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Date: Wed, 21 Apr 2021 22:43:14 -0700
From: Yonghong Song <yhs@...com>
To: Andrii Nakryiko <andrii@...nel.org>, <bpf@...r.kernel.org>,
<netdev@...r.kernel.org>, <ast@...com>, <daniel@...earbox.net>
CC: <kernel-team@...com>
Subject: Re: [PATCH v2 bpf-next 04/17] libbpf: mark BPF subprogs with hidden
visibility as static for BPF verifier
On 4/16/21 1:23 PM, Andrii Nakryiko wrote:
> Define __hidden helper macro in bpf_helpers.h, which is a short-hand for
> __attribute__((visibility("hidden"))). Add libbpf support to mark BPF
> subprograms marked with __hidden as static in BTF information to enforce BPF
> verifier's static function validation algorithm, which takes more information
> (caller's context) into account during a subprogram validation.
>
> Signed-off-by: Andrii Nakryiko <andrii@...nel.org>
> ---
> tools/lib/bpf/bpf_helpers.h | 8 ++++++
> tools/lib/bpf/btf.c | 5 ----
> tools/lib/bpf/libbpf.c | 45 ++++++++++++++++++++++++++++++++-
> tools/lib/bpf/libbpf_internal.h | 6 +++++
> 4 files changed, 58 insertions(+), 6 deletions(-)
>
> diff --git a/tools/lib/bpf/bpf_helpers.h b/tools/lib/bpf/bpf_helpers.h
> index 75c7581b304c..9720dc0b4605 100644
> --- a/tools/lib/bpf/bpf_helpers.h
> +++ b/tools/lib/bpf/bpf_helpers.h
> @@ -47,6 +47,14 @@
> #define __weak __attribute__((weak))
> #endif
>
> +/*
> + * Use __hidden attribute to mark a non-static BPF subprogram effectively
> + * static for BPF verifier's verification algorithm purposes, allowing more
> + * extensive and permissive BPF verification process, taking into account
> + * subprogram's caller context.
> + */
> +#define __hidden __attribute__((visibility("hidden")))
To prevent potential external __hidden macro definition conflict, how
about
#ifdef __hidden
#undef __hidden
#define __hidden __attribute__((visibility("hidden")))
#endif
> +
> /* When utilizing vmlinux.h with BPF CO-RE, user BPF programs can't include
> * any system-level headers (such as stddef.h, linux/version.h, etc), and
> * commonly-used macros like NULL and KERNEL_VERSION aren't available through
> diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
> index d30e67e7e1e5..d57e13a13798 100644
> --- a/tools/lib/bpf/btf.c
> +++ b/tools/lib/bpf/btf.c
> @@ -1605,11 +1605,6 @@ static void *btf_add_type_mem(struct btf *btf, size_t add_sz)
> btf->hdr->type_len, UINT_MAX, add_sz);
> }
>
> -static __u32 btf_type_info(int kind, int vlen, int kflag)
> -{
> - return (kflag << 31) | (kind << 24) | vlen;
> -}
> -
> static void btf_type_inc_vlen(struct btf_type *t)
> {
> t->info = btf_type_info(btf_kind(t), btf_vlen(t) + 1, btf_kflag(t));
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 9cc2d45b0080..ce5558d0a61b 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -71,6 +71,7 @@
> static struct bpf_map *bpf_object__add_map(struct bpf_object *obj);
> static const struct btf_type *
> skip_mods_and_typedefs(const struct btf *btf, __u32 id, __u32 *res_id);
> +static bool prog_is_subprog(const struct bpf_object *obj, const struct bpf_program *prog);
>
> static int __base_pr(enum libbpf_print_level level, const char *format,
> va_list args)
> @@ -274,6 +275,7 @@ struct bpf_program {
> bpf_program_clear_priv_t clear_priv;
>
> bool load;
> + bool mark_btf_static;
> enum bpf_prog_type type;
> enum bpf_attach_type expected_attach_type;
> int prog_ifindex;
> @@ -698,6 +700,15 @@ bpf_object__add_programs(struct bpf_object *obj, Elf_Data *sec_data,
> if (err)
> return err;
>
> + /* if function is a global/weak symbol, but has hidden
> + * visibility (or any non-default one), mark its BTF FUNC as
> + * static to enable more permissive BPF verification mode with
> + * more outside context available to BPF verifier
> + */
> + if (GELF_ST_BIND(sym.st_info) != STB_LOCAL
> + && GELF_ST_VISIBILITY(sym.st_other) != STV_DEFAULT)
Maybe we should check GELF_ST_VISIBILITY(sym.st_other) == STV_HIDDEN
instead?
> + prog->mark_btf_static = true;
> +
> nr_progs++;
> obj->nr_programs = nr_progs;
>
> @@ -2618,7 +2629,7 @@ static int bpf_object__sanitize_and_load_btf(struct bpf_object *obj)
> {
> struct btf *kern_btf = obj->btf;
> bool btf_mandatory, sanitize;
> - int err = 0;
> + int i, err = 0;
>
> if (!obj->btf)
> return 0;
> @@ -2632,6 +2643,38 @@ static int bpf_object__sanitize_and_load_btf(struct bpf_object *obj)
> return 0;
> }
>
> + /* Even though some subprogs are global/weak, user might prefer more
> + * permissive BPF verification process that BPF verifier performs for
> + * static functions, taking into account more context from the caller
> + * functions. In such case, they need to mark such subprogs with
> + * __attribute__((visibility("hidden"))) and libbpf will adjust
> + * corresponding FUNC BTF type to be marked as static and trigger more
> + * involved BPF verification process.
> + */
> + for (i = 0; i < obj->nr_programs; i++) {
> + struct bpf_program *prog = &obj->programs[i];
> + struct btf_type *t;
> + const char *name;
> + int j, n;
> +
> + if (!prog->mark_btf_static || !prog_is_subprog(obj, prog))
> + continue;
> +
> + n = btf__get_nr_types(obj->btf);
> + for (j = 1; j <= n; j++) {
> + t = btf_type_by_id(obj->btf, j);
> + if (!btf_is_func(t) || btf_func_linkage(t) != BTF_FUNC_GLOBAL)
> + continue;
> +
> + name = btf__str_by_offset(obj->btf, t->name_off);
> + if (strcmp(name, prog->name) != 0)
> + continue;
> +
> + t->info = btf_type_info(BTF_KIND_FUNC, BTF_FUNC_STATIC, 0);
> + break;
> + }
> + }
> +
> sanitize = btf_needs_sanitization(obj);
[...]
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