Re: [PATCH bpf-next v3 15/17] libbpf: Add support for custom exception callbacks

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Tue, Sep 12, 2023 at 4:32 PM Kumar Kartikeya Dwivedi
<memxor@gmail.com> wrote:
>
> Add support to libbpf to append exception callbacks when loading a
> program. The exception callback is found by discovering the declaration
> tag 'exception_callback:<value>' and finding the callback in the value
> of the tag.
>
> The process is done in two steps. First, for each main program, the
> bpf_object__sanitize_and_load_btf function finds and marks its
> corresponding exception callback as defined by the declaration tag on
> it. Second, bpf_object__reloc_code is modified to append the indicated
> exception callback at the end of the instruction iteration (since
> exception callback will never be appended in that loop, as it is not
> directly referenced).
>
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> ---
>  tools/lib/bpf/libbpf.c | 114 +++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 109 insertions(+), 5 deletions(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index afc07a8f7dc7..3a6108e3238b 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -436,9 +436,11 @@ struct bpf_program {
>         int fd;
>         bool autoload;
>         bool autoattach;
> +       bool sym_global;
>         bool mark_btf_static;
>         enum bpf_prog_type type;
>         enum bpf_attach_type expected_attach_type;
> +       int exception_cb_idx;
>
>         int prog_ifindex;
>         __u32 attach_btf_obj_fd;
> @@ -765,6 +767,7 @@ bpf_object__init_prog(struct bpf_object *obj, struct bpf_program *prog,
>
>         prog->type = BPF_PROG_TYPE_UNSPEC;
>         prog->fd = -1;
> +       prog->exception_cb_idx = -1;
>
>         /* libbpf's convention for SEC("?abc...") is that it's just like
>          * SEC("abc...") but the corresponding bpf_program starts out with
> @@ -871,14 +874,16 @@ bpf_object__add_programs(struct bpf_object *obj, Elf_Data *sec_data,
>                 if (err)
>                         return err;
>
> +               if (ELF64_ST_BIND(sym->st_info) != STB_LOCAL)
> +                       prog->sym_global = true;
> +
>                 /* if function is a global/weak symbol, but has restricted
>                  * (STV_HIDDEN or STV_INTERNAL) visibility, mark its BTF FUNC
>                  * as static to enable more permissive BPF verification mode
>                  * with more outside context available to BPF verifier
>                  */
> -               if (ELF64_ST_BIND(sym->st_info) != STB_LOCAL
> -                   && (ELF64_ST_VISIBILITY(sym->st_other) == STV_HIDDEN
> -                       || ELF64_ST_VISIBILITY(sym->st_other) == STV_INTERNAL))
> +               if (prog->sym_global && (ELF64_ST_VISIBILITY(sym->st_other) == STV_HIDDEN
> +                   || ELF64_ST_VISIBILITY(sym->st_other) == STV_INTERNAL))
>                         prog->mark_btf_static = true;
>
>                 nr_progs++;
> @@ -3142,6 +3147,86 @@ static int bpf_object__sanitize_and_load_btf(struct bpf_object *obj)
>                 }
>         }
>
> +       if (!kernel_supports(obj, FEAT_BTF_DECL_TAG))
> +               goto skip_exception_cb;
> +       for (i = 0; i < obj->nr_programs; i++) {

I'm not sure why you chose to do these very inefficient three nested
for loops, tbh. Can you please send a follow up patch to make this a
bit more sane? There is no reason to iterate over BTF multiple times.
In general BPF object's BTF can have tons of information (especially
with vmlinux.h), so minimizing unnecessary linear searches here is
worth doing.

How about this structure:


for each btf type in btf:
   if not decl_tag and not "exception_callback:" one, continue

   prog_name = <find from decl_tag's referenced func>
   subprog_name = <find from decl_Tag's name>

   prog = find_by_name(prog_name);
   subprog = find_by_name(subprog_name);

   <check conditions>

   <remember idx; if it's already set, emit human-readable error and
exit, don't rely on BPF verifier to complain >

Thanks.

> +               struct bpf_program *prog = &obj->programs[i];
> +               int j, k, n;
> +
> +               if (prog_is_subprog(obj, prog))
> +                       continue;
> +               n = btf__type_cnt(obj->btf);
> +               for (j = 1; j < n; j++) {
> +                       const char *str = "exception_callback:", *name;
> +                       size_t len = strlen(str);
> +                       struct btf_type *t;
> +
> +                       t = btf_type_by_id(obj->btf, j);
> +                       if (!btf_is_decl_tag(t) || btf_decl_tag(t)->component_idx != -1)
> +                               continue;
> +
> +                       name = btf__str_by_offset(obj->btf, t->name_off);
> +                       if (strncmp(name, str, len))
> +                               continue;
> +
> +                       t = btf_type_by_id(obj->btf, t->type);
> +                       if (!btf_is_func(t) || btf_func_linkage(t) != BTF_FUNC_GLOBAL) {
> +                               pr_warn("prog '%s': exception_callback:<value> decl tag not applied to the main program\n",
> +                                       prog->name);
> +                               return -EINVAL;
> +                       }
> +                       if (strcmp(prog->name, btf__str_by_offset(obj->btf, t->name_off)))
> +                               continue;
> +                       /* Multiple callbacks are specified for the same prog,
> +                        * the verifier will eventually return an error for this
> +                        * case, hence simply skip appending a subprog.
> +                        */
> +                       if (prog->exception_cb_idx >= 0) {
> +                               prog->exception_cb_idx = -1;
> +                               break;
> +                       }

you check this condition three times and handle it in three different
ways, it's bizarre. Why?


> +
> +                       name += len;
> +                       if (str_is_empty(name)) {
> +                               pr_warn("prog '%s': exception_callback:<value> decl tag contains empty value\n",
> +                                       prog->name);
> +                               return -EINVAL;
> +                       }
> +
> +                       for (k = 0; k < obj->nr_programs; k++) {
> +                               struct bpf_program *subprog = &obj->programs[k];
> +
> +                               if (!prog_is_subprog(obj, subprog))
> +                                       continue;
> +                               if (strcmp(name, subprog->name))
> +                                       continue;
> +                               /* Enforce non-hidden, as from verifier point of
> +                                * view it expects global functions, whereas the
> +                                * mark_btf_static fixes up linkage as static.
> +                                */
> +                               if (!subprog->sym_global || subprog->mark_btf_static) {
> +                                       pr_warn("prog '%s': exception callback %s must be a global non-hidden function\n",
> +                                               prog->name, subprog->name);
> +                                       return -EINVAL;
> +                               }
> +                               /* Let's see if we already saw a static exception callback with the same name */
> +                               if (prog->exception_cb_idx >= 0) {
> +                                       pr_warn("prog '%s': multiple subprogs with same name as exception callback '%s'\n",
> +                                               prog->name, subprog->name);
> +                                       return -EINVAL;
> +                               }
> +                               prog->exception_cb_idx = k;
> +                               break;
> +                       }
> +
> +                       if (prog->exception_cb_idx >= 0)
> +                               continue;
> +                       pr_warn("prog '%s': cannot find exception callback '%s'\n", prog->name, name);
> +                       return -ENOENT;
> +               }
> +       }
> +skip_exception_cb:
> +
>         sanitize = btf_needs_sanitization(obj);
>         if (sanitize) {
>                 const void *raw_data;
> @@ -6270,10 +6355,10 @@ static int
>  bpf_object__reloc_code(struct bpf_object *obj, struct bpf_program *main_prog,
>                        struct bpf_program *prog)
>  {
> -       size_t sub_insn_idx, insn_idx, new_cnt;
> +       size_t sub_insn_idx, insn_idx;
>         struct bpf_program *subprog;
> -       struct bpf_insn *insns, *insn;
>         struct reloc_desc *relo;
> +       struct bpf_insn *insn;
>         int err;
>
>         err = reloc_prog_func_and_line_info(obj, main_prog, prog);
> @@ -6582,6 +6667,25 @@ bpf_object__relocate(struct bpf_object *obj, const char *targ_btf_path)
>                                 prog->name, err);
>                         return err;
>                 }
> +
> +               /* Now, also append exception callback if it has not been done already. */
> +               if (prog->exception_cb_idx >= 0) {
> +                       struct bpf_program *subprog = &obj->programs[prog->exception_cb_idx];
> +
> +                       /* Calling exception callback directly is disallowed, which the
> +                        * verifier will reject later. In case it was processed already,
> +                        * we can skip this step, otherwise for all other valid cases we
> +                        * have to append exception callback now.
> +                        */
> +                       if (subprog->sub_insn_off == 0) {
> +                               err = bpf_object__append_subprog_code(obj, prog, subprog);
> +                               if (err)
> +                                       return err;
> +                               err = bpf_object__reloc_code(obj, prog, subprog);
> +                               if (err)
> +                                       return err;
> +                       }
> +               }
>         }
>         /* Process data relos for main programs */
>         for (i = 0; i < obj->nr_programs; i++) {
> --
> 2.41.0
>