Re: [PATCH bpf-next v3 4/8] bpf: Harden register offset checks for release helpers and kfuncs

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On Sat, Mar 05, 2022 at 03:25:56AM +0530, Kumar Kartikeya Dwivedi wrote:
> On Sat, Mar 05, 2022 at 03:13:33AM IST, Martin KaFai Lau wrote:
> > On Sat, Mar 05, 2022 at 02:18:56AM +0530, Kumar Kartikeya Dwivedi wrote:
> > > On Sat, Mar 05, 2022 at 01:58:30AM IST, Martin KaFai Lau wrote:

> > > > > +			verbose(env, "R%d must have zero offset when passed to release func\n",
> > > > > +				regno);
> > > > > +			return -EINVAL;
> > > > > +		}
> > > > > +		fixed_off_ok = release_reg ? false : true;
> > > > nit.
> > > > 		fixed_off_ok = !release_reg;
> > > >
> > > > but this is a bit moot here considering the reg->off
> > > > check has already been done for the release_reg case.
> > > >
> > >
> > > Yes, it would be a redundant check inside __check_ptr_off_reg, but we still need
> > > to call it for checking bad var_off.
> > Redundant check is fine.
> >
> > The intention and the net effect here is fixed_off is always
> > allowed for the remaining case, so may as well directly set
> > fixed_off_ok to true.  "fixed_off_ok = !release_reg;"
> > made me go back to re-read what else has not been handled
> > for the release_reg case but it could be just me being
> > slow here.
> >
> 
> Right, I can see why that may be confusing. I just set it to !release_reg to
> disable any other code that may be added using that bool later in the future.
hmm... If the concern is on future code,
how about using a comment to remind future cases instead
and directly set it to true?

/* All special cases were handled above, the remaining
 * PTR_TO_BTF_ID case always allows fixed off.
 */
fixed_off_ok = true;


> 
> > It will be useful to at least leave a comment here
> > on the redundant check and the remaining cases for
> > PTR_TO_BTF_ID actually always allow fixed_off.
> >
> 
> Yes, I will add a comment to make it clearer.



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