From: "Maciej W. Rozycki" <macro@linux-mips.org>
To: Rich Felker <dalias@libc.org>
Cc: Andy Lutomirski <luto@kernel.org>,
Linux MIPS Mailing List <linux-mips@linux-mips.org>,
LKML <linux-kernel@vger.kernel.org>,
Paul Burton <paul.burton@imgtec.com>,
David Daney <david.daney@cavium.com>,
Ralf Baechle <ralf@linux-mips.org>,
Paul Burton <paul.burton@mips.com>,
James Hogan <jhogan@kernel.org>
Subject: Re: Fixing MIPS delay slot emulation weakness?
Date: Sun, 16 Dec 2018 02:15:38 +0000 (GMT) [thread overview]
Message-ID: <alpine.LFD.2.21.1812160054300.11202@eddie.linux-mips.org> (raw)
In-Reply-To: <20181215225009.GB23599@brightrain.aerifal.cx>
On Sat, 15 Dec 2018, Rich Felker wrote:
> > A possibly nicer way to accomplish more or less the same thing would
> > be to allocate the area with _install_special_mapping() and arrange to
> > keep a reference to the struct page around.
> >
> > The really nice but less compatible fix would be to let processes or
> > even the whole system opt out by promising not to put anything in FPU
> > branch delay slots, of course.
>
> As I noted on Twitter when Mudge brought this topic back up, there's a
> much more compatible, elegant, and safe fix possible that does not
> involve any W+X memory. Emulate the delay slot in kernel-space. This
> is trivial to do safely for pretty much everything but loads/stores.
I think "trivial" is an understatement, you at least need to decode the
delay-slot instruction enough to tell privileged and user instructions
apart and send SIGILL where appropriate. Some user instructions send
exceptions too and you need to handle them accordingly.
OTOH, for things like ADDIUPC you need to interpret the instruction
anyway, as the value of the PC used for calculation will be wrong except
in the original location.
> For loads/stores, where you want them to execute with user privilege
> level, what you do is compute the effective address in kernel-space,
> then return to a fixed instruction in the vdso page that performs a
> generic load/store using the register the kernel put the effective
> address result in, then restores registers off the stack and jumps to
> the branch destination.
What about all the odd and especially vendor-specific load/store
instructions like ASET, SAA or SWAPW? Would we need to have all the
possible encodings provided in the VDSO?
Maciej
next prev parent reply other threads:[~2018-12-16 2:15 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-12-15 19:19 Fixing MIPS delay slot emulation weakness? Andy Lutomirski
2018-12-15 21:26 ` Paul Burton
2018-12-16 18:11 ` Rich Felker
2018-12-16 18:55 ` Andy Lutomirski
2018-12-15 22:50 ` Rich Felker
2018-12-16 2:15 ` Maciej W. Rozycki [this message]
2018-12-16 2:32 ` Rich Felker
2018-12-16 13:50 ` Maciej W. Rozycki
2018-12-16 18:13 ` Rich Felker
2018-12-16 18:59 ` Andy Lutomirski
2018-12-16 19:45 ` Maciej W. Rozycki
2018-12-17 0:59 ` Rich Felker
2018-12-17 1:55 ` Maciej W. Rozycki
2018-12-18 1:13 ` Aaro Koskinen
2018-12-19 4:32 ` Paul Burton
2018-12-19 21:12 ` Hugh Dickins
2018-12-20 17:56 ` Paul Burton
2018-12-20 17:45 ` [PATCH] MIPS: math-emu: Write-protect delay slot emulation pages Paul Burton
[not found] ` <20181220192616.42976218FE@mail.kernel.org>
2018-12-21 21:16 ` Paul Burton
2018-12-22 19:16 ` Sasha Levin
2018-12-23 16:16 ` Paul Burton
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