linux-kernel - Re: [PATCH] arm64/mm: Reject invalid NUMA option

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Message-ID: <20200427230920.3d606a2e@gandalf.local.home>
Date:   Mon, 27 Apr 2020 23:09:20 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Gavin Shan <gshan@...hat.com>
Cc:     Mark Rutland <mark.rutland@....com>, catalin.marinas@....com,
        linux-kernel@...r.kernel.org, shan.gavin@...il.com,
        will@...nel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] arm64/mm: Reject invalid NUMA option

On Mon, 27 Apr 2020 22:59:44 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Mon, 27 Apr 2020 22:54:06 -0400
> Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> > On Tue, 28 Apr 2020 10:59:14 +1000
> > Gavin Shan <gshan@...hat.com> wrote:
> >   
> > > Hi Mark,
> > > 
> > > On 4/24/20 8:11 PM, Mark Rutland wrote:    
> > > > [Adding Steve, who added str_has_prefix()]
> > > > 
> > > > On Fri, Apr 24, 2020 at 02:53:14PM +1000, Gavin Shan wrote:      
> > > >> The NUMA option is parsed by str_has_prefix() and the invalid option
> > > >> like "numa=o" can be regarded as "numa=off" wrongly.      
> > > > 
> > > > Are you certain that can pass? If that can happen, str_has_prefix() is
> > > > misnamed and does not seem to do what its kerneldoc says it does, as
> > > > "off" is not a prefix of "o".
> > > >       
> > > 
> > > Yes, It's possible. str_has_prefix() depends on strncmp(). In this particular
> > > case, it's equal to the snippet of code as below: strncmp() returns zero.
> > > str_has_prefix() returns 3.    
> > 
> > Wait! strncmp("o", "off", 3) returns zero?
> > 
> > That to me looks like a bug!
> > 
> > This means str_has_prefix() is broken in other areas as well.
> > 
> >   
> > > 
> > > int strncmp(const char *cs, const char *ct, size_t count)
> > > {
> > >          unsigned char c1, c2;
> > > 
> > >          while (count) {
> > >                  c1 = *cs++;
> > >                  c2 = *ct++;
> > >                  if (c1 != c2)
> > >                          return c1 < c2 ? -1 : 1;
> > >                  if (!c1)                             /* break after first character is compared */    
> > 
> > Crap! That is totally wrong!  
> 
> Looking at this again, it's not wrong. But how did we get here if c2 isn't
> zero as well?
> 

Could this be a bug in the implementation of strncmp() in
arch/arm64/lib/strncmp.S. As I don't know arm64 assembly, I have no idea
what it is trying to do.

But strncmp("o","off",3) returning zero *is* a bug.

-- Steve