libbpf: detect supported kernel BTF features and sanitize BTF

Depending on used versions of libbpf, Clang, and kernel, it's possible to
have valid BPF object files with valid BTF information, that still won't
load successfully due to Clang emitting newer BTF features (e.g.,
BTF_KIND_FUNC, .BTF.ext's line_info/func_info, BTF_KIND_DATASEC, etc), that
are not yet supported by older kernel.

This patch adds detection of BTF features and sanitizes BPF object's BTF
by substituting various supported BTF kinds, which have compatible layout:
  - BTF_KIND_FUNC -> BTF_KIND_TYPEDEF
  - BTF_KIND_FUNC_PROTO -> BTF_KIND_ENUM
  - BTF_KIND_VAR -> BTF_KIND_INT
  - BTF_KIND_DATASEC -> BTF_KIND_STRUCT

Replacement is done in such a way as to preserve as much information as
possible (names, sizes, etc) where possible without violating kernel's
validation rules.

v2->v3:
  - remove duplicate #defines from libbpf_util.h

v1->v2:
  - add internal libbpf_internal.h w/ common stuff
  - switch SK storage BTF to use new libbpf__probe_raw_btf()

Reported-by: Alexei Starovoitov <ast@fb.com>
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

1 files changed, 41 insertions, 32 deletions

diff --git a/tools/lib/bpf/libbpf_probes.c b/tools/lib/bpf/libbpf_probes.c
index a2c64a9ce1a6..5e2aa83f637a 100644
--- a/tools/lib/bpf/libbpf_probes.c
+++ b/tools/lib/bpf/libbpf_probes.c
@@ -15,6 +15,7 @@
 
 #include "bpf.h"
 #include "libbpf.h"
+#include "libbpf_internal.h"
 
 static bool grep(const char *buffer, const char *pattern)
 {
@@ -132,21 +133,43 @@ bool bpf_probe_prog_type(enum bpf_prog_type prog_type, __u32 ifindex)
         return errno != EINVAL && errno != EOPNOTSUPP;
 }
 
-static int load_btf(void)
+int libbpf__probe_raw_btf(const char *raw_types, size_t types_len,
+                          const char *str_sec, size_t str_len)
 {
-#define BTF_INFO_ENC(kind, kind_flag, vlen) \
-        ((!!(kind_flag) << 31) | ((kind) << 24) | ((vlen) & BTF_MAX_VLEN))
-#define BTF_TYPE_ENC(name, info, size_or_type) \
-        (name), (info), (size_or_type)
-#define BTF_INT_ENC(encoding, bits_offset, nr_bits) \
-        ((encoding) << 24 | (bits_offset) << 16 | (nr_bits))
-#define BTF_TYPE_INT_ENC(name, encoding, bits_offset, bits, sz) \
-        BTF_TYPE_ENC(name, BTF_INFO_ENC(BTF_KIND_INT, 0, 0), sz), \
-        BTF_INT_ENC(encoding, bits_offset, bits)
-#define BTF_MEMBER_ENC(name, type, bits_offset) \
-        (name), (type), (bits_offset)
-
-        const char btf_str_sec[] = "\0bpf_spin_lock\0val\0cnt\0l";
+        struct btf_header hdr = {
+                .magic = BTF_MAGIC,
+                .version = BTF_VERSION,
+                .hdr_len = sizeof(struct btf_header),
+                .type_len = types_len,
+                .str_off = types_len,
+                .str_len = str_len,
+        };
+        int btf_fd, btf_len;
+        __u8 *raw_btf;
+
+        btf_len = hdr.hdr_len + hdr.type_len + hdr.str_len;
+        raw_btf = malloc(btf_len);
+        if (!raw_btf)
+                return -ENOMEM;
+
+        memcpy(raw_btf, &hdr, sizeof(hdr));
+        memcpy(raw_btf + hdr.hdr_len, raw_types, hdr.type_len);
+        memcpy(raw_btf + hdr.hdr_len + hdr.type_len, str_sec, hdr.str_len);
+
+        btf_fd = bpf_load_btf(raw_btf, btf_len, NULL, 0, false);
+        if (btf_fd < 0) {
+                free(raw_btf);
+                return 0;
+        }
+
+        close(btf_fd);
+        free(raw_btf);
+        return 1;
+}
+
+static int load_sk_storage_btf(void)
+{
+        const char strs[] = "\0bpf_spin_lock\0val\0cnt\0l";
         /* struct bpf_spin_lock {
          *   int val;
          * };
@@ -155,7 +178,7 @@ static int load_btf(void)
          *   struct bpf_spin_lock l;
          * };
          */
-        __u32 btf_raw_types[] = {
+        __u32 types[] = {
                 /* int */
                 BTF_TYPE_INT_ENC(0, BTF_INT_SIGNED, 0, 32, 4),  /* [1] */
                 /* struct bpf_spin_lock */                      /* [2] */
@@ -166,23 +189,9 @@ static int load_btf(void)
                 BTF_MEMBER_ENC(19, 1, 0), /* int cnt; */
                 BTF_MEMBER_ENC(23, 2, 32),/* struct bpf_spin_lock l; */
         };
-        struct btf_header btf_hdr = {
-                .magic = BTF_MAGIC,
-                .version = BTF_VERSION,
-                .hdr_len = sizeof(struct btf_header),
-                .type_len = sizeof(btf_raw_types),
-                .str_off = sizeof(btf_raw_types),
-                .str_len = sizeof(btf_str_sec),
-        };
-        __u8 raw_btf[sizeof(struct btf_header) + sizeof(btf_raw_types) +
-                     sizeof(btf_str_sec)];
-
-        memcpy(raw_btf, &btf_hdr, sizeof(btf_hdr));
-        memcpy(raw_btf + sizeof(btf_hdr), btf_raw_types, sizeof(btf_raw_types));
-        memcpy(raw_btf + sizeof(btf_hdr) + sizeof(btf_raw_types),
-               btf_str_sec, sizeof(btf_str_sec));
 
-        return bpf_load_btf(raw_btf, sizeof(raw_btf), 0, 0, 0);
+        return libbpf__probe_raw_btf((char *)types, sizeof(types),
+                                     strs, sizeof(strs));
 }
 
 bool bpf_probe_map_type(enum bpf_map_type map_type, __u32 ifindex)
@@ -222,7 +231,7 @@ bool bpf_probe_map_type(enum bpf_map_type map_type, __u32 ifindex)
                 value_size = 8;
                 max_entries = 0;
                 map_flags = BPF_F_NO_PREALLOC;
-                btf_fd = load_btf();
+                btf_fd = load_sk_storage_btf();
                 if (btf_fd < 0)
                         return false;
                 break;