linux-kernel - Re: [PATCHv7 3/7] kho: drop notifiers

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Message-ID: <mafs0o6pwe1sy.fsf@kernel.org>
Date: Fri, 24 Oct 2025 17:50:53 +0200
From: Pratyush Yadav <pratyush@...nel.org>
To: Pasha Tatashin <pasha.tatashin@...een.com>
Cc: Pratyush Yadav <pratyush@...nel.org>,  akpm@...ux-foundation.org,
  brauner@...nel.org,  corbet@....net,  graf@...zon.com,  jgg@...pe.ca,
  linux-kernel@...r.kernel.org,  linux-kselftest@...r.kernel.org,
  linux-mm@...ck.org,  masahiroy@...nel.org,  ojeda@...nel.org,
  rdunlap@...radead.org,  rppt@...nel.org,  tj@...nel.org
Subject: Re: [PATCHv7 3/7] kho: drop notifiers

On Fri, Oct 24 2025, Pasha Tatashin wrote:

>> > -int kho_add_subtree(struct kho_serialization *ser, const char *name, void *fdt)
>> > +int kho_add_subtree(const char *name, void *fdt)
>> >  {
>> > -     int err = 0;
>> > -     u64 phys = (u64)virt_to_phys(fdt);
>> > -     void *root = page_to_virt(ser->fdt);
>> > +     struct kho_sub_fdt *sub_fdt;
>> > +     int err;
>> >
>> > -     err |= fdt_begin_node(root, name);
>> > -     err |= fdt_property(root, PROP_SUB_FDT, &phys, sizeof(phys));
>> > -     err |= fdt_end_node(root);
>> > +     sub_fdt = kmalloc(sizeof(*sub_fdt), GFP_KERNEL);
>> > +     if (!sub_fdt)
>> > +             return -ENOMEM;
>> >
>> > -     if (err)
>> > -             return err;
>> > +     INIT_LIST_HEAD(&sub_fdt->l);
>> > +     sub_fdt->name = name;
>> > +     sub_fdt->fdt = fdt;
>> >
>> > -     return kho_debugfs_fdt_add(&kho_out.dbg, name, fdt, false);
>> > +     mutex_lock(&kho_out.fdts_lock);
>> > +     list_add_tail(&sub_fdt->l, &kho_out.sub_fdts);
>> > +     err = kho_debugfs_fdt_add(&kho_out.dbg, name, fdt, false);
>>
>> I think you should remove sub_fdt from the list and kfree() it on error
>> here. Otherwise we signal an error to the caller and they might free
>> sub_fdt->fdt, which will later result in a use-after-free at
>> __kho_finalize().
>
> I think, it is better to simply do:
> WARN_ON_ONCE(kho_debugfs_fdt_add(...));
> Now debugfs is optional, and there is no reason to return an error to
> a caller if kho_debugfs_fdt_add() fails

Yeah, that works too.

>
>>
>> > +     mutex_unlock(&kho_out.fdts_lock);
>> > +
>> > +     return err;
>> >  }
>> >  EXPORT_SYMBOL_GPL(kho_add_subtree);
>> >
>> > -int register_kho_notifier(struct notifier_block *nb)
>> > +void kho_remove_subtree(void *fdt)
>> >  {
>> > -     return blocking_notifier_chain_register(&kho_out.chain_head, nb);
>> > -}
>> > -EXPORT_SYMBOL_GPL(register_kho_notifier);
>> > +     struct kho_sub_fdt *sub_fdt;
>> > +
>> > +     mutex_lock(&kho_out.fdts_lock);
>> > +     list_for_each_entry(sub_fdt, &kho_out.sub_fdts, l) {
>>
>> list_for_each_entry_safe() here since we delete.
>
> Not needed, we are breaking from the iterator when deleting.

Makes sense. Didn't think of that.

[...]

-- 
Regards,
Pratyush Yadav