linux-kernel - Re: [PATCH] alpha: fix FEN fault handling

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Message-ID: <Y7jdARsEQXPugR0t@ZenIV>
Date:   Sat, 7 Jan 2023 02:46:25 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Richard Henderson <rth@...ddle.net>
Cc:     linux-alpha@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] alpha: fix FEN fault handling

On Fri, Jan 06, 2023 at 05:55:14PM -0800, Richard Henderson wrote:
> On 1/6/23 16:59, Al Viro wrote:
> > Type 3 instruction fault (FPU insn with FPU disabled) is handled
> > by quietly enabling FPU and returning.  Which is fine, except that
> > we need to do that both for fault in userland and in the kernel;
> > the latter *can* legitimately happen - all it takes is this:
> > 
> > .global _start
> > _start:
> > 	call_pal 0xae
> > 	lda $0, 0
> > 	ldq $0, 0($0)
> > 
> > - call_pal CLRFEN to clear "FPU enabled" flag and arrange for
> > a signal delivery (SIGSEGV in this case).
> > 
> > Fixed by moving the handling of type 3 into the common part of
> > do_entIF(), before we check for kernel vs. user mode.
> > 
> > Incidentally, check for kernel mode is unidiomatic; the normal
> > way to do that is !user_mode(regs).  The difference is that
> > the open-coded variant treats any of bits 63..3 of regs->ps being
> > set as "it's user mode" while the normal approach is to check just
> > the bit 3.  PS is a 4-bit register and regs->ps always will have
> > bits 63..4 clear, so the open-code variant here is actually equivalent
> > to !user_mode(regs).  Harder to follow, though...
> > 
> > Reproducer above will crash any box where CLRFEN is not ignored by
> > PAL (== any actual hardware, AFAICS; PAL used in qemu doesn't
> > bother implementing that crap).
> 
> I didn't realize I'd forgotten this in qemu.  Anyway,
> 
> Reviewed-by: Richard Henderson <rth@...ddle.net>

Not sure it's worth bothering with in palcode-clipper - for Linux it's
useless (run out of timeslice and FEN will end up set, no matter what),
nothing in NetBSD or OpenBSD trees generates that call_pal, current
FreeBSD doesn't support alpha and their last version to do so hadn't
generated that call_pal either...  What else is out there?  OSF?