Re: [PATCH] memcg: Check more strictly to avoid ULLONG overflow by PAGE_ALIGN

[Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>]

On Mon, 15 Apr 2013 13:58:05 -0700
Andrew Morton <akpm@linux-foundation.org> wrote:

> On Fri, 12 Apr 2013 17:11:08 +0900 Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> wrote:
> 
> > > --- a/include/linux/res_counter.h
> > > +++ b/include/linux/res_counter.h
> > > @@ -54,7 +54,7 @@ struct res_counter {
> > >  	struct res_counter *parent;
> > >  };
> > >  
> > > -#define RESOURCE_MAX (unsigned long long)LLONG_MAX
> > > +#define RESOURCE_MAX (unsigned long long)ULLONG_MAX
> > >  
> > 
> > I don't think it's a good idea to change a user-visible value.
> 
> The old value was a mistake, surely.
> 

I introduced 'RESOURCE_MAX' in commit:c5b947b2, but I just used the
default value of the res_counter.limit. I'm not sure why it had been
initialized to (unsigned long long)LLONG_MAX.

> RESOURCE_MAX shouldn't be in this header file - that is far too general
> a name.  I suggest the definition be moved to res_counter.c.  And the
> (unsigned long long) cast is surely unneeded if we're to use
> ULLONG_MAX.
> 

Hmm, RESOUCE_MAX is now used outside of res_counter.c(e.g. tcp_memcontrol.c).
Adding Glauber Costa to the cc list.
Just changing the name to RES_COUNTER_MAX might be the choice.

> > >  /**
> > >   * Helpers to interact with userspace
> > > diff --git a/kernel/res_counter.c b/kernel/res_counter.c
> > > index ff55247..6c35310 100644
> > > --- a/kernel/res_counter.c
> > > +++ b/kernel/res_counter.c
> > > @@ -195,6 +195,12 @@ int res_counter_memparse_write_strategy(const char *buf,
> > >  	if (*end != '\0')
> > >  		return -EINVAL;
> > >  
> > > -	*res = PAGE_ALIGN(*res);
> > > +	/* Since PAGE_ALIGN is aligning up(the next page boundary),
> > > +	 * check the left space to avoid overflow to 0. */
> > > +	if (RESOURCE_MAX - *res < PAGE_SIZE - 1)
> > > +		*res = RESOURCE_MAX;
> > > +	else
> > > +		*res = PAGE_ALIGN(*res);
> > > +
> > 
> > Current interface seems strange because we can set a bigger value than
> > the value which means "unlimited".
> 
> I'm not sure what you mean by this?
> 
9223372036854775807(LLONG_MAX) means "unlimited" now. But we can set a bigger
value than that.

# cat /cgroup/memory/A/memory.memsw.limit_in_bytes
9223372036854775807
# echo 9223372036854775808 >/cgroup/memory/A/memory.memsw.limit_in_bytes
# cat /cgroup/memory/A/memory.memsw.limit_in_bytes
9223372036854775808

I feel "bigger than unlimited" is strange.

> > So, how about some thing like:
> > 
> > 	if (*res > RESOURCE_MAX)
> > 		return -EINVAL;
> > 	if (*res > PAGE_ALIGN(RESOURCE_MAX) - PAGE_SIZE)
> > 		*res = RESOURCE_MAX;
> > 	else
> > 		*res = PAGE_ALIGN(*res);
> > 
> 
> The first thing I'd do to res_counter_memparse_write_strategy() is to
> rename its second arg to `resp' then add a local called `res'.  Because
> that function dereferences res far too often.
> 
> Then,
> 
> -	*res = PAGE_ALIGN(*res);
> 	if (PAGE_ALIGN(res) >= res)
> 		res = PAGE_ALIGN(res);
> 	else
> 		res = RESOURCE_MAX;	/* PAGE_ALIGN wrapped to zero */
> 
> 	*resp = res;
> 	return 0;
> 	
> 	
Good idea :)


Thanks,
Daisuke Nishimura.

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