Re: [EXT] Re: [PATCH] examples/l3fwd: resolve stack buffer overflow issue

[Thomas Monjalon <thomas@monjalon.net>]

09/03/2022 16:24, Rahul Bhansali:
> Hi Thomas,
> 
> From: Thomas Monjalon <thomas@monjalon.net>
> > 11/01/2022 13:50, Rahul Bhansali:
> > >  		/* copy rest of the packets into the TX buffer. */
> > >  		len = num - n;
> > > +		if (len == 0)
> > > +			goto exit;
> > > +
> > 
> > I don't understand how it can fix something.
> > There is already  "while (j < len)" with j and len being 0, the loop should not be
> > effective in this case.
> 
> This Switch will execute Case statement first even before considering the while condition or anything else before case statement. While condition will be executed only after all switch cases are executed.

I don't know this construct. Is it part of the C standard?
We learn something everyday :)

> Hence in case of len = 0 and n > 28, it is throwing stack buffer overflow error.
> 
> Below is sample code to simulate the while loop behavior inside switch. Checked it for both x86 and arm64.
> https://godbolt.org/z/4Kecqbsde 
> 
> > 
> > >  		j = 0;
> > >  		switch (len % FWDSTEP) {
> > >  		while (j < len) {