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Date:   Tue, 28 Aug 2018 22:35:02 +0300
From:   Alexey Dobriyan <adobriyan@...il.com>
To:     "Ahmed S. Darwish" <darwish.07@...il.com>
Cc:     akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 11/13] proc: readdir /proc/*/task

On Tue, Aug 28, 2018 at 01:04:40PM +0000, Ahmed S. Darwish wrote:
> 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)
> > 
> > - 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
> >
> 
> See below:
> 
> > >  		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.
> > 
> > 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 .. (!)
> >
> 
> Ok, there's a "- p" in the end, so the length looks to be Ok.
> 
> Nonetheless, the whole patch series is introducing funny code
> like:
> 
> +/*
> + * Print an integer in decimal.
> + * "p" initially points PAST THE END OF THE BUFFER!
> + *
> + * DO NOT USE THESE FUNCTIONS!
> + *
> + * Do not copy these functions.
> + * Do not document these functions.
> + * Do not move these functions to lib/ or elsewhere.
> + * Do not export these functions to modules.
> + * Do not tell anyone about these functions.
> + */
> +noinline
> +char *_print_integer_u32(char *p, u32 x)
> +{
> +       do {
> +               *--p = '0' + (x % 10);
> +               x /= 10;
> +       } while (x != 0);
> +       return p;
> +}
> 
> And thus the code using these functions is throwing invalid
> past-the-stack pointers and strings with no NULL terminators
> like there's no tomorrow...
> 
> IMHO It's an accident waiting to happen to sprinkle pointers
> like that everywhere.

It is not if people will be prohibited from moving this code to lib/ and
"improving" it by adding more parameters.

> Are we really in a super hot path to justify all that?

/proc is very slow, try profiling just about anything involving /proc.

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