linux-hardening - Re: [PATCH -next, v2] sched: Use struct_size() helper in task_numa

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Date:   Tue, 18 Jan 2022 09:57:45 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Xiu Jianfeng <xiujianfeng@...wei.com>, mingo@...hat.com,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, gustavoars@...nel.org,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH -next, v2] sched: Use struct_size() helper in
 task_numa_group()

On Fri, Jan 14, 2022 at 07:50:47PM -0800, Kees Cook wrote:
> On Thu, Jan 13, 2022 at 10:18:57AM +0100, Peter Zijlstra wrote:

> > Then I would still much prefer something like:
> > 
> > 	unsigned int size = sizeof(*grp) +
> > 			    NR_NUMA_HINT_FAULT_STATS * numa_node_ids * sizeof(gfp->faults);
> > 
> > Which is still far more readable than some obscure macro. But again, the
> 
> I'm not sure it's _obscure_, but it is relatively new. It's even
> documented. ;)
> https://www.kernel.org/doc/html/latest/process/deprecated.html#open-coded-arithmetic-in-allocator-arguments

I'm one of those people who doesn't read documentation, I read code.

I also flat out refuse to read any documentation that isn't plain text.

> > I can't, nor do I want to, remember all these stupid little macros. Esp.
> > not for trivial things like this.
> 
> Well, the good news is that other folks will (and are) fixing them for
> you. :) Even if you never make mistakes with flexible arrays, other
> people do, and so we need to take on some improvements to the robustness
> of the kernel source tree-wide.

But nobody helps me read the code when I trip over crap like this :/ Why
do we have to have endless silly helpers for things that can be
trivially expressed in regular C? I appreciate things like
container_of() because if you write that out it's a mess, but this, very
much not so.

	struct_size(grp, faults, NR_NUMA_HINT_FAULTS_STATS * numa_node_ids);

vs

	sizeof(*gfp) + sizeof(grp->faults) * NR_NUMA_HINT_FAULT_STATS * nr_node_ids;

The latter wins hands down, instantly obvious what it does while with
the former I'd have to look up the macro.