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

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Date:   Fri, 6 Jan 2023 17:55:14 -0800
From:   Richard Henderson <rth@...ddle.net>
To:     Al Viro <viro@...iv.linux.org.uk>, linux-alpha@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH] alpha: fix FEN fault handling

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>


r~