linux-kernel - Re: [PATCH 08/18] arm64: convert raw syscall invocation to C

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Date:   Tue, 15 May 2018 12:01:58 +0200
From:   Dominik Brodowski <linux@...inikbrodowski.net>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Dave Martin <Dave.Martin@....com>,
        linux-arm-kernel@...ts.infradead.org, marc.zyngier@....com,
        catalin.marinas@....com, will.deacon@....com,
        linux-kernel@...r.kernel.org, james.morse@....com,
        viro@...iv.linux.org.uk, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 08/18] arm64: convert raw syscall invocation to C

On Tue, May 15, 2018 at 09:22:23AM +0100, Mark Rutland wrote:
> On Mon, May 14, 2018 at 10:24:45PM +0200, Dominik Brodowski wrote:
> > On Mon, May 14, 2018 at 12:41:10PM +0100, Mark Rutland wrote:
> > > I agree it would be nicer if it had a wrapper that took a pt_regs, even
> > > if it does nothing with it.
> > > 
> > > We can't use SYSCALL_DEFINE0() due to the fault injection muck, we'd
> > > need a ksys_ni_syscall() for our traps.c logic, and adding this
> > > uniformly would involve some arch-specific rework for x86, too, so I
> > > decided it was not worth the effort.
> > 
> > Couldn't you just open-code the "return -ENOSYS;" in traps.c?
> 
> I guess so. I was just worried that debug logic might be added to the generic
> ni_syscall() in future, and wanted to avoid potential divergence.
> 
> > Error injection has no reasonable stable ABI/API expectations, so that's not
> > a show-stopper either.
> 
> If people are happy with using SYSCALL_DEFINE0() for ni_syscall, I'm happy to
> do that -- it's just that we'll need a fixup for x86 as that will change the
> symbol name.

For me, it's less about using SYSCALL_DEFINE0() for ni_syscall, but more
about keeping the syscall invokation easy. Therefore, we do pass a pointer
struct pt_regs to sys_ni_syscall() on x86, even though it does not expect
it.

	/* this is a lie, but it does not hurt as sys_ni_syscall just returns -EINVAL */
	extern asmlinkage long sys_ni_syscall(const struct pt_regs *);

Thanks,
	Dominik