Commit 728ff167 authored by Emil Tsalapatis's avatar Emil Tsalapatis Committed by Alexei Starovoitov
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libbpf: Add gating for arena globals relocation feature 



Add feature gating for the arena globals relocation introduced in
commit c1f61171. The commit depends on a previous commit in the
same patchset that is absent from older kernels
(12a1fe6e "bpf/verifier: Do not limit maximum direct offset into arena map").

Without this commit, arena globals relocation with arenas >= 512MiB
fails to load and breaks libbpf's backwards compatibility.

Introduce a libbpf feature to check whether the running kernel allows for
full range ldimm64 offset, and only relocate arena globals if it does.

Fixes: c1f61171 ("libbpf: Move arena globals to the end of the arena")
Signed-off-by: default avatarEmil Tsalapatis <emil@etsalapatis.com>
Signed-off-by: default avatarAndrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20260210184532.255475-1-emil@etsalapatis.com


Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
parent 44331bd6

tools/lib/bpf/features.c

+64 −0
Original line number Diff line number Diff line
@@ -506,6 +506,67 @@ static int probe_kern_arg_ctx_tag(int token_fd) 
	return probe_fd(prog_fd);

}



static int probe_ldimm64_full_range_off(int token_fd)

{

	char log_buf[1024];

	int prog_fd, map_fd;

	int ret;

	LIBBPF_OPTS(bpf_map_create_opts, map_opts,

		.token_fd = token_fd,

		.map_flags = token_fd ? BPF_F_TOKEN_FD : 0,

	);

	LIBBPF_OPTS(bpf_prog_load_opts, prog_opts,

		.token_fd = token_fd,

		.prog_flags = token_fd ? BPF_F_TOKEN_FD : 0,

		.log_buf = log_buf,

		.log_size = sizeof(log_buf),

	);

	struct bpf_insn insns[] = {

		BPF_LD_MAP_VALUE(BPF_REG_1, 0, 1UL << 30),

		BPF_EXIT_INSN(),

	};

	int insn_cnt = ARRAY_SIZE(insns);



	map_fd = bpf_map_create(BPF_MAP_TYPE_ARRAY, "arr", sizeof(int), 1, 1, &map_opts);

	if (map_fd < 0) {

		ret = -errno;

		pr_warn("Error in %s(): %s. Couldn't create simple array map.\n",

			__func__, errstr(ret));

		return ret;

	}

	insns[0].imm = map_fd;



	prog_fd = bpf_prog_load(BPF_PROG_TYPE_TRACEPOINT, "global_reloc", "GPL", insns, insn_cnt, &prog_opts);

	ret = -errno;



	close(map_fd);

	close(prog_fd);



	if (prog_fd >= 0) {

		pr_warn("Error in %s(): Program loading unexpectedly succeeded.\n", __func__);

		return -EINVAL;

	}



	/*

	 * Feature is allowed if we're not failing with the error message

	 * "direct value offset of %u is not allowed" removed in

	 * 12a1fe6e12db ("bpf/verifier: Do not limit maximum direct offset into arena map").

	 * We should instead fail with "invalid access to map value pointer".

	 * Ensure we match with one of the two and we're not failing with a

	 * different, unexpected message.

	 */

	if (strstr(log_buf, "direct value offset of"))

		return 0;



	if (!strstr(log_buf, "invalid access to map value pointer")) {

		pr_warn("Error in %s(): Program unexpectedly failed with message: %s.\n",

			__func__, log_buf);

		return ret;

	}



	return 1;

}



typedef int (*feature_probe_fn)(int /* token_fd */);



static struct kern_feature_cache feature_cache;
@@ -581,6 +642,9 @@ static struct kern_feature_desc { 
	[FEAT_BTF_QMARK_DATASEC] = {

		"BTF DATASEC names starting from '?'", probe_kern_btf_qmark_datasec,

	},

	[FEAT_LDIMM64_FULL_RANGE_OFF] = {

		"full range LDIMM64 support", probe_ldimm64_full_range_off,

	},

};



bool feat_supported(struct kern_feature_cache *cache, enum kern_feature_id feat_id)

tools/lib/bpf/libbpf.c

+5 −2
Original line number Diff line number Diff line
@@ -3009,8 +3009,11 @@ static int init_arena_map_data(struct bpf_object *obj, struct bpf_map *map, 
	memcpy(obj->arena_data, data, data_sz);

	obj->arena_data_sz = data_sz;



	/* place globals at the end of the arena */

	/* place globals at the end of the arena (if supported) */

	if (kernel_supports(obj, FEAT_LDIMM64_FULL_RANGE_OFF))

		obj->arena_data_off = mmap_sz - data_alloc_sz;

	else

		obj->arena_data_off = 0;



	/* make bpf_map__init_value() work for ARENA maps */

	map->mmaped = obj->arena_data;

tools/lib/bpf/libbpf_internal.h

+2 −0
Original line number Diff line number Diff line
@@ -392,6 +392,8 @@ enum kern_feature_id { 
	FEAT_ARG_CTX_TAG,

	/* Kernel supports '?' at the front of datasec names */

	FEAT_BTF_QMARK_DATASEC,

	/* Kernel supports LDIMM64 imm offsets past 512 MiB. */

	FEAT_LDIMM64_FULL_RANGE_OFF,

	__FEAT_CNT,

};