[PATCH v2 0/5] x86/fpu: don't abuse x86_task_fpu(PF_USER_WORKER) in .regset_get() paths

[Oleg Nesterov <oleg@redhat.com>]

PF_USER_WORKER threads don't really differ from PF_KTHREAD threads
at least in that they never return to usermode and never use their
FPU state.

However, ptrace or coredump paths can access their FPU state and this
is the only reason why x86_task_fpu(PF_USER_WORKER) needs to work and
and discriminate PF_USER_WORKER from PF_KTHREAD. Unlike all other x86
FPU code paths which do not distinguish them.

OTOH, arch/x86/kernel/fpu/regset.c doesn't really need "struct fpu *",
the .regset_get() functions actually need a "struct fpstate *". If the
target task is PF_USER_WORKER, they can safely use &init_fpstate. So
this series adds the new simple helper

	static struct fpstate *get_fpstate(struct task_struct *task)
	{
		struct fpu *fpu;

		if (unlikely(task->flags & PF_USER_WORKER))
			return &init_fpstate;

		fpu = x86_task_fpu(task);
		if (task == current)
			fpu_sync_fpstate(fpu);
		return fpu->fpstate;
	}

which can be used instead of x86_task_fpu(task)->fpstate pattern in
arch/x86/kernel/fpu/regset.c.

However, there is an annoying complication: shstk_alloc_thread_stack()
can alloc the pointless shadow stack for PF_USER_WORKER thread and set
the ARCH_SHSTK_SHSTK flag. This means that ssp_get()->ssp_active() can
return true, and in this case it wouldn't be right to use the "unrelated"
init_fpstate.

That is why this series includes 4/5, and to me it looks like a cleanup
which makes sense regardless.

Link to V1: https://lore.kernel.org/all/20250814101340.GA17288@redhat.com/

Changes:

	- improve the subject/changelog in 1/5

	- drop "x86/shstk: add "task_struct *tsk" argument to reset_thread_features()"

	- rework 4/5 to not use reset_thread_features()

TODO:
	update the fpregs_soft_get() and user_regset.set() paths as well

Oleg.
---

 arch/x86/include/asm/shstk.h |  4 ++--
 arch/x86/kernel/fpu/regset.c | 46 ++++++++++++++++++++++++++------------------
 arch/x86/kernel/fpu/xstate.c | 12 ++++++------
 arch/x86/kernel/fpu/xstate.h |  4 ++--
 arch/x86/kernel/process.c    |  2 +-
 arch/x86/kernel/shstk.c      |  9 +++++++--
 6 files changed, 45 insertions(+), 32 deletions(-)