linux-kernel - Re: [PATCH v4 bpf-next 4/6] bpf: fs/xattr: Add BPF kfuncs to set and remove xattrs

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Message-ID: <E05F5DE1-A4A8-4727-837C-808E2DA27BB2@fb.com>
Date: Wed, 18 Dec 2024 04:38:49 +0000
From: Song Liu <songliubraving@...a.com>
To: Kumar Kartikeya Dwivedi <memxor@...il.com>
CC: Song Liu <songliubraving@...a.com>,
        Alexei Starovoitov
	<alexei.starovoitov@...il.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 v4 bpf-next 4/6] bpf: fs/xattr: Add BPF kfuncs to set and
 remove xattrs



> On Dec 17, 2024, at 10:32 AM, Kumar Kartikeya Dwivedi <memxor@...il.com> wrote:
> 
> On Tue, 17 Dec 2024 at 19:25, Song Liu <songliubraving@...a.com> wrote:
>> 
>> Hi Alexei,
>> 
>> Thanks for the review!
>> 
>>> On Dec 17, 2024, at 8:50 AM, Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:
>>> 
>>> On Mon, Dec 16, 2024 at 10:38 PM Song Liu <song@...nel.org> wrote:
>>>> 
>>>> Add the following kfuncs to set and remove xattrs from BPF programs:
>>>> 
>>>> bpf_set_dentry_xattr
>>>> bpf_remove_dentry_xattr
>>>> bpf_set_dentry_xattr_locked
>>>> bpf_remove_dentry_xattr_locked
>>>> 
>>>> The _locked version of these kfuncs are called from hooks where
>>>> dentry->d_inode is already locked.
>>> 
>>> ...
>>> 
>>>> + *
>>>> + * Setting and removing xattr requires exclusive lock on dentry->d_inode.
>>>> + * Some hooks already locked d_inode, while some hooks have not locked
>>>> + * d_inode. Therefore, we need different kfuncs for different hooks.
>>>> + * Specifically, hooks in the following list (d_inode_locked_hooks)
>>>> + * should call bpf_[set|remove]_dentry_xattr_locked; while other hooks
>>>> + * should call bpf_[set|remove]_dentry_xattr.
>>>> + */
>>> 
>>> the inode locking rules might change, so let's hide this
>>> implementation detail from the bpf progs by making kfunc polymorphic.
>>> 
>>> To struct bpf_prog_aux add:
>>> bool use_locked_kfunc:1;
>>> and set it in bpf_check_attach_target() if it's attaching
>>> to one of d_inode_locked_hooks
>>> 
>>> Then in fixup_kfunc_call() call some helper that
>>> if (prog->aux->use_locked_kfunc &&
>>>   insn->imm == special_kfunc_list[KF_bpf_remove_dentry_xattr])
>>>    insn->imm = special_kfunc_list[KF_bpf_remove_dentry_xattr_locked];
>>> 
>>> The progs will be simpler and will suffer less churn
>>> when the kernel side changes.
>> 
>> I was thinking about something in similar direction.
>> 
>> If we do this, shall we somehow hide the _locked version of the
>> kfuncs, so that the user cannot use it? If so, what's the best
>> way to do it?
> 
> Just don't add BTF_ID_FLAGS entries for them.
> You'd also need to make an extra call to add_kfunc_call to add its
> details before you can do the fixup.
> That allows find_kfunc_desc to work.
> I did something similar in earlier versions of resilient locks.
> In add_kfunc_call's end (instead of directly returning):
> func_id = get_shadow_kfunc_id(func_id, offset);
> if (!func_id)
>  return err;
> return add_kfunc_call(env, func_id, offset);
> 
> Then check in fixup_kfunc_call to find shadow kfunc id and substitute imm.
> Can use some other naming instead of "shadow".
> Probably need to take a prog pointer to make a decision to find the
> underlying kfunc id in your case.

Thanks for the hints! They helped a lot. 

I ended up doing this with a slightly different logic, which I 
think is cleaner. I will send v5 shortly. 

Song