[PATCH bpf-next v3 2/5] bpf: Add support for setting chain call sequence for programs

["Toke Høiland-Jørgensen" <toke@redhat.com>]

From: Toke Høiland-Jørgensen <toke@redhat.com>

This adds support for setting and deleting bpf chain call programs through
a couple of new commands in the bpf() syscall. The CHAIN_ADD and CHAIN_DEL
commands take two eBPF program fds and a return code, and install the
'next' program to be chain called after the 'prev' program if that program
returns 'retcode'. A retcode of -1 means "wildcard", so that the program
will be executed regardless of the previous program's return code.


The syscall command names are based on Alexei's prog_chain example[0],
which Alan helpfully rebased on current bpf-next. However, the logic and
program storage is obviously adapted to the execution logic in the previous
commit.

[0] https://git.kernel.org/pub/scm/linux/kernel/git/ast/bpf.git/commit/?h=prog_chain&id=f54f45d00f91e083f6aec2abe35b6f0be52ae85b&context=15

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
---
 include/uapi/linux/bpf.h |   10 ++++++
 kernel/bpf/syscall.c     |   78 ++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 88 insertions(+)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 1ce80a227be3..b03c23963af8 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -107,6 +107,9 @@ enum bpf_cmd {
 	BPF_MAP_LOOKUP_AND_DELETE_ELEM,
 	BPF_MAP_FREEZE,
 	BPF_BTF_GET_NEXT_ID,
+	BPF_PROG_CHAIN_ADD,
+	BPF_PROG_CHAIN_DEL,
+	BPF_PROG_CHAIN_GET,
 };
 
 enum bpf_map_type {
@@ -516,6 +519,13 @@ union bpf_attr {
 		__u64		probe_offset;	/* output: probe_offset */
 		__u64		probe_addr;	/* output: probe_addr */
 	} task_fd_query;
+
+	struct { /* anonymous struct used by BPF_PROG_CHAIN_* commands */
+		__u32		prev_prog_fd;
+		__u32		next_prog_fd;
+		__u32		retcode;
+		__u32		next_prog_id;   /* output: prog_id */
+	};
 } __attribute__((aligned(8)));
 
 /* The description below is an attempt at providing documentation to eBPF
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index b8a203a05881..be8112e08a88 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -2113,6 +2113,79 @@ static int bpf_prog_test_run(const union bpf_attr *attr,
 	return ret;
 }
 
+#define BPF_PROG_CHAIN_LAST_FIELD next_prog_id
+
+static int bpf_prog_chain(int cmd, const union bpf_attr *attr,
+			  union bpf_attr __user *uattr)
+{
+	struct bpf_prog *prog, *next_prog, *old_prog;
+	struct bpf_prog **array;
+	int ret = -EOPNOTSUPP;
+	u32 index, prog_id;
+
+	if (CHECK_ATTR(BPF_PROG_CHAIN))
+		return -EINVAL;
+
+	/* Index 0 is wildcard, encoded as ~0 by userspace */
+	if (attr->retcode == ((u32) ~0))
+		index = 0;
+	else
+		index = attr->retcode + 1;
+
+	if (index >= BPF_NUM_CHAIN_SLOTS)
+		return -E2BIG;
+
+	prog = bpf_prog_get(attr->prev_prog_fd);
+	if (IS_ERR(prog))
+		return PTR_ERR(prog);
+
+	/* If the chain_calls bit is not set, that's because the chain call flag
+	 * was not set on program load, and so we can't support chain calls.
+	 */
+	if (!prog->chain_calls)
+		goto out;
+
+	array = prog->aux->chain_progs;
+
+	switch (cmd) {
+	case BPF_PROG_CHAIN_ADD:
+		next_prog = bpf_prog_get(attr->next_prog_fd);
+		if (IS_ERR(next_prog)) {
+			ret = PTR_ERR(next_prog);
+			break;
+		}
+		old_prog = xchg(array + index, next_prog);
+		if (old_prog)
+			bpf_prog_put(old_prog);
+		ret = 0;
+		break;
+	case BPF_PROG_CHAIN_DEL:
+		old_prog = xchg(array + index, NULL);
+		if (old_prog) {
+			bpf_prog_put(old_prog);
+			ret = 0;
+		} else {
+			ret = -ENOENT;
+		}
+		break;
+	case BPF_PROG_CHAIN_GET:
+		old_prog = READ_ONCE(array[index]);
+		if (old_prog) {
+			prog_id = old_prog->aux->id;
+			if (put_user(prog_id, &uattr->next_prog_id))
+				ret = -EFAULT;
+			else
+				ret = 0;
+		} else
+			ret = -ENOENT;
+		break;
+	}
+
+out:
+	bpf_prog_put(prog);
+	return ret;
+}
+
 #define BPF_OBJ_GET_NEXT_ID_LAST_FIELD next_id
 
 static int bpf_obj_get_next_id(const union bpf_attr *attr,
@@ -2885,6 +2958,11 @@ SYSCALL_DEFINE3(bpf, int, cmd, union bpf_attr __user *, uattr, unsigned int, siz
 	case BPF_PROG_TEST_RUN:
 		err = bpf_prog_test_run(&attr, uattr);
 		break;
+	case BPF_PROG_CHAIN_ADD:
+	case BPF_PROG_CHAIN_DEL:
+	case BPF_PROG_CHAIN_GET:
+		err = bpf_prog_chain(cmd, &attr, uattr);
+		break;
 	case BPF_PROG_GET_NEXT_ID:
 		err = bpf_obj_get_next_id(&attr, uattr,
 					  &prog_idr, &prog_idr_lock);