On Thu, Feb 22, 2024 at 12:40:54PM -0800, 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().
[...]
> --- a/fs/sysfs/group.c
> +++ b/fs/sysfs/group.c
> @@ -33,10 +33,10 @@ static void remove_files(struct kernfs_node *parent,
>
> static umode_t __first_visible(const struct attribute_group *grp, struct kobject *kobj)
> {
> - if (grp->attrs && grp->is_visible)
> + if (grp->attrs && grp->attrs[0] && grp->is_visible)
> return grp->is_visible(kobj, grp->attrs[0], 0);
>
> - if (grp->bin_attrs && grp->is_bin_visible)
> + if (grp->bin_attrs && grp->bin_attrs[0] && grp->is_bin_visible)
> return grp->is_bin_visible(kobj, grp->bin_attrs[0], 0);
>
> return 0;
I'm wondering why 0 is returned by default and not SYSFS_GROUP_INVISIBLE.
An empty attribute list (containing just the NULL sentinel) will now
result in the attribute group being visible as an empty directory.
I thought the whole point was to hide such empty directories.
Was it a conscious decision to return 0?
Did you expect breakage if SYSFS_GROUP_INVISIBLE is returned?
Thanks,
Lukas
