Re: [PATCH bpf-next v5 1/8] bpf: Add bpf_kallsyms_lookup_name helper

From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Cc: bpf <bpf@vger.kernel.org>, "Song Liu" <songliubraving@fb.com>,
	"Alexei Starovoitov" <ast@kernel.org>,
	"Daniel Borkmann" <daniel@iogearbox.net>,
	"Andrii Nakryiko" <andrii@kernel.org>,
	"Martin KaFai Lau" <kafai@fb.com>, "Yonghong Song" <yhs@fb.com>,
	"Jesper Dangaard Brouer" <brouer@redhat.com>,
	"Toke Høiland-Jørgensen" <toke@redhat.com>,
	Networking <netdev@vger.kernel.org>
Subject: Re: [PATCH bpf-next v5 1/8] bpf: Add bpf_kallsyms_lookup_name helper
Date: Thu, 28 Oct 2021 11:10:22 -0700	[thread overview]
Message-ID: <CAEf4BzZa_vhXB3c8atNcTS6=krQvC25H7K7c3WWZhM=27ro=Wg@mail.gmail.com> (raw)
In-Reply-To: <20211028063501.2239335-2-memxor@gmail.com>

On Wed, Oct 27, 2021 at 11:35 PM Kumar Kartikeya Dwivedi
<memxor@gmail.com> 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.
>
> Acked-by: Song Liu <songliubraving@fb.com>
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> ---
>  include/linux/bpf.h            |  1 +
>  include/uapi/linux/bpf.h       | 16 ++++++++++++++++
>  kernel/bpf/syscall.c           | 27 +++++++++++++++++++++++++++
>  tools/include/uapi/linux/bpf.h | 16 ++++++++++++++++
>  4 files changed, 60 insertions(+)
>

[...]

> +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;
> +
> +       if (!bpf_dump_raw_ok(current_cred()))
> +               return -EPERM;
> +
> +       *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       = false,
> +       .ret_type       = RET_INTEGER,
> +       .arg1_type      = ARG_PTR_TO_MEM,
> +       .arg2_type      = ARG_CONST_SIZE,

can you make it ARG_CONST_SIZE_OR_ZERO? Not because zero makes sense,
but because it removes the need to prove to the verifier that the size
(which you can get at runtime) is > 0. Just less unnecessary fussing
with preventing Clang optimizations.

> +       .arg3_type      = ARG_ANYTHING,
> +       .arg4_type      = ARG_PTR_TO_LONG,
> +};
> +

[...]