From: Richard Henderson <rth@twiddle.net>
To: Al Viro <viro@zeniv.linux.org.uk>, linux-alpha@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Subject: Re: [PATCH] alpha: fix FEN fault handling
Date: Fri, 6 Jan 2023 17:55:14 -0800 [thread overview]
Message-ID: <84c0d4ea-09e2-4907-d03d-939d40fa3c96@twiddle.net> (raw)
In-Reply-To: <Y7jD8XDZGnQkTvMI@ZenIV>
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@twiddle.net>
r~
next prev parent reply other threads:[~2023-01-07 1:55 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-01-07 0:59 [PATCH] alpha: fix FEN fault handling Al Viro
2023-01-07 1:55 ` Richard Henderson [this message]
2023-01-07 2:46 ` Al Viro
2023-01-07 21:28 ` Al Viro
2023-01-07 23:03 ` Richard Henderson
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