bpf, libbpf: handle old kernels more graceful wrt global data sections

Andrii reported a corner case where e.g. global static data is present
in the BPF ELF file in form of .data/.bss/.rodata section, but without
any relocations to it. Such programs could be loaded before commit
d859900c4c56 ("bpf, libbpf: support global data/bss/rodata sections"),
whereas afterwards if kernel lacks support then loading would fail.

Add a probing mechanism which skips setting up libbpf internal maps
in case of missing kernel support. In presence of relocation entries,
we abort the load attempt.

Fixes: d859900c4c56 ("bpf, libbpf: support global data/bss/rodata sections")
Reported-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 86 insertions, 13 deletions

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index d817bf20f3d6..85315dedbde4 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -126,6 +126,8 @@ static inline __u64 ptr_to_u64(const void *ptr)
 struct bpf_capabilities {
         /* v4.14: kernel support for program & map names. */
         __u32 name:1;
+        /* v5.2: kernel support for global data sections. */
+        __u32 global_data:1;
 };
 
 /*
@@ -854,12 +856,15 @@ bpf_object__init_maps(struct bpf_object *obj, int flags)
          *
          * TODO: Detect array of map and report error.
          */
-        if (obj->efile.data_shndx >= 0)
-                nr_maps_glob++;
-        if (obj->efile.rodata_shndx >= 0)
-                nr_maps_glob++;
-        if (obj->efile.bss_shndx >= 0)
-                nr_maps_glob++;
+        if (obj->caps.global_data) {
+                if (obj->efile.data_shndx >= 0)
+                        nr_maps_glob++;
+                if (obj->efile.rodata_shndx >= 0)
+                        nr_maps_glob++;
+                if (obj->efile.bss_shndx >= 0)
+                        nr_maps_glob++;
+        }
+
         for (i = 0; data && i < nr_syms; i++) {
                 GElf_Sym sym;
 
@@ -971,6 +976,9 @@ bpf_object__init_maps(struct bpf_object *obj, int flags)
                 map_idx++;
         }
 
+        if (!obj->caps.global_data)
+                goto finalize;
+
         /*
          * Populate rest of obj->maps with libbpf internal maps.
          */
@@ -988,6 +996,7 @@ bpf_object__init_maps(struct bpf_object *obj, int flags)
                 ret = bpf_object__init_internal_map(obj, &obj->maps[map_idx++],
                                                     LIBBPF_MAP_BSS,
                                                     obj->efile.bss, NULL);
+finalize:
         if (!ret)
                 qsort(obj->maps, obj->nr_maps, sizeof(obj->maps[0]),
                       compare_bpf_map);
@@ -1333,11 +1342,17 @@ bpf_program__collect_reloc(struct bpf_program *prog, GElf_Shdr *shdr,
                 if (bpf_object__shndx_is_maps(obj, shdr_idx) ||
                     bpf_object__shndx_is_data(obj, shdr_idx)) {
                         type = bpf_object__section_to_libbpf_map_type(obj, shdr_idx);
-                        if (type != LIBBPF_MAP_UNSPEC &&
-                            GELF_ST_BIND(sym.st_info) == STB_GLOBAL) {
-                                pr_warning("bpf: relocation: not yet supported relo for non-static global \'%s\' variable found in insns[%d].code 0x%x\n",
-                                           name, insn_idx, insns[insn_idx].code);
-                                return -LIBBPF_ERRNO__RELOC;
+                        if (type != LIBBPF_MAP_UNSPEC) {
+                                if (GELF_ST_BIND(sym.st_info) == STB_GLOBAL) {
+                                        pr_warning("bpf: relocation: not yet supported relo for non-static global \'%s\' variable found in insns[%d].code 0x%x\n",
+                                                   name, insn_idx, insns[insn_idx].code);
+                                        return -LIBBPF_ERRNO__RELOC;
+                                }
+                                if (!obj->caps.global_data) {
+                                        pr_warning("bpf: relocation: kernel does not support global \'%s\' variable access in insns[%d]\n",
+                                                   name, insn_idx);
+                                        return -LIBBPF_ERRNO__RELOC;
+                                }
                         }
 
                         for (map_idx = 0; map_idx < nr_maps; map_idx++) {
@@ -1496,9 +1511,67 @@ bpf_object__probe_name(struct bpf_object *obj)
 }
 
 static int
+bpf_object__probe_global_data(struct bpf_object *obj)
+{
+        struct bpf_load_program_attr prg_attr;
+        struct bpf_create_map_attr map_attr;
+        char *cp, errmsg[STRERR_BUFSIZE];
+        struct bpf_insn insns[] = {
+                BPF_LD_MAP_VALUE(BPF_REG_1, 0, 16),
+                BPF_ST_MEM(BPF_DW, BPF_REG_1, 0, 42),
+                BPF_MOV64_IMM(BPF_REG_0, 0),
+                BPF_EXIT_INSN(),
+        };
+        int ret, map;
+
+        memset(&map_attr, 0, sizeof(map_attr));
+        map_attr.map_type = BPF_MAP_TYPE_ARRAY;
+        map_attr.key_size = sizeof(int);
+        map_attr.value_size = 32;
+        map_attr.max_entries = 1;
+
+        map = bpf_create_map_xattr(&map_attr);
+        if (map < 0) {
+                cp = libbpf_strerror_r(errno, errmsg, sizeof(errmsg));
+                pr_warning("Error in %s():%s(%d). Couldn't create simple array map.\n",
+                           __func__, cp, errno);
+                return -errno;
+        }
+
+        insns[0].imm = map;
+
+        memset(&prg_attr, 0, sizeof(prg_attr));
+        prg_attr.prog_type = BPF_PROG_TYPE_SOCKET_FILTER;
+        prg_attr.insns = insns;
+        prg_attr.insns_cnt = ARRAY_SIZE(insns);
+        prg_attr.license = "GPL";
+
+        ret = bpf_load_program_xattr(&prg_attr, NULL, 0);
+        if (ret >= 0) {
+                obj->caps.global_data = 1;
+                close(ret);
+        }
+
+        close(map);
+        return 0;
+}
+
+static int
 bpf_object__probe_caps(struct bpf_object *obj)
 {
-        return bpf_object__probe_name(obj);
+        int (*probe_fn[])(struct bpf_object *obj) = {
+                bpf_object__probe_name,
+                bpf_object__probe_global_data,
+        };
+        int i, ret;
+
+        for (i = 0; i < ARRAY_SIZE(probe_fn); i++) {
+                ret = probe_fn[i](obj);
+                if (ret < 0)
+                        return ret;
+        }
+
+        return 0;
 }
 
 static int
@@ -2100,6 +2173,7 @@ __bpf_object__open(const char *path, void *obj_buf, size_t obj_buf_sz,
 
         CHECK_ERR(bpf_object__elf_init(obj), err, out);
         CHECK_ERR(bpf_object__check_endianness(obj), err, out);
+        CHECK_ERR(bpf_object__probe_caps(obj), err, out);
         CHECK_ERR(bpf_object__elf_collect(obj, flags), err, out);
         CHECK_ERR(bpf_object__collect_reloc(obj), err, out);
         CHECK_ERR(bpf_object__validate(obj, needs_kver), err, out);
@@ -2193,7 +2267,6 @@ int bpf_object__load(struct bpf_object *obj)
 
         obj->loaded = true;
 
-        CHECK_ERR(bpf_object__probe_caps(obj), err, out);
         CHECK_ERR(bpf_object__create_maps(obj), err, out);
         CHECK_ERR(bpf_object__relocate(obj), err, out);
         CHECK_ERR(bpf_object__load_progs(obj), err, out);