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libbpf: Keep the fd_array fill index out of bpf_program__clone()

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bpf_program__clone() hands the kernel the fd_array of the object along
with obj->fd_array_cnt when the caller supplies neither. That counter is
not a count the kernel can use: it is the index of the next free slot
of an array built for the sparse fd_array API, where module BTF fds
start at slot 1 and slot 0 stays empty, since a kfunc insn with offset
0 names vmlinux BTF rather than a slot. A non-zero fd_array_cnt turns
the array into a continuous one, which the kernel resolves slot by
slot, and slot 0 holds 0. Cloning a program from an object that calls a
module kfunc thus fails with "fd 0 is not pointing to valid bpf_map or
btf", for every program of the object, which is what veristat runs into
on such an object. bpf_object__load() passes the array without a count.

Do the same in bpf_program__clone(), leaving the count to callers who
pass an array of their own, and rename the counter to fd_array_used so
that it no longer reads like the uapi field.

Fixes: 970bd2dced ("libbpf: Introduce bpf_program__clone()")
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Mykyta Yatsenko <yatsenko@meta.com>
Link: https://patch.msgid.link/20261019132203.3034698-4-memxor@gmail.com
This commit is contained in:
Kumar Kartikeya Dwivedi 2026-10-19 15:21:45 +02:00 committed by Alexei Starovoitov
parent c2e56c88f1
commit b1eaee8b9a
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1 changed files with 13 additions and 8 deletions
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21
tools/lib/bpf/libbpf.c
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@ -794,7 +794,12 @@ struct bpf_object {
int *fd_array;
size_t fd_array_cap;
size_t fd_array_cnt;
/*
* Slots handed out so far, slot 0 included. A kfunc insn names a module
* BTF slot through its offset, slot 0 stands for vmlinux BTF and stays
* empty, so the array goes to the kernel without a count.
*/
size_t fd_array_used;
struct usdt_manager *usdt_man;
@ -9716,22 +9721,22 @@ static int bpf_object__resolve_ksym_func_btf_id(struct bpf_object *obj,
/* set index for module BTF fd in fd_array, if unset */
if (mod_btf && !mod_btf->fd_array_idx) {
/* insn->off is s16 */
if (obj->fd_array_cnt == INT16_MAX) {
if (obj->fd_array_used == INT16_MAX) {
pr_warn("extern (func ksym) '%s': module BTF fd index %d too big to fit in bpf_insn offset\n",
ext->name, mod_btf->fd_array_idx);
return -E2BIG;
}
/* Cannot use index 0 for module BTF fd */
if (!obj->fd_array_cnt)
obj->fd_array_cnt = 1;
if (!obj->fd_array_used)
obj->fd_array_used = 1;
ret = libbpf_ensure_mem((void **)&obj->fd_array, &obj->fd_array_cap, sizeof(int),
obj->fd_array_cnt + 1);
obj->fd_array_used + 1);
if (ret)
return ret;
mod_btf->fd_array_idx = obj->fd_array_cnt;
mod_btf->fd_array_idx = obj->fd_array_used;
/* we assume module BTF FD is always >0 */
obj->fd_array[obj->fd_array_cnt++] = mod_btf->fd;
obj->fd_array[obj->fd_array_used++] = mod_btf->fd;
}
ext->is_set = true;
@ -10940,7 +10945,7 @@ int bpf_program__clone(struct bpf_program *prog, const struct bpf_prog_load_opts
attr.prog_ifindex = OPTS_GET(opts, prog_ifindex, 0) ?: prog->prog_ifindex;
attr.kern_version = OPTS_GET(opts, kern_version, 0) ?: obj->kern_version;
attr.fd_array = OPTS_GET(opts, fd_array, NULL) ?: obj->fd_array;
attr.fd_array_cnt = OPTS_GET(opts, fd_array_cnt, 0) ?: obj->fd_array_cnt;
attr.fd_array_cnt = OPTS_GET(opts, fd_array_cnt, 0);
attr.token_fd = OPTS_GET(opts, token_fd, 0) ?: obj->token_fd;
if (attr.token_fd)
attr.prog_flags |= BPF_F_TOKEN_FD;