arm64/fpsimd: signal32: Always save+flush state early

There are several issues with the way the native signal handling code
manipulates FPSIMD/SVE/SME state. To fix those issues, subsequent
patches will rework the native signal handling code to always save+flush
the current task's FPSIMD/SVE/SME state before manipulating that state.

In preparation for those changes, rework the compat signal handling code
to save+flush the current task's FPSIMD state before manipulating it.

Subsequent patches will remove fpsimd_signal_preserve_current_state()
and fpsimd_update_current_state(). Compat tasks can only have FPSIMD
state, and cannot have any SVE or SME state. Thus, the SVE state
manipulation present in fpsimd_signal_preserve_current_state() and
fpsimd_update_current_state() is not necessary, and it is safe to
directly manipulate current->thread.uw.fpsimd_state once it has been
saved+flushed.

Use fpsimd_save_and_flush_current_state() to save+flush the state for
both signal delivery and signal return, before the state is manipulated
in any way. While it would be safe for compat_restore_vfp_context() to
use fpsimd_flush_task_state(current), there are several extant issues in
the native signal code resulting from incorrect use of
fpsimd_flush_task_state(), and for consistency it is preferable to use
fpsimd_save_and_flush_current_state().

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Will Deacon <will@kernel.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20250409164010.3480271-12-mark.rutland@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Author: mrutland-arm

arch/arm64/kernel/signal32.c

@@ -103,7 +103,7 @@ static int compat_preserve_vfp_context(struct compat_vfp_sigframe __user *frame)
 	 * Note that this also saves V16-31, which aren't visible
 	 * in AArch32.
 	 */
-	fpsimd_signal_preserve_current_state();
+	fpsimd_save_and_flush_current_state();
 
 	/* Place structure header on the stack */
 	__put_user_error(magic, &frame->magic, err);
@@ -169,14 +169,17 @@ static int compat_restore_vfp_context(struct compat_vfp_sigframe __user *frame)
 	fpsimd.fpsr = fpscr & VFP_FPSCR_STAT_MASK;
 	fpsimd.fpcr = fpscr & VFP_FPSCR_CTRL_MASK;
 
+	if (err)
+		return -EFAULT;
+
 	/*
 	 * We don't need to touch the exception register, so
 	 * reload the hardware state.
 	 */
-	if (!err)
-		fpsimd_update_current_state(&fpsimd);
+	fpsimd_save_and_flush_current_state();
+	current->thread.uw.fpsimd_state = fpsimd;
 
-	return err ? -EFAULT : 0;
+	return 0;
 }
 
 static int compat_restore_sigframe(struct pt_regs *regs,