>
> The FPSIMD/SVE/SME registers eagerly saved to memory (and so when a task
> is scheduled out, the value in memory will be up-to-date), but they're
> lazily restored (so the value in registers can be transiently stale
> while the task is running), and there's a special case when scheduling a
> task in where we attempt to spot if the CPU registers happen to be
> up-to-date with the task.
>
> The gist of this is that when manipulating the FPSIMD/SVE/SME regsets of
> a task:
>
> * For reads, we know that the value in memory is up-to-date unless the
>   task is the current task.
>
> * For writes (which can only occur for a tracee which is not the current
>   task), we need to update some tracking data to prevent context-switch
>   from reusing stale values on a CPU. That's what
>   fpsimd_flush_task_state() does.
>
> Pretty much all other regsets don't need the "flush" on writes, since
> the value in memory will be loaded when the task is next scheduled in.

That is good to know, thanks! I actually meant to write TLS/TPIDR as
that's a more comparable case (not saved/restored on exception
entry/return), but as you pointed out above a task must be scheduled out
before its register state is inspected so there's no concern here.

- Kevin