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

[Rahul Bhansali]

> -----Original Message-----
> From: Thomas Monjalon <thomas at monjalon.net>
> Sent: Thursday, March 10, 2022 12:37 AM
> To: Rahul Bhansali <rbhansali at marvell.com>
> Cc: dev at dpdk.org; david.marchand at redhat.com; Conor Walsh
> <conor.walsh at intel.com>
> Subject: Re: [EXT] Re: [PATCH] examples/l3fwd: resolve stack buffer overflow
> issue
> 
> 09/03/2022 16:24, Rahul Bhansali:
> > Hi Thomas,
> >
> > From: Thomas Monjalon <thomas at 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 :)
Yeah, this is new learning for me as well 😊
It’s the way switch works and make it faster than If-else conditions executions by directly transferring control to respective case first and then do rest of the things.
Ref document: https://docs.microsoft.com/en-us/cpp/c-language/switch-statement-c?view=msvc-170

> 
> > 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://urldefense.proofpoint.com/v2/url?u=https-3A__godbolt.org_z_4Ke
> > cqbsde&d=DwICAg&c=nKjWec2b6R0mOyPaz7xtfQ&r=bm7kwlFq6L5uO69sS-
> 08RKSWPEU
> > tAQMUQjqHDFDtmpY&m=3GLnWHKqJB7pB5Jc36-gFYv-q-
> 3lyEtAFIK3Zt_TMRHhsAGJPIM
> > sAYTAunXt-TCf&s=L93OhPE9w3nl-Tf16rsvJ_OIC9Jar3Q7Be6vX9KfKfc&e=
> >
> > >
> > > >  		j = 0;
> > > >  		switch (len % FWDSTEP) {
> > > >  		while (j < len) {
> 
> 
> 
>