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

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Message-ID: <mafs0jz0ke1qd.fsf@kernel.org>
Date: Fri, 24 Oct 2025 17:52:26 +0200
From: Pratyush Yadav <pratyush@...nel.org>
To: Pratyush Yadav <pratyush@...nel.org>
Cc: Pasha Tatashin <pasha.tatashin@...een.com>,  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, Pratyush Yadav wrote:

> 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.

On a second thought, maybe pr_warn() instead of WARN_ON()? This isn't an
assertion since the debugfs creation can fail for many reasons. It isn't
expected to always succeed. So a full WARN_ON() splat seems overkill.

[...]

-- 
Regards,
Pratyush Yadav