Re: [PATCH bpf-next v1 1/6] bpf: Add bpf_kallsyms_lookup_name helper

[Song Liu <song@kernel.org>]

On Tue, Oct 5, 2021 at 5:29 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 weak and typeless ksym vars.
>
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

LGTM.

Acked-by: Song Liu <songliubraving@fb.com>

> ---
>  include/linux/bpf.h            |  1 +
>  include/uapi/linux/bpf.h       | 14 ++++++++++++++
>  kernel/bpf/syscall.c           | 24 ++++++++++++++++++++++++
>  tools/include/uapi/linux/bpf.h | 14 ++++++++++++++
>  4 files changed, 53 insertions(+)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index d604c8251d88..17206aae329d 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -2107,6 +2107,7 @@ extern const struct bpf_func_proto bpf_for_each_map_elem_proto;
>  extern const struct bpf_func_proto bpf_btf_find_by_name_kind_proto;
>  extern const struct bpf_func_proto bpf_sk_setsockopt_proto;
>  extern const struct bpf_func_proto bpf_sk_getsockopt_proto;
> +extern const struct bpf_func_proto bpf_kallsyms_lookup_name_proto;
>
>  const struct bpf_func_proto *tracing_prog_func_proto(
>    enum bpf_func_id func_id, const struct bpf_prog *prog);
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 6fc59d61937a..bbd0a3f4e5f6 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -4909,6 +4909,19 @@ union bpf_attr {
>   *     Return
>   *             The number of bytes written to the buffer, or a negative error
>   *             in case of failure.
> + *
> + * long bpf_kallsyms_lookup_name(const char *name, int name_sz, int flags, u64 *res)
> + *     Description
> + *             Get the address of a kernel symbol, returned in *res*. *res* is
> + *             set to 0 if the symbol is not found.
> + *     Return
> + *             On success, zero. On error, a negative value.
> + *
> + *             **-EINVAL** if *flags* is not zero.
> + *
> + *             **-EINVAL** if string *name* is not the same size as *name_sz*.
> + *
> + *             **-ENOENT** if symbol is not found.
>   */
>  #define __BPF_FUNC_MAPPER(FN)          \
>         FN(unspec),                     \
> @@ -5089,6 +5102,7 @@ union bpf_attr {
>         FN(task_pt_regs),               \
>         FN(get_branch_snapshot),        \
>         FN(trace_vprintk),              \
> +       FN(kallsyms_lookup_name),       \
>         /* */
>
>  /* integer value in 'imm' field of BPF_CALL instruction selects which helper
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index 4e50c0bfdb7d..073ca9ebe58b 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -4753,6 +4753,28 @@ static const struct bpf_func_proto bpf_sys_close_proto = {
>         .arg1_type      = ARG_ANYTHING,
>  };
>
> +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,
> +       .ret_type       = RET_INTEGER,
> +       .arg1_type      = ARG_PTR_TO_MEM,
> +       .arg2_type      = ARG_CONST_SIZE,
> +       .arg3_type      = ARG_ANYTHING,
> +       .arg4_type      = ARG_PTR_TO_LONG,
> +};
> +
>  static const struct bpf_func_proto *
>  syscall_prog_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
>  {
> @@ -4763,6 +4785,8 @@ syscall_prog_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
>                 return &bpf_btf_find_by_name_kind_proto;
>         case BPF_FUNC_sys_close:
>                 return &bpf_sys_close_proto;
> +       case BPF_FUNC_kallsyms_lookup_name:
> +               return &bpf_kallsyms_lookup_name_proto;
>         default:
>                 return tracing_prog_func_proto(func_id, prog);
>         }
> diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
> index 6fc59d61937a..bbd0a3f4e5f6 100644
> --- a/tools/include/uapi/linux/bpf.h
> +++ b/tools/include/uapi/linux/bpf.h
> @@ -4909,6 +4909,19 @@ union bpf_attr {
>   *     Return
>   *             The number of bytes written to the buffer, or a negative error
>   *             in case of failure.
> + *
> + * long bpf_kallsyms_lookup_name(const char *name, int name_sz, int flags, u64 *res)
> + *     Description
> + *             Get the address of a kernel symbol, returned in *res*. *res* is
> + *             set to 0 if the symbol is not found.
> + *     Return
> + *             On success, zero. On error, a negative value.
> + *
> + *             **-EINVAL** if *flags* is not zero.
> + *
> + *             **-EINVAL** if string *name* is not the same size as *name_sz*.
> + *
> + *             **-ENOENT** if symbol is not found.
>   */
>  #define __BPF_FUNC_MAPPER(FN)          \
>         FN(unspec),                     \
> @@ -5089,6 +5102,7 @@ union bpf_attr {
>         FN(task_pt_regs),               \
>         FN(get_branch_snapshot),        \
>         FN(trace_vprintk),              \
> +       FN(kallsyms_lookup_name),       \
>         /* */
>
>  /* integer value in 'imm' field of BPF_CALL instruction selects which helper
> --
> 2.33.0
>