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.
Further, it's critical that BPF_LINK_GET_FD_BY_ID only ever allows to create
bpf_link FD that's O_RDONLY. This is to protect processes owning bpf_link and
thus allowed to perform modifications on them (like LINK_UPDATE), from other
processes that got bpf_link ID from GET_NEXT_ID API. In the latter case, only
querying bpf_link information (implemented later in the series) will be
allowed.
Signed-off-by: Andrii Nakryiko <andriin@xxxxxx>
---
include/uapi/linux/bpf.h | 2 ++
kernel/bpf/syscall.c | 56 ++++++++++++++++++++++++++++++++++++++++
2 files changed, 58 insertions(+)
diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index eccfd1dea951..407c086bc9e4 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -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 {
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 8b3a7d5814ae..527ec16702be 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -3719,6 +3719,55 @@ 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 open_flags
+
+static int bpf_link_get_fd_by_id(const union bpf_attr *attr)
+{
+ struct bpf_link *link;
+ u32 id = attr->link_id;
+ int f_flags;
+ int fd, err;
+
+ if (CHECK_ATTR(BPF_LINK_GET_FD_BY_ID) ||
+ /* links are not allowed to be open by ID as writable */
+ attr->open_flags & ~BPF_F_RDONLY)
+ return -EINVAL;
+
+ if (!capable(CAP_SYS_ADMIN))
+ return -EPERM;
+
+ f_flags = bpf_get_file_flag(attr->open_flags);
+ if (f_flags < 0)
+ return f_flags;
+
+ 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, f_flags);
+ 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;
@@ -3836,6 +3885,13 @@ 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;
--
2.24.1
