netdev - Re: [PATCH bpf-next v3 1/8] bpf: Add bpf_kallsyms_lookup

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Message-ID: <20211020180418.5sgoickl4pucrk52@ast-mbp.dhcp.thefacebook.com>
Date:   Wed, 20 Oct 2021 11:04:18 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Kumar Kartikeya Dwivedi <memxor@...il.com>
Cc:     bpf@...r.kernel.org, Song Liu <songliubraving@...com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>, Yonghong Song <yhs@...com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        netdev@...r.kernel.org
Subject: Re: [PATCH bpf-next v3 1/8] bpf: Add bpf_kallsyms_lookup_name helper

On Fri, Oct 15, 2021 at 02:26:37AM +0530, Kumar Kartikeya Dwivedi wrote:
> This helper allows us to get the address of a kernel symbol from inside
> a BPF_PROG_TYPE_SYSCALL prog (used by gen_loader), so that we can
> relocate typeless ksym vars.
...
> +BPF_CALL_4(bpf_kallsyms_lookup_name, const char *, name, int, name_sz, int, flags, u64 *, res)
> +{
> +	if (flags)
> +		return -EINVAL;
> +
> +	if (name_sz <= 1 || name[name_sz - 1])
> +		return -EINVAL;
> +
> +	*res = kallsyms_lookup_name(name);
> +	return *res ? 0 : -ENOENT;
> +}
> +
> +const struct bpf_func_proto bpf_kallsyms_lookup_name_proto = {
> +	.func		= bpf_kallsyms_lookup_name,
> +	.gpl_only	= true,

When libbpf is processing of typeless ksyms it parses /proc/kallsyms.
There is no gpl-ness in the action of reading this file.
Hence above should be '= false' for consistency between
light and regular skeleton.

But different check is necessary.
This helper is available to syscall_bpf prog type (which is CAP_BPF)
and shouldn't be used to bypass kptr checks.
So bpf_kallsyms_lookup_name() should probably have:
if (!bpf_dump_raw_ok(current_cred())))
  return -EPERM;

The rest of the patches look great.
Thank you for working on this!