Re: [PATCH 3/3] fs/file.c: move sanity_check from alloc_fd() to put_unused_fd()

[Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>]

On Tue, 2024-06-18 at 10:35 +0200, Michal Hocko wrote:
> On Mon 17-06-24 11:04:41, Tim Chen wrote:
> > diff --git a/kernel/sys.c b/kernel/sys.c
> > index 3a2df1bd9f64..b4e523728c3e 100644
> > --- a/kernel/sys.c
> > +++ b/kernel/sys.c
> > @@ -1471,6 +1471,7 @@ static int do_prlimit(struct task_struct *tsk, unsigned int resource,
> >                 return -EINVAL;
> >         resource = array_index_nospec(resource, RLIM_NLIMITS);
> >  
> > +       task_lock(tsk->group_leader);
> >         if (new_rlim) {
> >                 if (new_rlim->rlim_cur > new_rlim->rlim_max)
> >                         return -EINVAL;
> 
> This is clearly broken as it leaves the lock behind on the error, no?

Thanks for pointing that out.  Need unlock before return. I don't like
adding task_lock in alloc_fd path if there are multiple alloc_fd going
on causing contention.  

The race with rlimit change should be a very rare thing.  Should be
sufficient that patch one check for fd not going beyond the observed
rlimit.

Tim