Re: FP handling in signal.c and traps.c

[Atsushi Nemoto <nemoto@toshiba-tops.co.jp>]

>>>>> On Fri, 17 Aug 2001 22:56:02 +0200, "Kevin D. Kissell" <kevink@mips.com> said:
kevink> I attach a diff relative to the current OSS repository for a
kevink> proposed patch to fix the signal holes discussed over the past
kevink> few days.

Thanks for your patch.  I tried this patch and it seems to work fine,
but I think still there is a hole in it.

After patching it, codes in restore_sigcontext becomes:

	if (owned_fp) {
		/* Can't tell if signal handler used FP, must restore */
		err |= restore_fp_context(sc);
	} else {
		if (current == last_task_used_math) {
		/* Signal handler acquired FPU - give it back */
			last_task_used_math = NULL;
			regs->cp0_status &= ~ST0_CU1;
			if (current->used_math) {
			/* Undo possible contamination of thread state */
				restore_thread_fp_context(sc);
			}
		}
	}

But this should be:

	if (owned_fp) {
		/* Can't tell if signal handler used FP, must restore */
		err |= restore_fp_context(sc);
	} else {
		if (current == last_task_used_math) {
		/* Signal handler acquired FPU - give it back */
			last_task_used_math = NULL;
			regs->cp0_status &= ~ST0_CU1;
		}
		if (current->used_math) {
			/* Undo possible contamination of thread state */
			restore_thread_fp_context(sc);
		}
	}

This change fix a hole in case that:

- The signaled thread used the FPU but not owns it.
- and context switch occur in the signal handler.
- and other thread takes the FPU (the signal handler loses the FPU).

In this case, last_task_used_math is not current at
restore_sigcontext, but we must restore the saved fp context.

---
Atsushi Nemoto