Re: [PATCH v2 bpf-next 04/17] libbpf: mark BPF subprogs with hidden visibility as static for BPF verifier

[Yonghong Song <yhs@fb.com>]

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@kernel.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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