linux-kernel - Re: [PATCH] reboot: fix parsing of reboot cpu number

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Message-ID: <CAFnufp1-0jj8Tq=NLRgFnUP5_j4GooguLz1AVTu16L8XR4e6Sw@mail.gmail.com>
Date:   Fri, 16 Oct 2020 18:45:08 +0200
From:   Matteo Croce <mcroce@...ux.microsoft.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     linux-kernel@...r.kernel.org, Guenter Roeck <linux@...ck-us.net>,
        Arnd Bergmann <arnd@...db.de>, Mike Rapoport <rppt@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Robin Holt <robinmholt@...il.com>,
        Fabian Frederick <fabf@...net.be>, stable@...r.kernel.org
Subject: Re: [PATCH] reboot: fix parsing of reboot cpu number

On Fri, Oct 16, 2020 at 2:20 PM Petr Mladek <pmladek@...e.com> wrote:
>
> On Wed 2020-10-14 23:27:46, Matteo Croce wrote:
> > From: Matteo Croce <mcroce@...rosoft.com>
> >
> > The kernel cmdline reboot= argument allows to specify the CPU used
> > for rebooting, with the syntax `s####` among the other flags, e.g.
> >
> >   reboot=soft,s4
> >   reboot=warm,s31,force
> >
> > In the early days the parsing was done with simple_strtoul(), later
> > deprecated in favor of the safer kstrtoint() which handles overflow.
> >
> > But kstrtoint() returns -EINVAL if there are non-digit characters
> > in a string, so if this flag is not the last given, it's silently
> > ignored as well as the subsequent ones.
> >
> > To fix it, use _parse_integer() which still handles overflow, but
> > restores the old behaviour of parsing until a non-digit character
> > is found.
>
> Hmm, _parse_integer() is an internal function. And even the comment
> says "Don't you dare use this function."
>
> I guess the it is because the base must be hardcoded. And
> KSTRTOX_OVERFLOW bit must be handled.
>

I know, but it's the only function that I know which has the same
behaviour of simple_strtoul(). Other than sscanf, which uses
simple_strtoul().
I didn't know that simple_strtoul() is no longer obsolete, I will
revert to it then.

> I suggest to go back to simple_strtoul(). It is not longer obsolete.
> It still exists because it is needed for exactly this purpose,
> see the comment in include/linux/kernel.h
>
> The potentional overflow is not a big deal. The result will be
> that the system will reboot on another rCPU than expected. But
> it might happen also with any typo.
>
> > While at it, limit the CPU number to num_possible_cpus(),
> > because setting it to a value lower than INT_MAX but higher
> > than NR_CPUS produces the following error on reboot and shutdown:
>
> Great catch! Please, fix this in a separate patch.
>

Ok, thanks!

-- 
per aspera ad upstream