Re: [PATCH 4/4][PoC][RFC] Allow to trace fd usage with rlimit-events

[Krzysztof Opasiak]

Hi,

On 10/19/2017 01:05 AM, Al Viro wrote:
> On Wed, Oct 18, 2017 at 10:32:30PM +0200, Krzysztof Opasiak wrote:
>
> > @@ -417,7 +417,7 @@ static int task_get_unused_fd_flags(struct binder_proc *proc, int flags)
> >   	rlim_cur = task_rlimit(proc->tsk, RLIMIT_NOFILE);
> >   	unlock_task_sighand(proc->tsk, &irqs);
> >   
> > -	return __alloc_fd(files, 0, rlim_cur, flags);
> > +	return __alloc_fd(proc->tsk, 0, rlim_cur, flags);
>
> Who said that proc->files will remain equal to proc->tsk->files?
>
> > -static void __put_unused_fd(struct files_struct *files, unsigned int fd)
> > +static void __put_unused_fd(struct task_struct *owner, unsigned int fd)
> >   {
> > +	struct files_struct *files = owner->files;
> >   	struct fdtable *fdt = files_fdtable(files);
> >   	__clear_open_fd(fd, fdt);
> >   	if (fd < files->next_fd)
> >   		files->next_fd = fd;
> > +
> > +	if (rlimit_noti_watch_active(owner, RLIMIT_NOFILE)) {
> > +		unsigned int count;
> > +
> > +		count = count_open_fds(fdt);
> > +		rlimit_noti_res_changed(owner, RLIMIT_NOFILE, count + 1, count);
> > +	}
> >   }
>
> [... and similar for other __...fd() primitives]
> This is blatantly wrong - you *CAN'T* modify files_struct unless it's
> 	a) yours (i.e. current->files) or
> 	b) you've had its refcount incremented for you by some process that
> did, at the time, have current->files pointing to it.
>
> There is a reason why binder keeps ->files explicitly, rather than going through
> ->tsk->files.

Your are perfectly right! Thank you very much for catching this.

To be honest, initially I just added the struct task_struct ptr to the 
argument list keeping that in mind. Then when I was cleaning up patches 
before sending I found this to look a litlle bit odd and forgot that I 
did this on purpose because tsk->files can be reassigned and that's why 
I removed the files param.

I'll fix this for v2. Thanks once again.

Best regards,
--
Krzysztof Opasiak
Samsung R&D Institute Poland
Samsung Electronics