Re: [PATCH] reboot: Fix variable assignments in type_store

From: Petr Mladek <pmladek@suse.com>
To: Matteo Croce <mcroce@linux.microsoft.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Nathan Chancellor <natechancellor@gmail.com>,
	Arnd Bergmann <arnd@arndb.de>, Kees Cook <keescook@chromium.org>,
	linux-kernel@vger.kernel.org, Guenter Roeck <linux@roeck-us.net>,
	Pavel Tatashin <pasha.tatashin@soleen.com>,
	Mike Rapoport <rppt@kernel.org>,
	Tyler Hicks <tyhicks@linux.microsoft.com>,
	Nick Desaulniers <ndesaulniers@google.com>,
	clang-built-linux@googlegroups.com
Subject: Re: [PATCH] reboot: Fix variable assignments in type_store
Date: Wed, 18 Nov 2020 12:47:02 +0100	[thread overview]
Message-ID: <X7UJtlLqJ0ZljhXG@alley> (raw)
In-Reply-To: <CAFnufp3NBHUQ8fT1TEw5BvazGQjP9KmsU3y7ioJScWQ9qGiq1A@mail.gmail.com>

On Fri 2020-11-13 22:28:18, Matteo Croce wrote:
> On Fri, Nov 13, 2020 at 9:06 PM Petr Mladek <pmladek@suse.com> wrote:
> >
> > On Fri 2020-11-13 03:58:49, Matteo Croce wrote:
> > > On Fri, Nov 13, 2020 at 3:46 AM Andrew Morton <akpm@linux-foundation.org> wrote:
> > > >
> > > > On Fri, 13 Nov 2020 02:38:18 +0100 Matteo Croce <mcroce@linux.microsoft.com> wrote:
> > > >
> > > > > At this point, since 'pci' enables BOOT_CF9_FORCE type and
> > > > > BOOT_CF9_SAFE is not user selectable, should I simply leave only
> > > > > 'pci'?
> > > > >
> > > > > This way, we'll have the same set of options for both sysfs and kernel cmdline.
> > > >
> > > > Well, you're the reboot expert ;)
> > > >
> > >
> > > So honored! :)
> > >
> > > > But my $0.02 is yes, let's keep the command-line and sysfs interfaces
> > > > in sync and cover it all in documentation.  It would of course be
> > > > problematic to change the existing reboot= interface.
> > > >
> > > > I assume that means doing this?
> > > >
> > > > - #define BOOT_CF9_FORCE_STR     "cf9_force"
> > > > + #define BOOT_CF9_FORCE_STR     "pci"
> > > > - #define BOOT_CF9_SAFE_STR      "cf9_safe"
> > >
> > > Either BOOT_PCI_STR or BOOT_CF9_FORCE_STR, I have no strong preference.
> > >
> > > The syntax is 'pci' while the enum BOOT_CF9_FORCE, so we can't please both.
> >
> > The question is whether we should modify/allow to set these values at
> > all.
> >
> > Anyway, we must prevent them on non-x86 architectures because
> > the reboot behavior would be undefined there. They could probably
> > make a mess even on many x86-architectures.
> >
> 
> That's right, but the same can be obtained by passing 'reboot=pci'  on
> non x86 machines: the cmdline parsing is generic and will set
> reboot_type on all arches.

Fair enough. Ah, I mixed reboot_type and reboot_mode and looked
into wrong part of reboot_setup().

> I looked at the reboot_type usage, there isn't any reference outside
> arch/x86. In fact, the parameter is just ignored:
> 
> # uname -m
> aarch64
> # cat /proc/cmdline
> console=ttyS0,115200n8 reboot=pci
> # reboot -ff
> Rebooting.
> [   43.893833] reboot: Restarting system

Good to know. Thanks for checking.

> The same applies for reboot_force, the only flags available on
> different architectures are reboot_mode and reboot_cpu.
> We could hide some handlers for some architectures. We save some
> space, and avoid letting the user set flags which do nothing.

I am fine with the current patchset after all. We could always make
it more safe when people hit it in the real life. All these
worries were because I thought that this interface allowed
to set values that were not possible before.

> > Anyway, we should get input from some x86-experts about the BOOT_CF9
> > values.
> 
> Sure, x86@kernel.org ?

Yes but I do not resist on it any longer. Just if you were going to send
another version just by chance then it would be nice to CC x86.

Best Regards,
Petr