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

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