linux-kernel - Re: [PATCH 1/2] CPU, NUMA topology ABIs: clarify the overflow issue of sysfs pagebuf

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Message-ID: <YIrU0GMcnk6/5mbP@kroah.com>
Date:   Thu, 29 Apr 2021 17:46:24 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Tian Tao <tiantao6@...ilicon.com>, corbet@....net,
        song.bao.hua@...ilicon.com, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: Re: [PATCH 1/2] CPU, NUMA topology ABIs: clarify the overflow issue
 of sysfs pagebuf

On Thu, Apr 29, 2021 at 07:21:13AM -0700, Dave Hansen wrote:
> On 4/29/21 12:03 AM, Tian Tao wrote:
> > diff --git a/Documentation/ABI/stable/sysfs-devices-node b/Documentation/ABI/stable/sysfs-devices-node
> > index 484fc04..82dfe64 100644
> > --- a/Documentation/ABI/stable/sysfs-devices-node
> > +++ b/Documentation/ABI/stable/sysfs-devices-node
> > @@ -47,7 +47,10 @@ What:		/sys/devices/system/node/nodeX/cpulist
> >  Date:		October 2002
> >  Contact:	Linux Memory Management list <linux-mm@...ck.org>
> >  Description:
> > -		The CPUs associated to the node.
> > +		The CPUs associated to the node. The format is like 0-3,
> > +		8-11, 14,17. maximum size is PAGE_SIZE, so the tail
> > +		of the string will be trimmed while its size is larger
> > +		than PAGE_SIZE.
> 
> I think it's pretty arguable that truncating output on a real system is
> an ABI break.  Doing this would make the interface rather useless.
> 
> Don't we need a real solution rather than throwing up our hands?
> 
> Do we think >PAGE_SIZE data out of a sysfs file is a worse ABI break or
> something?

There is a real way to get > PAGE_SIZE out of a sysfs file.  The LED
developers had to do this when they ran into this same exact problem.
Make it a binary sysfs file and promise to NEVER create such a file
again in the future :)

thanks,

gre gk-h