Re: [syzbot] WARNING in c_start

[Yury Norov <yury.norov@gmail.com>]

Add people from other threads discussing this.

On Sat, Oct 15, 2022 at 01:53:19PM +0200, Borislav Petkov wrote:
> On Sat, Oct 15, 2022 at 08:39:19PM +0900, Tetsuo Handa wrote:
> > That's an invalid command line. The correct syntax is:
> > 
> > #syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
> 
> The fix is not in Linus' tree yet.
> 
> > Andrew Jones proposed a fix for x86 and riscv architectures [2]. But
> > other architectures have the same problem. And fixing all callers will
> > not be in time for this merge window.
> 
> Why won't there be time? That's why the -rcs are for.
> 
> Also, that thing fires only when CONFIG_DEBUG_PER_CPU_MAPS is enabled.
> 
> So no, we will take Andrew's fixes for all arches in time for 6.1.

Summarizing things:

1. cpumask_check() was introduced to make sure that the cpu number
passed into cpumask API belongs to a valid range. But the check is
broken for a very long time. And because of that there are a lot of
places where cpumask API is used wrongly.

2. Underlying bitmap functions handle that correctly - when user
passes out-of-range CPU index, the nr_cpu_ids is returned, and this is
what expected by client code. So if DEBUG_PER_CPU_MAPS config is off,
everything is working smoothly.

3. I fixed all warnings that I was aware at the time of submitting the
patch. 2 follow-up series are on review: "[PATCH v2 0/4] net: drop
netif_attrmask_next*()" and "[PATCH 0/9] lib/cpumask: simplify
cpumask_next_wrap()". Also, Andrew Jones, Alexander Gordeev and Guo Ren
proposed fixes for c_start() in arch code.

4. The code paths mentioned above are all known to me that violate
cpumask_check() rules. (Did I miss something?)

With all that, I agree with Borislav. Unfortunately, syzcall didn't CC
me about this problem with c_start(). But I don't like the idea to revert
cpumask_check() fix. This way we'll never clean that mess. 

If for some reason those warnings are unacceptable for -rcs (and like
Boris, I don't understand why), than instead of reverting commits, I'd
suggest moving cpumask sanity check from DEBUG_PER_CPU_MAPS under a new
config, say CONFIG_CPUMASK_DEBUG, which will be inactive until people will
fix their code. I can send a patch shortly, if we'll decide going this way.

How people would even realize that they're doing something wrong if
they will not get warned about it?

Thanks,
Yury