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

On 21/01/2026 16:37, Mark Rutland wrote:

On Wed, Jan 21, 2026 at 03:59:22PM +0100, Kevin Brodsky wrote:

quoted

On 21/01/2026 14:56, Joey Gouly wrote:

quoted

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!

quoted

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