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Date:   Tue, 28 Aug 2018 22:31:54 +0300
From:   Alexey Dobriyan <adobriyan@gmail.com>
To:     "Ahmed S. Darwish" <darwish.07@gmail.com>
Cc:     akpm@linux-foundation.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 11/13] proc: readdir /proc/*/task
Message-ID: <20180828193154.GA4847@avx2>
References: <20180827231503.26899-1-adobriyan@gmail.com>
 <20180827231503.26899-11-adobriyan@gmail.com>
 <20180828123622.GA2087@darwi-kernel>
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On Tue, Aug 28, 2018 at 12:36:22PM +0000, Ahmed S. Darwish wrote:
> On Tue, Aug 28, 2018 at 02:15:01AM +0300, Alexey Dobriyan wrote:
> > ---
> >  fs/proc/base.c | 8 ++++----
> >  1 file changed, 4 insertions(+), 4 deletions(-)
> >
> 
> Missing description and S-o-b. Further comments below..
> 
> > diff --git a/fs/proc/base.c b/fs/proc/base.c
> > index 33f444721965..668e465c86b3 100644
> > --- a/fs/proc/base.c
> > +++ b/fs/proc/base.c
> > @@ -3549,11 +3549,11 @@ static int proc_task_readdir(struct file *file, struct dir_context *ctx)
> >  	for (task = first_tid(proc_pid(inode), tid, ctx->pos - 2, ns);
> >  	     task;
> >  	     task = next_tid(task), ctx->pos++) {
> > -		char name[10 + 1];
> > -		unsigned int len;
> > +		char name[10], *p = name + sizeof(name);
> > +
> 
> Multiple issues:
> 
> - len should be 11, as was in the original code
>   (0xffffffff = 4294967295, 10 letters)

len should be 10.

> - while we're at it, let's use a constant for the '11' instead of
>   mysterious magic numbers
> 
> - 'p' is clearly overflowing the stack here
> 
> >  		tid = task_pid_nr_ns(task, ns);
> > -		len = snprintf(name, sizeof(name), "%u", tid);
> > -		if (!proc_fill_cache(file, ctx, name, len,
> > +		p = _print_integer_u32(p, tid);
> > +		if (!proc_fill_cache(file, ctx, p, name + sizeof(name) - p,
> 
> You're replacing snprintf() code __that did proper len checking__
> with code that does not. That's not good.

Yes, the whole point of the patch is to skip length checking.

> I can't see how the fourth proc_fill_cache() parameter, ``name +
> sizeof(name)'' safely ever replace the original 'len' parameter.
> It's a pointer value .. (!)
> 
> Overall this looks like a broken patch submitted by mistake.