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Date:   Tue, 14 Dec 2021 16:50:15 +0100
From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     David Vernet <void@manifault.com>
Cc:     Miroslav Benes <mbenes@suse.cz>, Petr Mladek <pmladek@suse.com>,
        linux-doc@vger.kernel.org, live-patching@vger.kernel.org,
        linux-kernel@vger.kernel.org, jpoimboe@redhat.com,
        jikos@kernel.org, joe.lawrence@redhat.com, corbet@lwn.net,
        yhs@fb.com, songliubraving@fb.com
Subject: Re: [PATCH] livepatch: Fix leak on klp_init_patch_early failure path
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References: <20211213191734.3238783-1-void@manifault.com>
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On Tue, Dec 14, 2021 at 07:26:33AM -0800, David Vernet wrote:
> Miroslav Benes <mbenes@suse.cz> wrote on Tue [2021-Dec-14 10:17:08 +0100]:
> > It would help to share warning outputs (or whatever) from DEBUG_KOBJECTS.
> 
> This is the output when running kpatch load livepatch-sample.ko if an extra
> 'struct klp_obj' entry is added that has a name but no funcs:
> 
> Without patch:
> 
> [   67.285762] livepatch_sample: tainting kernel with TAINT_LIVEPATCH
> [   67.286107] kobject: 'livepatch_sample' (00000000cf89f7b6): kobject_add_internal: parent: 'module', set: 'module'
> [   67.286113] kobject: 'holders' (00000000858b03bf): kobject_add_internal: parent: 'livepatch_sample', set: '<NULL>'
> [   67.286126] kobject: 'notes' (00000000f2a3a4ce): kobject_add_internal: parent: 'livepatch_sample', set: '<NULL>'
> [   67.297856] kobject: 'holders' (00000000858b03bf): kobject_cleanup, parent 00000000cf89f7b6
> [   67.297859] kobject: 'holders' (00000000858b03bf): auto cleanup kobject_del
> [   67.297861] kobject: 'holders' (00000000858b03bf): calling ktype release
> [   67.297862] kobject: (00000000858b03bf): dynamic_kobj_release
> [   67.297863] kobject: 'holders': free name
> [   67.297865] kobject: 'notes' (00000000f2a3a4ce): kobject_cleanup, parent 00000000cf89f7b6
> [   67.297866] kobject: 'notes' (00000000f2a3a4ce): auto cleanup kobject_del
> [   67.297867] kobject: 'notes' (00000000f2a3a4ce): calling ktype release
> [   67.297868] kobject: (00000000f2a3a4ce): dynamic_kobj_release
> [   67.297869] kobject: 'notes': free name
> [   67.297874] kobject: 'livepatch_sample' (00000000cf89f7b6): kobject_cleanup, parent 000000002555fa2d
> [   67.297876] kobject: 'livepatch_sample' (00000000cf89f7b6): auto cleanup kobject_del
> [   67.297877] kobject: 'livepatch_sample' (00000000cf89f7b6): calling ktype release
> [   67.297878] kobject: 'livepatch_sample': free name
> [   99.445938] kobject: '0:40' (000000002a98d11d): kobject_add_internal: parent: 'bdi', set: 'devices'
> [   99.445954] kobject: '0:40' (000000002a98d11d): kobject_uevent_env
> [   99.445957] kobject: '0:40' (000000002a98d11d): fill_kobj_path: path = '/devices/virtual/bdi/0:40'
> 
> With patch:
> 
> [  162.275251] livepatch_sample: tainting kernel with TAINT_LIVEPATCH
> [  162.275985] kobject: 'livepatch_sample' (00000000e688ee30): kobject_add_internal: parent: 'module', set: 'module'
> [  162.275993] kobject: 'holders' (000000004eee7860): kobject_add_internal: parent: 'livepatch_sample', set: '<NULL>'
> [  162.276010] kobject: 'notes' (00000000c4f390ab): kobject_add_internal: parent: 'livepatch_sample', set: '<NULL>'
> [  162.276028] kobject: '(null)' (000000003acccf72): kobject_cleanup, parent 0000000000000000
> [  162.276031] kobject: '(null)' (000000003acccf72): calling ktype release
> [  162.276033] kobject: '(null)' (00000000aeae6326): kobject_cleanup, parent 0000000000000000
> [  162.276035] kobject: '(null)' (00000000aeae6326): calling ktype release
> [  162.276037] kobject: '(null)' (0000000093b68297): kobject_cleanup, parent 0000000000000000
> [  162.276039] kobject: '(null)' (0000000093b68297): calling ktype release
> [  162.294063] kobject: 'holders' (000000004eee7860): kobject_cleanup, parent 00000000e688ee30
> [  162.294070] kobject: 'holders' (000000004eee7860): auto cleanup kobject_del
> [  162.294074] kobject: 'holders' (000000004eee7860): calling ktype release
> [  162.294078] kobject: (000000004eee7860): dynamic_kobj_release
> [  162.294081] kobject: 'holders': free name
> [  162.294086] kobject: 'notes' (00000000c4f390ab): kobject_cleanup, parent 00000000e688ee30
> [  162.294090] kobject: 'notes' (00000000c4f390ab): auto cleanup kobject_del
> [  162.294094] kobject: 'notes' (00000000c4f390ab): calling ktype release
> [  162.294097] kobject: (00000000c4f390ab): dynamic_kobj_release
> [  162.294100] kobject: 'notes': free name
> [  162.294114] kobject: 'livepatch_sample' (00000000e688ee30): kobject_cleanup, parent 00000000f9317c72
> [  162.294118] kobject: 'livepatch_sample' (00000000e688ee30): auto cleanup kobject_del
> [  162.294123] kobject: 'livepatch_sample' (00000000e688ee30): calling ktype release
> [  162.294126] kobject: 'livepatch_sample': free name
> 
> The extra lines are of course the kobject: '(null)' entries, for which we
> *don't* see auto cleanup kobject_del being called. So it seems that what's
> there now is probably not actually leaking memory, and the question is
> really whether or not the documentation in kobject.c is the source of truth
> (i.e. whether the code needs to be "fixed" to honor the API contract).
> 
> > I think that this might be, once again, a false positive. We use kobjects 
> > differently than what the kobject implementation and its documentation 
> > assume.
> 
> I'm curious to hear what Greg says. As Petr pointed out, it seems that the
> documentation for kobjects is inconsistent. If we're going by the function
> comment header in kobject.c then what we have now should probably be
> considered a bug? If we're going by what's in
> Documentation/core-api/kobject.rst, I think what we have now is correct.

I do not understand, what is the problem here?  I have been ignoring
this thread :)

> I do think it's a bit of a leaky abstraction to assume that the
> implementation doesn't allocate anything, but I also see Petr's point that
> it could be useful to make it explicit that kobject_init() doesn't allocate
> anything, and instead just affords callers the option of using
> kobject_put() if they want to the objects' destructors to be invoked.

kobject_init() does allocate things internally, where does it say it
does not?  What is trying to be "fixed" here?

confused,

greg k-h