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

[Mark Rutland]

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.

> 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.

Mark.