bpf: add support for persistent maps/progs

This work adds support for "persistent" eBPF maps/programs. The term
"persistent" is to be understood that maps/programs have a facility
that lets them survive process termination. This is desired by various
eBPF subsystem users.

Just to name one example: tc classifier/action. Whenever tc parses
the ELF object, extracts and loads maps/progs into the kernel, these
file descriptors will be out of reach after the tc instance exits.
So a subsequent tc invocation won't be able to access/relocate on this
resource, and therefore maps cannot easily be shared, f.e. between the
ingress and egress networking data path.

The current workaround is that Unix domain sockets (UDS) need to be
instrumented in order to pass the created eBPF map/program file
descriptors to a third party management daemon through UDS' socket
passing facility. This makes it a bit complicated to deploy shared
eBPF maps or programs (programs f.e. for tail calls) among various
processes.

We've been brainstorming on how we could tackle this issue and various
approches have been tried out so far, which can be read up further in
the below reference.

The architecture we eventually ended up with is a minimal file system
that can hold map/prog objects. The file system is a per mount namespace
singleton, and the default mount point is /sys/fs/bpf/. Any subsequent
mounts within a given namespace will point to the same instance. The
file system allows for creating a user-defined directory structure.
The objects for maps/progs are created/fetched through bpf(2) with
two new commands (BPF_OBJ_PIN/BPF_OBJ_GET). I.e. a bpf file descriptor
along with a pathname is being passed to bpf(2) that in turn creates
(we call it eBPF object pinning) the file system nodes. Only the pathname
is being passed to bpf(2) for getting a new BPF file descriptor to an
existing node. The user can use that to access maps and progs later on,
through bpf(2). Removal of file system nodes is being managed through
normal VFS functions such as unlink(2), etc. The file system code is
kept to a very minimum and can be further extended later on.

The next step I'm working on is to add dump eBPF map/prog commands
to bpf(2), so that a specification from a given file descriptor can
be retrieved. This can be used by things like CRIU but also applications
can inspect the meta data after calling BPF_OBJ_GET.

Big thanks also to Alexei and Hannes who significantly contributed
in the design discussion that eventually let us end up with this
architecture here.

Reference: https://lkml.org/lkml/2015/10/15/925
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 27 insertions, 3 deletions

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index d7783cb04d86..0d3313d02a7e 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -111,7 +111,7 @@ static const struct file_operations bpf_map_fops = {
         .release = bpf_map_release,
 };
 
-static int bpf_map_new_fd(struct bpf_map *map)
+int bpf_map_new_fd(struct bpf_map *map)
 {
         return anon_inode_getfd("bpf-map", &bpf_map_fops, map,
                                 O_RDWR | O_CLOEXEC);
@@ -174,7 +174,7 @@ struct bpf_map *__bpf_map_get(struct fd f)
         return f.file->private_data;
 }
 
-static struct bpf_map *bpf_map_get(u32 ufd)
+struct bpf_map *bpf_map_get(u32 ufd)
 {
         struct fd f = fdget(ufd);
         struct bpf_map *map;
@@ -548,7 +548,7 @@ static const struct file_operations bpf_prog_fops = {
         .release = bpf_prog_release,
 };
 
-static int bpf_prog_new_fd(struct bpf_prog *prog)
+int bpf_prog_new_fd(struct bpf_prog *prog)
 {
         return anon_inode_getfd("bpf-prog", &bpf_prog_fops, prog,
                                 O_RDWR | O_CLOEXEC);
@@ -674,6 +674,24 @@ free_prog_nouncharge:
         return err;
 }
 
+#define BPF_OBJ_LAST_FIELD bpf_fd
+
+static int bpf_obj_pin(const union bpf_attr *attr)
+{
+        if (CHECK_ATTR(BPF_OBJ))
+                return -EINVAL;
+
+        return bpf_obj_pin_user(attr->bpf_fd, u64_to_ptr(attr->pathname));
+}
+
+static int bpf_obj_get(const union bpf_attr *attr)
+{
+        if (CHECK_ATTR(BPF_OBJ) || attr->bpf_fd != 0)
+                return -EINVAL;
+
+        return bpf_obj_get_user(u64_to_ptr(attr->pathname));
+}
+
 SYSCALL_DEFINE3(bpf, int, cmd, union bpf_attr __user *, uattr, unsigned int, size)
 {
         union bpf_attr attr = {};
@@ -734,6 +752,12 @@ SYSCALL_DEFINE3(bpf, int, cmd, union bpf_attr __user *, uattr, unsigned int, siz
         case BPF_PROG_LOAD:
                 err = bpf_prog_load(&attr);
                 break;
+        case BPF_OBJ_PIN:
+                err = bpf_obj_pin(&attr);
+                break;
+        case BPF_OBJ_GET:
+                err = bpf_obj_get(&attr);
+                break;
         default:
                 err = -EINVAL;
                 break;