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bpf: Support GET_FD_BY_ID and GET_NEXT_ID for bpf_link

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Add support to look up bpf_link by ID and iterate over all existing bpf_links
in the system. GET_FD_BY_ID code handles not-yet-ready bpf_link by checking
that its ID hasn't been set to non-zero value yet. Setting bpf_link's ID is
done as the very last step in finalizing bpf_link, together with installing
FD. This approach allows users of bpf_link in kernel code to not worry about
races between user-space and kernel code that hasn't finished attaching and
initializing bpf_link.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200429001614.1544-4-andriin@fb.com
This commit is contained in:
Andrii Nakryiko 2020-04-28 17:16:07 -07:00 committed by Alexei Starovoitov
parent a3b80e1078
commit 2d602c8cf4
2 changed files with 51 additions and 0 deletions
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2
include/uapi/linux/bpf.h
View file

@ -113,6 +113,8 @@ enum bpf_cmd {
BPF_MAP_DELETE_BATCH,
BPF_LINK_CREATE,
BPF_LINK_UPDATE,
BPF_LINK_GET_FD_BY_ID,
BPF_LINK_GET_NEXT_ID,
};
enum bpf_map_type {

49
kernel/bpf/syscall.c
View file

@ -3713,6 +3713,48 @@ static int link_update(union bpf_attr *attr)
return ret;
}
static int bpf_link_inc_not_zero(struct bpf_link *link)
{
return atomic64_fetch_add_unless(&link->refcnt, 1, 0) ? 0 : -ENOENT;
}
#define BPF_LINK_GET_FD_BY_ID_LAST_FIELD link_id
static int bpf_link_get_fd_by_id(const union bpf_attr *attr)
{
struct bpf_link *link;
u32 id = attr->link_id;
int fd, err;
if (CHECK_ATTR(BPF_LINK_GET_FD_BY_ID))
return -EINVAL;
if (!capable(CAP_SYS_ADMIN))
return -EPERM;
spin_lock_bh(&link_idr_lock);
link = idr_find(&link_idr, id);
/* before link is "settled", ID is 0, pretend it doesn't exist yet */
if (link) {
if (link->id)
err = bpf_link_inc_not_zero(link);
else
err = -EAGAIN;
} else {
err = -ENOENT;
}
spin_unlock_bh(&link_idr_lock);
if (err)
return err;
fd = bpf_link_new_fd(link);
if (fd < 0)
bpf_link_put(link);
return fd;
}
SYSCALL_DEFINE3(bpf, int, cmd, union bpf_attr __user *, uattr, unsigned int, size)
{
union bpf_attr attr;
@ -3830,6 +3872,13 @@ SYSCALL_DEFINE3(bpf, int, cmd, union bpf_attr __user *, uattr, unsigned int, siz
case BPF_LINK_UPDATE:
err = link_update(&attr);
break;
case BPF_LINK_GET_FD_BY_ID:
err = bpf_link_get_fd_by_id(&attr);
break;
case BPF_LINK_GET_NEXT_ID:
err = bpf_obj_get_next_id(&attr, uattr,
&link_idr, &link_idr_lock);
break;
default:
err = -EINVAL;
break;