linux-kernel - Re: [PATCH v11 bpf-next 5/7] bpf: Use btf_kfunc_id_set.remap logic for bpf_dynptr_from

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Message-ID: <FA63868F-D254-438C-9265-7BE40D970D99@fb.com>
Date: Thu, 30 Jan 2025 21:24:15 +0000
From: Song Liu <songliubraving@...a.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
CC: Song Liu <songliubraving@...a.com>, Song Liu <song@...nel.org>,
        bpf
	<bpf@...r.kernel.org>,
        Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
        LKML
	<linux-kernel@...r.kernel.org>,
        LSM List
	<linux-security-module@...r.kernel.org>,
        Kernel Team <kernel-team@...a.com>,
        Andrii Nakryiko <andrii@...nel.org>, Eddy Z <eddyz87@...il.com>,
        Alexei
 Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin
 KaFai Lau <martin.lau@...ux.dev>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
        KP Singh
	<kpsingh@...nel.org>,
        Matt Bobrowski <mattbobrowski@...gle.com>,
        Liam
 Wisehart <liamwisehart@...a.com>,
        Shankaran Gnanashanmugam
	<shankaran@...a.com>
Subject: Re: [PATCH v11 bpf-next 5/7] bpf: Use btf_kfunc_id_set.remap logic
 for bpf_dynptr_from_skb


> On Jan 30, 2025, at 12:23 PM, Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:

[...]

>>> For all these reasons I don't like this approach.
>>> This "generality" doesn't make it cleaner or easier to extend.
>>> For the patch 6... just repeat what specialize_kfunc()
>>> currently does for dynptr ?
>> 
>> Yes, specialize_kfunc() can handle this. But we will need to use
>> d_inode_locked_hooks from 6/7 in specialize_kfunc(). It works,
>> but it is not clean (to me).
> 
> I'm missing why that would be necessary to cross the layers
> so much. I guess the code will tell.
> Pls send an rfc to illustrate the unclean part.

The actual code is actually a lot cleaner than I thought. We just
need to use the bpf_lsm_has_d_inode_locked() helper in verifier.c. 

Thanks,
Song

> 
>> I will revise this set so that the polymorphism logic in handled
>> in specialize_kfunc(). For longer term, maybe we should discuss
>> "move some logic from verifier core to kfuncs" in the upcoming
>> LSF/MM/BPF?
> 
> imo such topic is too narrow and detail oriented.
> There is not much to gain from discussing it at lsfmm.
> email works well for such discussions.