linux-kernel - Re: [PATCH v5 bpf-next 4/5] bpf: fs/xattr: Add BPF kfuncs to set and remove xattrs

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Message-ID: <BF2BF0EC-90C2-4BFC-B1F3-D842AE1B7761@fb.com>
Date: Wed, 18 Dec 2024 21:47:34 +0000
From: Song Liu <songliubraving@...a.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
CC: Song Liu <song@...nel.org>, bpf <bpf@...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>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau
	<martin.lau@...ux.dev>,
        KP Singh <kpsingh@...nel.org>,
        Matt Bobrowski
	<mattbobrowski@...gle.com>,
        Paul Moore <paul@...l-moore.com>, James Morris
	<jmorris@...ei.org>,
        "Serge E . Hallyn" <serge@...lyn.com>,
        Kumar Kartikeya
 Dwivedi <memxor@...il.com>
Subject: Re: [PATCH v5 bpf-next 4/5] bpf: fs/xattr: Add BPF kfuncs to set and
 remove xattrs

Hi Alexei, 

Thanks for the review!

> On Dec 18, 2024, at 1:20 PM, Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:
> 
> On Tue, Dec 17, 2024 at 8:48 PM Song Liu <song@...nel.org> wrote:
>> 
>> 
>> BTF_KFUNCS_START(bpf_fs_kfunc_set_ids)
>> @@ -170,6 +330,10 @@ BTF_ID_FLAGS(func, bpf_put_file, KF_RELEASE)
>> BTF_ID_FLAGS(func, bpf_path_d_path, KF_TRUSTED_ARGS)
>> BTF_ID_FLAGS(func, bpf_get_dentry_xattr, KF_SLEEPABLE | KF_TRUSTED_ARGS)
>> BTF_ID_FLAGS(func, bpf_get_file_xattr, KF_SLEEPABLE | KF_TRUSTED_ARGS)
>> +BTF_ID_FLAGS(func, bpf_set_dentry_xattr, KF_SLEEPABLE | KF_TRUSTED_ARGS)
>> +BTF_ID_FLAGS(func, bpf_remove_dentry_xattr, KF_SLEEPABLE | KF_TRUSTED_ARGS)
>> +BTF_ID_FLAGS(func, bpf_set_dentry_xattr_locked, KF_SLEEPABLE | KF_TRUSTED_ARGS)
>> +BTF_ID_FLAGS(func, bpf_remove_dentry_xattr_locked, KF_SLEEPABLE | KF_TRUSTED_ARGS)
>> BTF_KFUNCS_END(bpf_fs_kfunc_set_ids)
> 
> The _locked() versions shouldn't be exposed to bpf prog.
> Don't add them to the above set.
> 
> Also we need to somehow exclude them from being dumped into vmlinux.h
> 
>> static int bpf_fs_kfuncs_filter(const struct bpf_prog *prog, u32 kfunc_id)
>> @@ -186,6 +350,37 @@ static const struct btf_kfunc_id_set bpf_fs_kfunc_set = {
>>        .filter = bpf_fs_kfuncs_filter,
>> };

[...]

>> + */
>> +static void remap_kfunc_locked_func_id(struct bpf_verifier_env *env, struct bpf_insn *insn)
>> +{
>> +       u32 func_id = insn->imm;
>> +
>> +       if (bpf_lsm_has_d_inode_locked(env->prog)) {
>> +               if (func_id == special_kfunc_list[KF_bpf_set_dentry_xattr])
>> +                       insn->imm =  special_kfunc_list[KF_bpf_set_dentry_xattr_locked];
>> +               else if (func_id == special_kfunc_list[KF_bpf_remove_dentry_xattr])
>> +                       insn->imm = special_kfunc_list[KF_bpf_remove_dentry_xattr_locked];
>> +       } else {
>> +               if (func_id == special_kfunc_list[KF_bpf_set_dentry_xattr_locked])
>> +                       insn->imm =  special_kfunc_list[KF_bpf_set_dentry_xattr];
> 
> This part is not necessary.
> _locked() shouldn't be exposed and it should be an error
> if bpf prog attempts to use invalid kfunc.

I was implementing this in different way than the solution you and Kumar
suggested. Instead of updating this in add_kfunc_call, check_kfunc_call, 
and fixup_kfunc_call, remap_kfunc_locked_func_id happens before 
add_kfunc_call. Then, for the rest of the process, the verifier handles
_locked version and not _locked version as two different kfuncs. This is
why we need the _locked version in bpf_fs_kfunc_set_ids. I personally 
think this approach is a lot cleaner. 

I think the missing piece is to exclude the _locked version from 
vmlinux.h. Maybe we can achieve this by adding a different DECL_TAG 
to these kfuncs?

Does this make sense?

Thanks,
Song