bpf: Introduce dynamic program extensions

Introduce dynamic program extensions. The users can load additional BPF
functions and replace global functions in previously loaded BPF programs while
these programs are executing.

Global functions are verified individually by the verifier based on their types only.
Hence the global function in the new program which types match older function can
safely replace that corresponding function.

This new function/program is called 'an extension' of old program. At load time
the verifier uses (attach_prog_fd, attach_btf_id) pair to identify the function
to be replaced. The BPF program type is derived from the target program into
extension program. Technically bpf_verifier_ops is copied from target program.
The BPF_PROG_TYPE_EXT program type is a placeholder. It has empty verifier_ops.
The extension program can call the same bpf helper functions as target program.
Single BPF_PROG_TYPE_EXT type is used to extend XDP, SKB and all other program
types. The verifier allows only one level of replacement. Meaning that the
extension program cannot recursively extend an extension. That also means that
the maximum stack size is increasing from 512 to 1024 bytes and maximum
function nesting level from 8 to 16. The programs don't always consume that
much. The stack usage is determined by the number of on-stack variables used by
the program. The verifier could have enforced 512 limit for combined original
plus extension program, but it makes for difficult user experience. The main
use case for extensions is to provide generic mechanism to plug external
programs into policy program or function call chaining.

BPF trampoline is used to track both fentry/fexit and program extensions
because both are using the same nop slot at the beginning of every BPF
function. Attaching fentry/fexit to a function that was replaced is not
allowed. The opposite is true as well. Replacing a function that currently
being analyzed with fentry/fexit is not allowed. The executable page allocated
by BPF trampoline is not used by program extensions. This inefficiency will be
optimized in future patches.

Function by function verification of global function supports scalars and
pointer to context only. Hence program extensions are supported for such class
of global functions only. In the future the verifier will be extended with
support to pointers to structures, arrays with sizes, etc.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/bpf/20200121005348.2769920-2-ast@kernel.org

1 files changed, 38 insertions, 3 deletions

diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
index 7657ede7aee2..eb64c245052b 100644
--- a/kernel/bpf/trampoline.c
+++ b/kernel/bpf/trampoline.c
@@ -5,6 +5,12 @@
 #include <linux/filter.h>
 #include <linux/ftrace.h>
 
+/* dummy _ops. The verifier will operate on target program's ops. */
+const struct bpf_verifier_ops bpf_extension_verifier_ops = {
+};
+const struct bpf_prog_ops bpf_extension_prog_ops = {
+};
+
 /* btf_vmlinux has ~22k attachable functions. 1k htab is enough. */
 #define TRAMPOLINE_HASH_BITS 10
 #define TRAMPOLINE_TABLE_SIZE (1 << TRAMPOLINE_HASH_BITS)
@@ -194,8 +200,10 @@ static enum bpf_tramp_prog_type bpf_attach_type_to_tramp(enum bpf_attach_type t)
 	switch (t) {
 	case BPF_TRACE_FENTRY:
 		return BPF_TRAMP_FENTRY;
-	default:
+	case BPF_TRACE_FEXIT:
 		return BPF_TRAMP_FEXIT;
+	default:
+		return BPF_TRAMP_REPLACE;
 	}
 }
 
@@ -204,12 +212,31 @@ int bpf_trampoline_link_prog(struct bpf_prog *prog)
 	enum bpf_tramp_prog_type kind;
 	struct bpf_trampoline *tr;
 	int err = 0;
+	int cnt;
 
 	tr = prog->aux->trampoline;
 	kind = bpf_attach_type_to_tramp(prog->expected_attach_type);
 	mutex_lock(&tr->mutex);
-	if (tr->progs_cnt[BPF_TRAMP_FENTRY] + tr->progs_cnt[BPF_TRAMP_FEXIT]
-	    >= BPF_MAX_TRAMP_PROGS) {
+	if (tr->extension_prog) {
+		/* cannot attach fentry/fexit if extension prog is attached.
+		 * cannot overwrite extension prog either.
+		 */
+		err = -EBUSY;
+		goto out;
+	}
+	cnt = tr->progs_cnt[BPF_TRAMP_FENTRY] + tr->progs_cnt[BPF_TRAMP_FEXIT];
+	if (kind == BPF_TRAMP_REPLACE) {
+		/* Cannot attach extension if fentry/fexit are in use. */
+		if (cnt) {
+			err = -EBUSY;
+			goto out;
+		}
+		tr->extension_prog = prog;
+		err = bpf_arch_text_poke(tr->func.addr, BPF_MOD_JUMP, NULL,
+					 prog->bpf_func);
+		goto out;
+	}
+	if (cnt >= BPF_MAX_TRAMP_PROGS) {
 		err = -E2BIG;
 		goto out;
 	}
@@ -240,9 +267,17 @@ int bpf_trampoline_unlink_prog(struct bpf_prog *prog)
 	tr = prog->aux->trampoline;
 	kind = bpf_attach_type_to_tramp(prog->expected_attach_type);
 	mutex_lock(&tr->mutex);
+	if (kind == BPF_TRAMP_REPLACE) {
+		WARN_ON_ONCE(!tr->extension_prog);
+		err = bpf_arch_text_poke(tr->func.addr, BPF_MOD_JUMP,
+					 tr->extension_prog->bpf_func, NULL);
+		tr->extension_prog = NULL;
+		goto out;
+	}
 	hlist_del(&prog->aux->tramp_hlist);
 	tr->progs_cnt[kind]--;
 	err = bpf_trampoline_update(prog->aux->trampoline);
+out:
 	mutex_unlock(&tr->mutex);
 	return err;
 }