netdev - Re: [PATCH v2 bpf-next 2/4] libbpf: Fix relocation of kfunc ksym in ld

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Message-ID: <CAEf4BzbpG2bxNyr_Qmado_fMwZ8Czjy+uTTNxmfwE-PUbrL+mg@mail.gmail.com>
Date:   Fri, 17 Mar 2023 15:49:38 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     davem@...emloft.net, daniel@...earbox.net, andrii@...nel.org,
        martin.lau@...nel.org, void@...ifault.com, davemarchevsky@...a.com,
        tj@...nel.org, memxor@...il.com, netdev@...r.kernel.org,
        bpf@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH v2 bpf-next 2/4] libbpf: Fix relocation of kfunc ksym in
 ld_imm64 insn.

On Fri, Mar 17, 2023 at 1:19 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> From: Alexei Starovoitov <ast@...nel.org>
>
> void *p = kfunc; -> generates ld_imm64 insn.
> kfunc() -> generates bpf_call insn.
>
> libbpf patches bpf_call insn correctly while only btf_id part of ld_imm64 is
> set in the former case. Which means that pointers to kfuncs in modules are not
> patched correctly and the verifier rejects load of such programs due to btf_id
> being out of range. Fix libbpf to patch ld_imm64 for kfunc.
>
> Signed-off-by: Alexei Starovoitov <ast@...nel.org>
> ---
>  tools/lib/bpf/libbpf.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index a557718401e4..4c34fbd7b5be 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -7533,6 +7533,12 @@ static int bpf_object__resolve_ksym_func_btf_id(struct bpf_object *obj,
>         ext->is_set = true;
>         ext->ksym.kernel_btf_id = kfunc_id;
>         ext->ksym.btf_fd_idx = mod_btf ? mod_btf->fd_array_idx : 0;
> +       /* Also set kernel_btf_obj_fd to make sure that bpf_object__relocate_data()
> +        * populates FD into ld_imm64 insn when it's used to point to kfunc.
> +        * {kernel_btf_id, btf_fd_idx} -> fixup bpf_call.
> +        * {kernel_btf_id, kernel_btf_obj_fd} -> fixup ld_imm64.
> +        */
> +       ext->ksym.kernel_btf_obj_fd = mod_btf ? mod_btf->fd : 0;
>         pr_debug("extern (func ksym) '%s': resolved to kernel [%d]\n",
>                  ext->name, kfunc_id);

we should report module name here as well, I'll send a patch for both
func and var ksyms

>
> --
> 2.34.1
>