Commit 8837fe5d authored by Daniel Borkmann's avatar Daniel Borkmann Committed by Alexei Starovoitov
Browse files

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
d859900c ("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: d859900c ("bpf, libbpf: support global data/bss/rodata sections")
Reported-by: default avatarAndrii Nakryiko <andriin@fb.com>
Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
parent a21b48a2

tools/lib/bpf/libbpf.c

+86 −13
Original line number Diff line number Diff line
@@ -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->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);
@@ -1333,12 +1342,18 @@ 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) {

			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++) {

				if (maps[map_idx].libbpf_type != type)
@@ -1495,10 +1510,68 @@ bpf_object__probe_name(struct bpf_object *obj) 
	return 0;

}



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);

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