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

[xiujianfeng]

在 2022/1/15 11:50, Kees Cook 写道:
> On Thu, Jan 13, 2022 at 10:18:57AM +0100, Peter Zijlstra wrote:
> > On Tue, Jan 11, 2022 at 10:14:25AM -0500, Steven Rostedt wrote:
> > > On Tue, 11 Jan 2022 12:30:42 +0100
> > > Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > >
> > > > > > >   	if (unlikely(!deref_curr_numa_group(p))) {
> > > > > > > -		unsigned int size = sizeof(struct numa_group) +
> > > > > > > -				    NR_NUMA_HINT_FAULT_STATS *
> > > > > > > -				    nr_node_ids * sizeof(unsigned long);
> > > > > > > +		unsigned int size = struct_size(grp, faults,
> > > > > > > +						NR_NUMA_HINT_FAULT_STATS * nr_node_ids);
> > > > > > Again, why?! The old code was perfectly readable, this, not so much.
> > > > > Because it is unsafe,
> > > > Unsafe how? Changelog doesn't mention anything, nor do you. In fact,
> > > > Changelog says there is no functional change, which makes me hate the
> > > > thing for obscuring something that was simple.
> > > If for some reason faults changes in size, the original code must be
> > > updated whereas the new code is robust enough to not need changing.
> I think this alone is reason enough. :)
>
> > 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
>
> That said, the original patch is incomplete: it should be using size_t
> for "size".
thanks, I will send a v3 patch with this change and more detailed commit 
message.
> > It is a fairly useful and common pattern to have a small structure and
> > an array in the same memory allocation.
> >
> > Think hash-tables, the structure contains the size of the table and some
> > other things, like for example a seed for the hash function or a lock,
> > and then the table itself as an array.
> Right, the use of flexible arrays is very common in the kernel. So much
> so that we've spent years fixing all the ancient "fake flexible arrays"
> scattered around the kernel messing up all kinds of compile-time and
> run-time flaw mitigations. Flexible array manipulations are notoriously
> prone to mistakes (overflows in allocation, mismatched bounds storage
> sizes, array index overflows, etc). These helpers (with more to come)
> help remove some of the foot-guns that C would normally impart to them.
>
> > 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.
>
> -Kees