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@xxxxxxxxxx>
---
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);
[...]
