RE: [PATCH 1/3] sysfs: Fix crash on empty group attributes array

[Dan Williams <dan.j.williams@intel.com>]

Dan Williams wrote:
> It turns out that arch/x86/events/intel/core.c makes use of "empty"
> attributes.
> 
> 	static struct attribute *empty_attrs;
> 
> 	__init int intel_pmu_init(void)
> 	{
> 	        struct attribute **extra_skl_attr = &empty_attrs;
> 	        struct attribute **extra_attr = &empty_attrs;
> 	        struct attribute **td_attr    = &empty_attrs;
> 	        struct attribute **mem_attr   = &empty_attrs;
> 	        struct attribute **tsx_attr   = &empty_attrs;
> 		...
> 
> That breaks the assumption __first_visible() that expects that if
> grp->attrs is set then grp->attrs[0] must also be set and results in
> backtraces like:
> 
>     BUG: kernel NULL pointer dereference, address: 00rnel mode
>     #PF: error_code(0x0000) - not-present ] PREEMPT SMP NOPTI
>     CPU: 1 PID: 1 Comm: swapper/IP: 0010:exra_is_visible+0x14/0x20
>      ? exc_page_fault+0x68/0x190
>      internal_create_groups+0x42/0xa0
>      pmu_dev_alloc+0xc0/0xe0
>      perf_event_sysfs_init+0x580000000000 ]---
>     RIP: 0010:exra_is_visible+0x14/0
> 
> Check for non-empty attributes array before calling is_visible().
> 
> Reported-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
> Closes: https://github.com/thesofproject/linux/pull/4799#issuecomment-1958537212
> Fixes: 70317fd24b41 ("sysfs: Introduce a mechanism to hide static attribute_groups")
> Cc: Marc Herbert <marc.herbert@intel.com>

Ok, this one is now

Tested-by: Marc Herbert <marc.herbert@intel.com>

...per:

https://github.com/thesofproject/linux/pull/4799#issuecomment-1960469389

Thanks for all the help Marc!