Re: [PATCH] mm: page_counter: reject empty string in page_counter_memparse()

[Michal Hocko]

On Tue 18-08-26 08:06:22, Shakeel Butt wrote:
> On Tue, Aug 18, 2026 at 09:45:56AM +0200, Michal Hocko wrote:
> > On Mon 17-08-26 09:16:40, Shakeel Butt wrote:
> > > On Mon, Aug 17, 2026 at 12:26:52PM +0800, Tao Cui wrote:
> > > > From: Tao Cui <cuitao@xxxxxxxxxx>
> > > > 
> > > > memparse() consumes no characters on an empty input and leaves the
> > > > end pointer at the terminating NUL.  The only validation in
> > > > page_counter_memparse() checks for trailing characters, so an empty
> > > > input slips through and the limit becomes 0.
> > > > 
> > > > All limit write callbacks of the memory controller strstrip() the
> > > > input before calling this helper, so a script that writes an unset
> > > > variable hits this path:
> > > > 
> > > >   LIMIT=
> > > >   echo "$LIMIT" > $CG/memory.max
> > > >   echo $?
> > > >   0
> > > >   cat $CG/memory.max
> > > >   0
> > > > 
> > > > Nothing reports the mistake: the limit is now 0 and the OOM killer
> > > > goes after every task in the cgroup.  The same happens for
> > > > memory.min, memory.low, memory.high, memory.swap.high,
> > > > memory.swap.max and memory.zswap.max, where 0 silently removes the
> > > > protection or disables swap and zswap.
> > > > 
> > > > Reject the input when no characters were consumed, which is the one
> > > > case the trailing-character check cannot catch.
> > > > 
> > > > Fixes: 3e32cb2e0a12 ("mm: memcontrol: lockless page counters")
> > > > Signed-off-by: Tao Cui <cuitao@xxxxxxxxxx>
> > > > ---
> > > >  mm/page_counter.c | 2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > 
> > > > diff --git a/mm/page_counter.c b/mm/page_counter.c
> > > > index 661e0f2a5127..d14db705b04f 100644
> > > > --- a/mm/page_counter.c
> > > > +++ b/mm/page_counter.c
> > > > @@ -281,7 +281,7 @@ int page_counter_memparse(const char *buf, const char *max,
> > > >  	}
> > > >  
> > > >  	bytes = memparse(buf, &end);
> > > > -	if (*end != '\0')
> > > > +	if (*end != '\0' || end == buf)
> > > >  		return -EINVAL;
> > > 
> > > I wonder if someone started depending on this behavior. In that case it is
> > > better to return error instead of silently ignore, so we will hear complains
> > > loudly. This looks good to me.
> > 
> > This is backward incompatible change and I am wondering why should we
> > even risk regression.
> 
> Mainly I was wondering if this is intentional or unintentional. If this us
> unintentional, can we fix it without anyone noticing?

My guess would be this was just omission. Those happen and over years we
have learned that userspace is quite creative at using those.

> However if we are ok with this then let's make is formal and make this a
> documented behavior. I don't have any strong opinion either way but I think you
> are saying it safer to just assume this is intentional. Fine with me.

My main question is why should we even bother to change this in the
first place? Is that reason stronger than a theoretical breakage of
userspace that we might learn much later?
-- 
Michal Hocko
SUSE Labs