linux-kernel - Re: [PATCH] drivers/base: Fix unsigned comparison to -1 in CPUMAP_FILE_MAX

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Message-ID: <YxeCXALgsIGxiSlG@lorien.usersys.redhat.com>
Date:   Tue, 6 Sep 2022 13:24:44 -0400
From:   Phil Auld <pauld@...hat.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Barry Song <21cnbao@...il.com>,
        Tian Tao <tiantao6@...ilicon.com>,
        Yury Norov <yury.norov@...il.com>,
        feng xiangjun <fengxj325@...il.com>, stable@...r.kernel.org
Subject: Re: [PATCH] drivers/base: Fix unsigned comparison to -1 in
 CPUMAP_FILE_MAX_BYTES

On Tue, Sep 06, 2022 at 07:02:37PM +0200 Greg Kroah-Hartman wrote:
> On Tue, Sep 06, 2022 at 12:04:30PM -0400, Phil Auld wrote:
> > As PAGE_SIZE is unsigned long, -1 > PAGE_SIZE when NR_CPUS <= 3.
> > This leads to very large file sizes:
> > 
> > topology$ ls -l
> > total 0
> > -r--r--r-- 1 root root 18446744073709551615 Sep  5 11:59 core_cpus
> 
> Yeah, lots of CPUs!  :)
>

Yep, apparently things like lscpu fail with this size. 


> > -r--r--r-- 1 root root                 4096 Sep  5 11:59 core_cpus_list
> > -r--r--r-- 1 root root                 4096 Sep  5 10:58 core_id
> > -r--r--r-- 1 root root 18446744073709551615 Sep  5 10:10 core_siblings
> > -r--r--r-- 1 root root                 4096 Sep  5 11:59 core_siblings_list
> > -r--r--r-- 1 root root 18446744073709551615 Sep  5 11:59 die_cpus
> > -r--r--r-- 1 root root                 4096 Sep  5 11:59 die_cpus_list
> > -r--r--r-- 1 root root                 4096 Sep  5 11:59 die_id
> > -r--r--r-- 1 root root 18446744073709551615 Sep  5 11:59 package_cpus
> > -r--r--r-- 1 root root                 4096 Sep  5 11:59 package_cpus_list
> > -r--r--r-- 1 root root                 4096 Sep  5 10:58 physical_package_id
> > -r--r--r-- 1 root root 18446744073709551615 Sep  5 10:10 thread_siblings
> > -r--r--r-- 1 root root                 4096 Sep  5 11:59 thread_siblings_list
> > 
> > Adjust the inequality to catch the case when NR_CPUS is configured
> > to a small value.
> > 
> > Fixes: 7ee951acd31a ("drivers/base: fix userspace break from using bin_attributes for cpumap and cpulist")
> > Reported-by: feng xiangjun <fengxj325@...il.com>
> > Signed-off-by: Phil Auld <pauld@...hat.com>
> > Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > Cc: "Rafael J. Wysocki" <rafael@...nel.org>
> > Cc: Yury Norov <yury.norov@...il.com>
> > Cc: stable@...r.kernel.org
> > Cc: feng xiangjun <fengxj325@...il.com>
> > ---
> >  include/linux/cpumask.h | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> > 
> > diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
> > index bd047864c7ac..7b1349612d6d 100644
> > --- a/include/linux/cpumask.h
> > +++ b/include/linux/cpumask.h
> > @@ -1127,9 +1127,10 @@ cpumap_print_list_to_buf(char *buf, const struct cpumask *mask,
> >   * cover a worst-case of every other cpu being on one of two nodes for a
> >   * very large NR_CPUS.
> >   *
> > - *  Use PAGE_SIZE as a minimum for smaller configurations.
> > + *  Use PAGE_SIZE as a minimum for smaller configurations while avoiding
> > + *  unsigned comparison to -1.
> >   */
> > -#define CPUMAP_FILE_MAX_BYTES  ((((NR_CPUS * 9)/32 - 1) > PAGE_SIZE) \
> > +#define CPUMAP_FILE_MAX_BYTES  ((((NR_CPUS * 9)/32) > PAGE_SIZE + 1) \
> >  					? (NR_CPUS * 9)/32 - 1 : PAGE_SIZE)
> >  #define CPULIST_FILE_MAX_BYTES  (((NR_CPUS * 7)/2 > PAGE_SIZE) ? (NR_CPUS * 7)/2 : PAGE_SIZE)
> >  
> > -- 
> > 2.31.1
> > 
> 
> Nice catch.  What type of systems did you run this on to verify it will
> work?
>

Feng ran it on his 2 cpu laptop.  I ran it on my usual test bed (80 cpus) but
configured both with NR_CPUS at 8192 and at 2.

It's not really dependent on the actual system. It's a compile time config
option.  I failed to set it small when testing the first one.

The fix here is just maths :) 

With NR_CPUS == 2:

# ls -l /sys/devices/system/cpu/cpu0/topology/
total 0
-r--r--r--. 1 root root 4096 Sep  6 11:40 cluster_cpus
-r--r--r--. 1 root root 4096 Sep  6 11:40 cluster_cpus_list
-r--r--r--. 1 root root 4096 Sep  6 11:40 cluster_id
-r--r--r--. 1 root root 4096 Sep  6 11:40 core_cpus
-r--r--r--. 1 root root 4096 Sep  6 11:40 core_cpus_list
-r--r--r--. 1 root root 4096 Sep  6 11:40 core_id
-r--r--r--. 1 root root 4096 Sep  6 11:29 core_siblings
-r--r--r--. 1 root root 4096 Sep  6 11:40 core_siblings_list
...



Cheers,
Phil

> thanks,
> 
> greg k-h
> 

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