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libbpf: Add gating for arena globals relocation feature

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Add feature gating for the arena globals relocation introduced in
commit c1f61171d4. The commit depends on a previous commit in the
same patchset that is absent from older kernels
(12a1fe6e12 "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: c1f61171d4 ("libbpf: Move arena globals to the end of the arena")
Signed-off-by: Emil Tsalapatis <emil@etsalapatis.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20260210184532.255475-1-emil@etsalapatis.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Emil Tsalapatis 2026-02-10 13:45:32 -05:00 • committed by Alexei Starovoitov
parent 44331bd6a6
commit 728ff16791
3 changed files with 71 additions and 2 deletions
Download patch file Download diff file Expand all files Collapse all files

64
tools/lib/bpf/features.c
View file

@ -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)

7
tools/lib/bpf/libbpf.c
View file

@ -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 */
obj->arena_data_off = mmap_sz - data_alloc_sz;
/* 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;

2
tools/lib/bpf/libbpf_internal.h
View file

@ -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,
};