arm64/fpsimd: ptrace/prctl: Ensure VL changes do not resurrect stale data

The SVE/SME vector lengths can be changed via prctl/ptrace syscalls.
Changes to the SVE/SME vector lengths are documented as preserving the
lower 128 bits of the Z registers (i.e. the bits shared with the FPSIMD
V registers). To ensure this, vec_set_vector_length() explicitly copies
register values from a task's saved SVE state to its saved FPSIMD state
when dropping the task to FPSIMD-only.

The logic for this was not updated when when FPSIMD/SVE state tracking
was changed across commits:

  baa8515 ("arm64/fpsimd: Track the saved FPSIMD state type separately to TIF_SVE")
  a0136be (arm64/fpsimd: Load FP state based on recorded data type")
  bbc6172 ("arm64/fpsimd: SME no longer requires SVE register state")
  8c845e2 ("arm64/sve: Leave SVE enabled on syscall if we don't context switch")

Since the last commit above, a task's FPSIMD/SVE state may be stored in
FPSIMD format while TIF_SVE is set, and the stored SVE state is stale.
When vec_set_vector_length() encounters this case, it will erroneously
clobber the live FPSIMD state with stale SVE state by using
sve_to_fpsimd().

Fix this by using fpsimd_sync_from_effective_state() instead.

Related issues with streaming mode state will be addressed in subsequent
patches.

Fixes: 8c845e2 ("arm64/sve: Leave SVE enabled on syscall if we don't context switch")
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: David Spickett <david.spickett@arm.com>
Cc: Luis Machado <luis.machado@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20250508132644.1395904-15-mark.rutland@arm.com
Signed-off-by: Will Deacon <will@kernel.org>

Author: mrutland-arm

arch/arm64/kernel/fpsimd.c

@@ -852,7 +852,7 @@ int vec_set_vector_length(struct task_struct *task, enum vec_type type,
 	fpsimd_flush_task_state(task);
 	if (test_and_clear_tsk_thread_flag(task, TIF_SVE) ||
 	    thread_sm_enabled(&task->thread)) {
-		sve_to_fpsimd(task);
+		fpsimd_sync_from_effective_state(task);
 		task->thread.fp_type = FP_STATE_FPSIMD;
 	}